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  • 1995-1999  (2,415)
  • 1996  (2,415)
  • General Chemistry  (1,786)
  • Life and Medical Sciences  (629)
  • 101
    Electronic Resource
    Electronic Resource
    Studies on the Topography of Biomembranes: Regioselective Photolabelling in Vesicles with the Tandem Use of Cholesterol and a Photoactivable Transmembrane Phospholipidic Probe (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 129-138 
    Details
    ISSN: 0947-6539
    Keywords: bilayers ; cholesterol ; membrane ; probes ; phospholipids ; photolabelling ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2H NMR, DSC and UV studies of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) vesicles have demonstrated the favourable physicochemical properties of the transmembrane phospholipid probes 1 a and 1 b for membrane topographical studies. In particular, in the presence of a physiological amount of cholesterol, only one transmembrane conformation is observed. The use of 1 a and cholesterol together for photolabelling experiments in DMPC vesicles led to a remarkable improvement in the regioselectivity of cross-linking between 1 a and DMPC, and between 1 a and cholesterol: the myristoyl chains functionalized at C11, C12 and C13 made up 95% of the total photolabelled myristates, and cholesterol was principally functionalized at the C25 position on the side-chain. This indicates the formation of a highly ordered bilayer structure and proves directly the orientation of cholesterol perpendicular to the membrane plane with its chain terminal buried in the middle of the bilayer.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020204
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  • 102
    Electronic Resource
    Electronic Resource
    Ab Initio Quantum Mechanical Model Calculations on the Catalytic Mechanism of Aspartylglucosaminidase (AGA): A Serine Protease-Like Mechanism with an N-terminal Threonine and Substrate-Assisted Catalysis (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1548-1551 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; enzymatic catalysis ; enzyme models ; mechanistic studies ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio quantum mechanical model calculations were used in studying the acylation and deacylation steps of the catalytic mechanism of aspartylglucosaminidase (AGA). AGA catalyses the hydrolysis of an amide linkage between oligosaccharide and asparagine by utilising an N-terminal threonine as a catalytic amino acid. Results are reported for the model enzyme reaction at the MP2/6-31+G*//HF/6-31 + G* + ΔZPE level. Contribution of aqueous solvation to the relative energies was estimated by using the continuum solvation model of Tomasi. The serine protease-like catalytic mechanism was found to be feasible for AGA. The protonated α-amino group of the substrate of AGA was suggested to enhance the catalysis by stabilising the anionic oxygen of the substrate, which is formed in the reaction, and by lowering the pKa of the nucleophilic oxygen of the N-terminal threonine. Finally, the similarities in the catalytic mechanisms of AGA and other amidohydrolases were discussed.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021212
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  • 103
    Electronic Resource
    Electronic Resource
    Mineralization of Gold Nanoparticles in a Block Copolymer Microemulsion (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1552-1555 
    Details
    ISSN: 0947-6539
    Keywords: block copolymers ; micelles ; mineralization ; nanostructures ; thin films ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Controlled mineralization of gold nanoparticles has been performed in a microemulsion of polystyrene-block-poly(2-vinylpyridine). The starting point was the formation of a thermodynamically stable dispersion of HAuCl4 in inverse micelles of the block copolymer in toluene, which became metastable when the gold was reduced. Kinetic control of the transformation allowed the following stages of the mineralization/coagulation process to be stabilized: 1) one gold particle per micelle, 2) aggregated micelles containing two or three gold particles, and 3) a state in which empty micelles coexist with larger polymer-stabilized gold particles. Distinctive variations in the spectra were observed depending on the particle size and whether two particles had formed a couple with orientation-dependent dipolar interactions.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021213
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  • 104
    Electronic Resource
    Electronic Resource
    Asymmetric Control in the Pictet-Spengler Reaction by Means of N-Protected Amino Acids as Chiral Auxiliary Groups (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1566-1571 
    Details
    ISSN: 0947-6539
    Keywords: amino acids ; asymmetric syntheses ; carbolines ; chiral auxiliaries ; Pictet-Spengler reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Aromatic and aliphatic Schiff bases of tryptamine react with Fmoc- or phthaloyl-protected amino acid chlorides to form N-acyliminium intermediates, which, in the presence of titanium alkoxides at room temperature, undergo Pictet-Spengler reactions to give tetrahydro-β-carbolines with diastereomeric ratios of up to 99:1. The chiral auxiliary can be removed from the Pictet-Spengler adducts by means of a simple reduction. To rationalize the observed stereoselectivity a transition-state model is proposed in which the titanium atom coordinates both the carbonyl group of the N-acyliminium ion and the amino acid protecting group.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021215
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  • 105
    Electronic Resource
    Electronic Resource
    Substrate Uptake and Activation by Dimolybdenum and Ditungsten Hexaalkoxides-Factors Influencing the Cleavage of C-X Multiple Bonds in the Reactions Between [Mo2(OR)6] (R = tBu or CH2tBu) and Ar2C=S, Et2NC≡N, and P(nBu)3Dedicated to Professor R. R. Schrock at the start of his sixth decade (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 110-117 
    Details
    ISSN: 0947-6539
    Keywords: kinetics ; metal-metal bonds ; molybdenum complexes ; multiple bonds ; tungsten complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction between diarylthiones and [Mo2(OCH2tBu)6] in hydrocarbon solvents yielded [Mo2(OCH2tBu)6-(μ-S)(=CAr2)] by cleavage of the C=S double bond. The Lewis base adduct [Mo2 (OCH2tBu)6(μ-S) (=CPh2) (PMe3)] (2) has been crystallographically characterized; it contains six- and five-coordinate Mo atoms linked through μ-S and μ-OR groups. The rate of the cleavage of the C=S bond has been studied by variable-temperature 1H NMR in [D8]toluene. A Hammett plot shows that both electron-donating and electron-releasing substituents in the aryl groups enhance the rate relative to Ph2C=S. The activation parameters for cleavage of the C=S bonds in Ph2C=S, (p-MeOC6H4)2C=S, and (m-CF3C6H4)2C=S exhibit essentially identical values for ΔS
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020119
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  • 106
    Electronic Resource
    Electronic Resource
    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020101
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  • 107
    Electronic Resource
    Electronic Resource
    What Do Ab Initio Calculations Predict for the Structure of Lithiated Azaenolates of Peptides? (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 9-18 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; azaenolates ; NMR chemical shifts ; peptides ; Ramachandran maps ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structures and conformations of the azaenolate lithium salts of amides (formamide, acetamide, and N-methylacetamide) and of the dipeptide model N-formylalaninamide were investigated by means of ab initio MO theory. Four possible structures of the lithiated C-enolates of acetamide were also included in the study. All structures were calculated at the HF/6-31+G(d) and MP2(fc)/6-31 + G(d)/HF/6-31 + G(d) levels; the lithiated azaenolates of formamide were also investigated at higher theoretical levels (up to MP4(fc)/6-311 + G(d,p)/MP2(fc)/6-311 + G(d,p)). For the lithiated azaenolates of all amides investigated, the most stable structure contains a four-membered ring in which the lithium ion is complexed by the oxygen and nitrogen atoms; the substituents attached to the carbon and nitrogen atoms of the azaenolate are in a cis arrangement. The lithiated azaenolates of acetamide are predicted to be more stable than the corresponding C-enolates. To simulate solvation, calculations on complexes of the lithiated azaenolates of formamide with up to three molecules dimethyl ether were also performed, and all azaenolates of amides were also reoptimized by ab initio reaction-field calculations. Both solvation models reduce the preference for lithium-chelated cis structures. The Ramachandran maps of the dilithiated bis(azaenolate) of N-formylalaninamide (having cis or trans arrangements of the azaenolate substituents) were scanned by MNDO calculations for conformational accessible regions. Thirteen stable structures were subsequently optimized at the HF/6-31 + G(d) ab initio level. The global minimum resembles a peptide in C7 conformation, but other conformations, not known for peptides, are close in energy. The structures of dimers of the lithiated azaenolates of N-methylacetamide and of glycinaldehyde were also calculated. The NMR chemical shielding of carbon, nitrogen, and oxygen atoms in all structures were predicted ab initio by using the gauge-including atomic orbital (GIAO) method.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020106
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  • 108
    Electronic Resource
    Electronic Resource
    Ruthenium-Catalyzed cis-Dihydroxylation of Alkenes: Scope and Limitations (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 50-57 
    Details
    ISSN: 0947-6539
    Keywords: alkenes ; catalysis ; dihydroxylations ; electrophilicity ; ruthenium compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidative ruthenium catalysis (0.07 molequiv RuCl3·(H2o)3, 1.5 molequiv NaIO4, EtOAc/CH3CH3CN/H2O 3:3:1), beyond the usual C—C bond cleavage to give dicarbonyls, has been shown to syn-dihydroxylate a wide range of alkenes (except for strained bicyclic alkenes, sterically hindered trisubstituted alkenes, and most tetrasubstituted alkenes) to give vicinal diols rapidly (within minutes) and efficiently. The minor products are the usual oxidative fission products, namely, ketones and aldehydes or carboxylic acids, and sometimes ketols. Longer reaction times lower the yields of most diols, probably owing to oxidative glycol cleavage. Reactions with substrates containing one or more electron-with-drawing groups in conjugation with or adjacent to the alkene moiety are generally slower but give better yields. The diastereoselectivity of the present “flash” dihydroxylation, anti to the existing α-stereogenic center, with cycloalkenes is excellent whereas that with acyclic alkenes is moderate to poor. Sodium metaperiodate is still the best co-oxidant for the catalytic reaction. Aqueous acetonitrile (approximately 86%) as an alternative solvent system was found to give better yields of 1,2-diols than the original solvent system in some cases.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020111
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  • 109
    Electronic Resource
    Electronic Resource
    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020201
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  • 110
    Electronic Resource
    Electronic Resource
    A 2,3-Connected Tellurium Net and the Cs3 Te22 Phase (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 390-397 
    Details
    ISSN: 0947-6539
    Keywords: band structures ; hypervalent bonding ; semiempirical calculations ; tellurium compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bonding in the recently reported Cs3 Te22 phase, which contains both Te8 rings and remarkable Te6 sheets, is studied by approximate molecular orbital theory. Our focus is on the geometric and electronic features of the unique 2,3-connected Te net found as a substructure in this phase. The calculations show that both the linear and T-shaped Te geometries in the 2,3-connected Te net of Cs3 Te22 are determined by their particular electron count. Both types of tellurium atoms are hypervalent; we make connections to other well known hypervalent molecules, such as XeF2, I3-, and BrF3. Several possible variations and distortions of this net are discussed, all of which are found to be less stable. The discrete crown-shaped Te8 units that appear in the phase show normal covalent bonding and should occur in smaller molecular entities, too. According to our computations, Cs3 Te22 should be metallic. Two structurally related phases, CsTe7 and Cs2 Te15, are suggested.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020407
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  • 111
    Electronic Resource
    Electronic Resource
    Zirconium(II)- and Hafnium(II)-Assisted Reductive Coupling of Coordinated Carbonyl Groups Leading to Ketenylidene Complexes of Zirconium(IV) and Hafnium(IV) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 412-419 
    Details
    ISSN: 0947-6539
    Keywords: carbon-carbon coupling ; carbonyl complexes ; hafnium complexes ; ketenylidene complexes ; zirconium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The biscyclopentadienyldicarbonyl derivatives of zirconium(II) and hafnium(II) ([MCp2(CO)2]; M=Zr, Hf) promote the reductive coupling of coordinated carbon monoxide to give, in the presence of N,N-dialkylcarbamtes of the tetravalent metals [M(O2CNR2)4], the ketenylidene complexes [M3Cp2(μ2-CCO)-(μ3-O)(O2CNR2)6] (1 a: M=Zr, R=Et; 1 b: M=Zr, R=iPr; 2: M=Hf, R=iPr). The yields of the isolated zirconium complexes are as high as 60%, while that of the hafnium derivative is 40%. The X-ray crystal structure analysis of 1 b shows that it consists of trinuclear molecules, with the three zirconium atoms held together by the bidentate C2O ligand, by the tridentate bridging oxide and by the bidentate carbamato groups. Labelling experiments with [ZrCp2(13CO)2] show that the ketenylidene ligand originates from the coordinated CO groups. These ketenylidene complexes, which are rare examples of compounds containing a CCO but no CO ligand, are characterized by an intense IR band at about 2015 cm-1, associated with the bridging C2O ligand. The ketenylidene group of 1 b was readily removed and replaced by a bidentate μ-oxo ligand of the same hapticity by reaction with carbon dioxide or acetone or by thermal decomposition. The resulting product [Zr3Cp2(μ-O)(μ3-O)-(O2CNiPr2)6] (3) crystallizes in the same space group as 1 b and with similar cell constants and bond parameters.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020410
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  • 112
    Electronic Resource
    Electronic Resource
    9-Borabarbaralanes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 458-461 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; barbaralanes ; borabarbaralanes ; Cope rearrangement ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of MgCOT(thf)x with tBuBF2 or PhBCl2 affords the first 9-borabarbaralanes 2 (C8H8BR, a: R = tBu; b: R=Ph). With the aminoboron dihalides BCl2NiPr2 and BCl2N(SiMe3)tBu 9-borabicyclo[4.2.1]-nona-2,4,7-trienes 3 (a: R=NiPr2, b: R=N(SiMe3)tBu) and the trans-9-borabicyclo[4.3.0]nona-2,4,7-triene 4 are obtained. The bicyclic compounds 3a and 3 b are converted into 9-borabarbaralanes 2c and 2 d, respectively, by irradiation in solution as well as by heating. All 9-borabarbaralanes 2 are fluxional in solution. In the crystalline state, the B-phenyl derivative 2b displays a well-ordered van der Waals crystal structure. The theoretical prediction that the degenerate Cope rearrangement in barbaralanes will be retarded by π-acceptor groups in the 9 position has been verified. Quantum chemical calculations employing density functional theory support and help interpret the experimental findings. The isoelectronic 9-barbaralyl cations, in contrast, have such high Cope barriers that other rearrangement pathways are followed instead.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020416
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  • 113
    Electronic Resource
    Electronic Resource
    Di(polyprenyl) phosphates as models for primitive membrane constituents: Synthesis and phase properties (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 789-799 
    Details
    ISSN: 0947-6539
    Keywords: amphiphiles ; cytomimetic chemistry ; phosphitylation ; polyprenols ; vesicles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Identical- and mixed-chain di(polyprenyl) phosphates, derived from 3-methyl-2-buten-1-ol, (E)-geraniol, (E,E)-farnesol, (E,E,E)-geranylgeraniol, (R)-citronellol, or (R)-tetrahydrogeraniol have been synthesized by phosphitylation and oxidation with iodine/water. These phosphates gave organized systems in water, in particular vesicles, the physical properties of which were studied by photon correlation spectroscopy, electron microscopy, entrapment of a fluorescent dye (calcein), 31P NMR, stopped-flow/light-scattering studies, and fluorescence polarization with lipophilic probes. Interdigitation of mixed-length phosphates was not found to have any favorable effect on the stability of the vesicles. The C〉15 vesicles were formed despite the fact that they are composed of only one molecular species and are devoid of any reinforcer, but those with two geranyl chains appeared to be rather fragile. Phosphates containing C5 chains are soluble in water. The results obtained lend weight to the hypothesis that polyprenyl phosphates might have been primitive membrane constituents, and stimulate the search for appropriate reinforcers.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020708
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  • 114
    Electronic Resource
    Electronic Resource
    Novel sulfur-containing inorganic aprotic superacids as effective initiators of low-temperature cracking and isomerization of alkanes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 812-814 
    Details
    ISSN: 0947-6539
    Keywords: alkanes ; cracking ; isomerizations ; sulfur compounds ; superacids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel family of inorganic aprotic superacids has been found. Oxochlorides and chloride of sulfur combined with aluminium halides (SOCl2·n AlX3, SO2Cl2·n AlX3, MeSO2Cl·n AlX3, CF3SO2Cl·n AlX3, S2Cl2·n AlX3; n = 2 or 3, X = Br or Cl) are found to be effective promoters for the cracking and isomerization of alkanes at room temperature.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020711
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  • 115
    Electronic Resource
    Electronic Resource
    Cyclobis(paraquat-4,4′-biphenylene)-an organic molecular square(Molecular Meccano), Part 8: for Part 7, see P. R. Ashton, P. T. Glink, J. F. Stoddart, P. A. Tasker, A. J. P. White, D. J. Williams, Chem. Eur. J. 1996, 2, 729-736. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 877-893 
    Details
    ISSN: 0947-6539
    Keywords: catenanes ; second-sphere coordination ; self-assembly ; template syntheses ; topological stereoisomerism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Template-directed syntheses of cyclobis(paraquat-4,4′-biphenylene) (1)- a Molecular square-have been achieved by use of π-electron-rich macrocyclic hydroquinone-based and acyclic ferrocene-based templates. In particular, the use of a polyether-disubstituted ferrocene derivative as a template permits synthesis of 1 (which is accessible only in very low yields without templates) on a preparative scale. Furthermore, the use of a macrocyclic hydroquinone-based polyether template in corporating an ester function in one polyether chain-an (oriented) macrocycle-affords a 1 : 1 mixture of two topologically stereoisomeric [3]catenanes. Ester hydrolysis of the π-electron-rich macrocyclic components mechanically interlocked with 1 within the catenated structures releases the tetracationic cyclophane in quantitative yield as a result of the degradation of the [3]catenanes. The molecular square has been characterized by X-ray crystallography, FAB mass spectrometry, 1H NMR and 13C NMR spectroscopies, and elemental analysis. The binding properties of 1 and of the smaller cyclophane cyclobis(paraquat-p-phenylene) toward a series of π-electronrich guests have also been investigated with the above techniques and UV/VIS spectroscopy. The self-assembly of the resulting supramolecular complexes in solution and in the solid state is driven mainly by π-π stacking interactions and hydrogen-bonding interactions, as well as by edge-to-face T-type interactions. In particular, the complexation of ferrocene or a ferrocene-based derivative within the cavity of 1 suggests the possibility of constructing functioning ferrocene-based molecular and supramolecular devices that can be controlled electrochemically in the form of catenanes, rotaxanes, and pseudorotaxanes.
    Additional Material: 20 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020718
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  • 116
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    Electronic Resource
    Resonance-assisted O-H ⃛ O hydrogen bonding: Its role in the crystalline self-recognition of β-diketone enols and its structural and IR characterization (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 925-934 
    Details
    ISSN: 0947-6539
    Keywords: crystal engineering ; diketone enols ; hydrogen bonds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The fact that hydrogen bonding is normally stronger than other nonbonding attractive forces can be exploited for the rational design of molecular crystals with known packing features and specific physical properties (crystal engineering). In the present paper the problem of obtaining homodromous molecular chains controlled by strong O-H ⃛ O interactions is investigated, particular attention being paid to β-chains, that is, infinite hydrogen-bonded chains of β-diketone enol fragments ⃛ O=C—C=C—OH ⃛, which are linked by stronger-than-usual resonance-assisted hydrogen bonds (RAHBs) and are intrinsically interesting as prototypes of a large family of switching proton bistate molecular devices. Accordingly, the crystal and molecular structures of thirteen new compounds containing the 1,3-cyclopentanedione and 1,3-cyclohexanedione fragment (or their heterocyclic analogues) were determined, and most of them were found to give the expected β-chain packing pattern. Comparison with literature data makes it possible to identify seven fundamental β-chain patterns, which can be shown to be selected by reason of the relative encumbrances of the substituents. Furthermore, a general analysis of all functional groups able to form strong O—H ⃛ O bonds reveals a semiquantitative correspondence between the O—H ⃛ O measurable parameters (O ⃛ O, H ⃛ O and O—H distances, and ṽ(O—H) IR stretching frequencies) and the hydrogen bond energy EHB, and a hierarchy of chemical functionalities that are well characterized by limited EHB ranges and that, in decreasing order of energy, can direct the crystal packing process.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020804
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  • 117
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    Opening and closure of the fullerene cage in cis-bisimino adducts of C60: The influence of the addition pattern and the addendDedicated to Professor Friedrich Hund on the ocdasion of his 100th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 935-943 
    Details
    ISSN: 0947-6539
    Keywords: clusters ; fullerenes ; nitrenes ; regio-selectivity ; valence isomerization ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis, isolation, and spectroscopic characterization of the bisimino[60]fullerenes C60(NCOOR)2 (1a: R = Et, 1b. R = tBu) with a cis-1 addition pattern as well as of their regioisomeric analogues 2-7 with different addition patterns are described. Whereas compounds 2-7 are typical fulleroaziridines and obey the rule of the minimization of [5,6] double bonds, the cis-1 isomers 1a and 1b represent the first examples of fullerene derivatives with open transannular [6,6] bonds. Characteristic features within the fullerene framework of these valence isomers VI are the presence of a doubly bridged open 14-membered ring with a phenanthrene perimeter as well as of an 8-membered 1,4-diazocine heterocycle. Moreover, it is shown that, by transforming cis-1-C60(NCOOtBu)2 (1b) into cis-1-C60(NH)2 (1c), the fullerene cage can be closed in an intraring 2π→2s̰ isomerization to valence isomer V. These are the first chemical modifications of the fullerene core that allow the synthesis of both open and closed valence isomers with the same addition pattern. Density-functional as well as AM1 calculations corroborate the experimental findings that it depends on the addition pattern as well as on the nature of the addends whether or not the transannular [6,6] bonds are closed. Only in the cis-1 adducts C60(NR)2 that prefer planar imino bridges (e.g., carbamates or amides) are the open forms VI more stable than the closed isomers V.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020805
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    4,4-dimethylbicyclo[3.1.0]hexa-1(6),2-diene - a highly strained 1,3-bridged cyclopropene (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 967-973 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; carbenes ; cyclopropenes ; matrix isolation ; rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photochemical rearrangement of 4,4-dimethylcyclogexa-2,5-dienylidene (3) to 4,4-dimethylbicyclo-[3.1.0]hexa-1(6),2-diene (5) was investigated by means of the matrix isolation technique. Carbene 3 was generated in argon matrices at 10 K by photolysis of the corresponding diazop compound γ 〉550 nm) and was characterized by IR and UV/Vis spectroscopy and its characteristic thermal reaction with triplet oxygen. Long-wavelength irradiation (γ 〉 515 nm) induced an irreversible rearrangement of carbene 3 to the highly strained cyclopropene 5. This is in contrast to the previously investigated 1 H-bicyclo[3.1.0]hexa-3,5-dien-2-one (1), which is thermally labile even at 10 K and rearranges back to 4-oxo-2,5-cyclohexa-dienylidene. The experimental findings were confirmed by ab initio calculations at the MP 2/6-31 G(d) level. MP 2 and experimental IR frequencies are in good agreement. The strain energies of 5 and 1 are 75 and 78 kcalmol-1, more than 20 kcal mol-1 larger than that of cyclopropene. The somewhat greater stability of 5 than 1 is a consequence of better π delocalization in the diene unit of the bicyclic system.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020809
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  • 119
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    A straightforward, highly stereoselective synthesis of protected isostatine derivatives (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1001-1006 
    Details
    ISSN: 0947-6539
    Keywords: aminopolyols ; asymmetric aminations ; asymmetric epoxidation ; asymmetric hydroxylation ; didemnins ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel strategy for the synthesis of isostatine derivatives has been developed. Contrary to previous approaches to isostatine, a non-proteinogenic amino acid that is an essential component of the didemnins, the present synthesis does not require the intermediacy of the expensive amino acid D-allo-isoleucine, the starting material being commercially available enantiopure (S)-2-methyl-1-butanol. Steps in the sequence include catalytic asymmetric epoxidation, regioselective titanium-promoted opening of an epoxy alcohol with an ammonia equivalent, stereospecific generation of an N-Bocamino epoxide and its nucleophilic opening by a cyanide anion. Application of this method has permitted the enantioselective preparation of isostatine methyl ester and, for the first time, of isostatine amide, both in fully protected form.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020814
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  • 120
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    High-spin → low-spin relaxation in [Fe(bpp)2](CF3SO3)2 H2O after LIESST and thermal spin-state trapping - dynamics of spin transition versus dynamics of phase transitionDedicated to Professor Gerhard Herberich on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1134-1138 
    Details
    ISSN: 0947-6539
    Keywords: iron complexes ; magnetic susceptibility ; Mössbauer spectroscopy ; phasetransitions ; spin crossover ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The iron(II) complex [Fe(bpp)2]-(CF3SO3)2 H2O (bpp = 2,6-bis(pyrazolyl-3-yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T1/2 (where the fraction of HS species γHS = 0.5) are 147 K and ≍285 K in the cooling and heating directions, respectively. The compound shows the LIESST and reverse-LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mössbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal spin-state trapping was monitored at temperatures between 104 and 118 K by magnetic susceptibility measurements. The relaxation mechanisms of the HS states generated by LIESST and thermal spinstate trapping are completely different. We suggest that the HS → LS relaxation after thermal spin-state trapping is triggered by an additional structural phase transition of the system.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020913
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  • 121
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    Repetitive Strategy for Exponential Growth of Hydroxy-Functionalized Dendrons (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1330-1334 
    Details
    ISSN: 0947-6539
    Keywords: dendrimers ; exponential growth ; repetitive syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of dendrons with orthogonally protected functional groups, which allows the construction of dendrons in an exponential fashion, is described. The sequence has been carried out on a gram scale for the first, second, and fourth generation. The dendrons have a relatively loose and flexible structure, which should make further growth feasible. Owing to the dendrons' adjustable and broadly applicable substitution pattern, they are the first representatives of a future construction kit for the systematic synthesis of layered dendrimers. The coupling step involves urethane formation.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021021
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  • 122
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    A Paradigm Case for the Merging of Glycal and Enzymatic Assembly Methods in Glycoconjugate Synthesis: A Highly Efficient Chemo-Enzymatic Synthesis of GM3 (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1359-1362 
    Details
    ISSN: 0947-6539
    Keywords: anhydrosugars ; enzymatic catalysis ; gangliosides ; glycoconjugates ; glycosidations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A concise, regio- and stereoselective synthesis of the ganglioside GM3 has been achieved. Use of an α-1,2-oxirane derived from lactal facilitated installation of the anomeric β-ceramide side chain. Introduction of the sialic acid residue at C3, of the lactose was accomplished regiospecifically by enzymatically mediated sialyl transfer from CMP-Neu-5-Ac.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021105
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  • 123
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    Synthesis and Structure of a Bis(Double Helicate) and its Cryptatoclathrate“Adamantanoid Chelate Complexes”, Part 6; for Part 5, see ref. [4]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1363-1367 
    Details
    ISSN: 0947-6539
    Keywords: clathrates ; cryptates ; helicates ; selfassembly ; zinc complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structures of the hitherto unknown bis(double helicate) 10 and its cryptatoclathrate (10)2·2THF were unequivocally determined by X-ray diffraction. Bis(double helicate) 10 is formed in a one-pot synthesis starting from CH-acidic bis (tetrazolylmethyl ketone) 9 and Zn(OAc)2. The formation of racemic, homochiral 10 from [Zn2L32] fragments, which are formed in a self-assembly process, is governed by chiral self-recognition. According to NMR studies only 10 is present in solution. 13C CP/MAS NMR spectroscopy and X-ray analysis confirm aggregation of 10 with two molecules of THF to yield the inclusion compound (10)2·2 THF in the solid state.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960021106
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  • 124
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    Se2NBr3, Se2NCl5, Se2NCl-6: New Nitride Halides of Selenium(III) and Selenium(IV) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1373-1378 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; crystal structure ; halogen compounds ; nitrides ; selenium compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Se2NBr3, a planar molecule shaped liked dividers, was formed by the reaction of SeBr4 with N(SiMe3)3, which reduced the selenium to the trivalent state. Under the same conditions, Se2NCl5 was obtained by the reaction of SeCl4 with N(SiMe3)3, retaining the tetravalent state of selenium. The molecule is formed by two edge-sharing pseudotrigonal bipyramids with selenium as their central atoms. Se2NCl5 reacts with PPh4Cl to form the Se2NCl-6 ion by the addition of a Cl- ion to only one of the two selenium atoms, resulting in pseudooctahedral coordination of this atom. Ab initio calculations for all the compounds reported confirm the experimentally determined structures.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021108
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  • 125
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    Investigation of the Self-Assembly Pathway of Pentanuclear Helicates by Electrospray Mass Spectrometry (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1395-1398 
    Details
    ISSN: 0947-6539
    Keywords: bipyridine ligands ; helicates ; kinetics ; mass spectrometry ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-assembly of the pentanuclear double helicates Hh, Ha and He from the corresponding oligobipyridine strands Lh, La and Le and Cu1 ions has been investigated by NMR and electrospray mass spectrometry (ESMS). Where as Hh is assembled rapidly (in less than 20 min), He (about 20 h) and especially Ha (about 60 h) form much more slowly. The rate decreases strongly with increasing steric bulk of the substituents in the 4,4′-positions on the bipyridine units; this indicates that the search processes (wrapping, unwrapping) that lead to the final helicate are strongly hindered by the size of the substituents. The ESMS data give information about the species present in solution under different conditions and allow the formulation of possible formation pathways, which may involve, in particular, helicates of hairpin type.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021111
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  • 126
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    Phosphorus-Containing Dendrimers: Synthesis of Macromolecules with Multiple Tri- and Tetrafunctionalization (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1417-1426 
    Details
    ISSN: 0947-6539
    Keywords: dendrimers ; macromolecules ; phosphorus compounds ; substitutions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variety of dendrimers up to generation 7 possessing terminal P(S)Cl2, (3-[G1]-3-[G7]) or P(O)Cl2, (7-[G1]-7-[G3], 7-[G3, 7-[G7) have been tri- and tetrafunctionalized. Selective monosubstitution of P(X)Cl2 (X = S, 0) termini with allylamine or propargylamine gave the trifunctionalized dendrimers 5-[G1], 5-[G7], 8-[G1], 8-[G3], 10[G1], 11-[G4], ll-[G4], 12-[G1]-12-[G3], 12-[G5], and 12-[G7]. Reaction of dendrimers 4-[G1] and 4-[G4], possessing terminal P(S)[N-allyl) fragments, with propargylamine afforded trifunctionalized dendrimers l4-[G1 and l4-[G4]. Multiply trifunctionalized macromolecules 13-[G2] and 13-[G3] with P(S)(NH-allyl)(NH-propargyl) moieties at the surface were prepared by treatment of 8-[G2] and 8-[G3] with propargylamine. Dendrimers 15-[G1]-l5-[G3] and l6-[G1]-l6-[G3] with P(O)(NH-allyl)(OC6H4CHO) and P(O)(NH-propargyl)(OC6H4CHO) termini were also synthesized. Reaction of hydrazine or cyanomethylenetriphenylphosphorane (17) with compounds 15-[G1], 15-[G3], and 16-[G1]-16-[G3] led to the multiply tetrafunctionalized dendrimers 18-[G1], 18-[G3], 19-[G1], 20-[G1]-20-[G3], 21-[G1], and 21-[G2].
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021114
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    New Ruthenium(II) Complexes of Functionalized Monoanionic Aryldiamine N,C,N'-Terdentate Ligands: Syntheses of [RuII{2,6-(Me2NCH2)2-4-R-C6H2}-(terpy)]+Cl-; X-ray Structure of a Dimeric Organolithium Compound, [Li{2,6-(Me2NCH2)2-4-Ph-C6H2}]2 (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1440-1445 
    Details
    ISSN: 0947-6539
    Keywords: aryldiamines ; chelate ligands ; organometallic compounds ; ruthenium complexes ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The new anionic functionalized aryldiamine ligands [2,6-(Me2NCH2)2-4-R-C6H2]- (R = Me3SiC≡C, C6H5, Me3-Si), formally derived from [2,6-(Me2-NCH2)2C6H3]-, have been prepared as their lithium compounds. The compound [Li{2,6-(Me2NCH2)2-4-Ph-C6H2}]2 crystallizes in the monoclinic space group C2/c (no. 15) with a = 13.1225(5), b = 13.5844(7), c = 18.9859(12) Å, β = 105.329(5)°, V = 3264.0(3) Å3. Z = 4. The structure refinement converged to R1 = 0.0374 for 2037 observed reflections [Fo〉4σ(Fo)] and wR2 = 0.0922 for 2560 unique data. The organolithium compounds have been used in transmetalation reactions to give the corresponding functionalized organoruthenium(II) complexes [RuII{2,6-(Me2NCH2)2-4-R-C6H2}-(terpy)]+Cl- (terpy = 2,2′;6′,2′-terpyridine). The RuII species with R = HC°C has also been synthesized.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021117
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  • 128
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    Synthesis and Molecular Structure of the First Neutral Transition-Metal Complex Containing a Linear M=C=C=C=C=CR2 chainDedicated to Professor Max Schmidt on the occasion of his 70th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 19-23 
    Details
    ISSN: 0947-6539
    Keywords: carbon ligands ; diynes ; iridium compounds ; metallacumulenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of [IrH2Cl(PiPr3)2] (2) with the pentadiyne derivative HC≡C—C≡C—CPh2OH (1) yields the five-coordinate compound [IrH(C≡C—C≡CPh2OH)Cl(PiPr3)2] (3), which photochemically rearranges to give the vinylidene-substituted isomer trans-[IrCl(=C=CH-C≡CPh2OH)(PiPr3)2] (4). On treatment of 3 with pyridine, the octahedral complex [IrH (C≡C—C≡CPh2OH)Cl(py)(PiPr3)2] (5) is formed. While attempts to eliminate water from 4 (or 5) failed, the reaction of 3 with one equivalent of trifluorosulfonic acid anhydride and two equivalents of triethylamine led to the formation of trans-[IrCl(=C=C=C=C=CPh2)(PiPr3)2] (6), which is the first neutral transition-metal complex containing a C5R2 ligand. The X-ray structural analysis of 6 reveals a perfect square-planar coordination geometry around the metal centre and an almost linear IrC5 chain with Ir-C—C and C—C—C bond angles of 174-179°.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020107
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  • 129
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    Isolation of Selected Exchange Processes in Nuclear Magnetic Resonance (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 45-49 
    Details
    ISSN: 0947-6539
    Keywords: exchange processes ; kinetics ; NMR spectroscopy ; zirconium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have shown that it is possible to inhibit the transfer of magnetization in a system with several exchanging sites in dynamic equilibrium, as in a mixture of cis- and trans-ZrCl4L2 with excess free ligand L (L = (CH3O)3PO). The forward and backward reaction rates involving two selected sites can be studied while the effect of competing exchange processes is “quenched”. This can be achieved either by selective inversion of the magnetization of the two chosen sites in the course of the reaction interval, or alternatively by inversion of the magnetization of all other sites in the exchange network. The rate of exchange from the free to the cis site was determined to be kcis←free = 0.018 s-1. In the usual methods, this process would tend to be overshadowed by the almost two hundred times faster competing exchange process from the cis to the trans site (Ktrans←cis = 3.32 s-1).
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020110
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  • 130
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    Centropentaindan, a Fenestrane Bearing Five Mutually Fused Indan Units: Syntheses, Molecular Structure, and Bridgehead SubstitutionBenzoannelated Centropolyquinanes, Part 20; for Part 19, see ref. [8a], for Part 21, see ref. [7]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 58-67 
    Details
    ISSN: 0947-6539
    Keywords: centropolyindans ; cyclodehydrogenation ; fenestranes ; polycycles ; triquinacenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Syntheses, properties, and some reactions of a novel centropolycyclic hydrocarbon, centropentaindan (1), are described. The molecular structure of 1 consists of a tetrabenzo[5.5.5.5]fenestrane bearing a fifth centrically fused indan unit. Independent syntheses have been developed employing i) twofold cyclodehydrogenation of tribenzotriquinacene (7) with Pd/C at 500°C, which gives 1 in 50% yield, and ii) a two-step procedure by tetrabromination of di-fuso-centrotriindan (8) followed by condensation with two molecules of benzene, which provides 1 in 88% yield. Some bridgehead-substituted centropentaindans are described, including the highly labile dibromo derivative 19, and the centrohexacyclic, topologically nonplanar endo-peroxide 26 and endo-disulfide 28. The notably rigid ring fusion in the molecular framework of 1 is shown both by X-ray structural analysis and by particular steric interactions of the two mutually compressed bridgehead substituents.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020112
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  • 131
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    Fluorescent Sensors for Transition Metals Based on Electron-Transfer and Energy-Transfer Mechanisms (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 75-82 
    Details
    ISSN: 0947-6539
    Keywords: copper complexes ; electron transfer ; energy transfer ; fluorescent sensors ; nickel complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fluorescent sensors for 3d divalent metal ions have been designed by means of a supramolecular approach: an anthracene fragment (the signalling subunit) has been linked to either a cyclic or a noncyclic quadridentate ligand (the receptor). Occurrence of the metal-receptor interaction is signalled through the quenching of anthracene fluorescence. When the receptor (i.e., the dioxotetramine subunit of sensors 2 and 3) is able to promote the one-electron oxidation of the metal, quenching takes place through a photoinduced metal-to-fluorophore electron-transfer mechanism. In the case of sensors containing a tetraamine binding subunit (4 and 5), quenching proceeds by an energy-transfer process. Selective metal binding and recognition can be achieved by varying the pH, and metal ions can be distinguished (e.g., CuII from NiII) by spectrofluorimetric titration experiments in buffered solutions. Whereas systems 2, 3 and 5 show reversible metal binding behaviour, the cyclam-containing system 4 irreversibly incorporates transition metals (due to the kinetic macrocyclic effect) and cannot work properly as a sensor.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020114
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 125-128 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020203
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  • 133
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    The “Peroxo Perrhenic Acid” H4Re2O13: An Oxygen-Rich Metal Peroxide and Oxidation CatalystMultiple Bonds between Transition Metals and Main-Group Elements, Part 149. Part 148: W. A. Herrmann, F. E. Kühn, C. C. Romão, J. Organomet. Chem. 1995, 495, 209-213.Dedicated to Professor Herbert Roesky on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 168-173 
    Details
    ISSN: 0947-6539
    Keywords: catalysis ; crystal structure ; oxidations ; rhenium peroxo complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rhenium oxides Re2O7 and ReO3 react with hydrogen peroxide solutions yielding peroxo complexes that efficiently catalyze the oxidation of olefins, aromatics, and certain organometallic compounds. In contrast, related oxides of molybdenum (MoO3) and vanadium (V2O5) do not activate H2O2 under comparable conditions. The active rhenium peroxo complex was isolated from the system Re2O7/H2O2: the crystalline red-orange, explosive compound of formula H4Re2O13 is the most oxygen rich rhenium compound isolated to date. Its structure resembles a “peroxo perrhenic acid”. The binuclear compound could be isolated in the form of a diglyme adduct, structurally defined as two corner-sharing pentagonal bipyramids with apical oxo and aquo ligands; the equatorial positions are occupied by the bridging oxygen and by n2-peroxo groups (two [O2]2- ligands per rhenium). In contrast to the known complex [CH3ReO(O2)2].H2O, the new peroxo species [O{ReO(O2)2.H2O}2] decomposes hydrolytically during the catalytic cycle and can thus not compete in terms of catalytic activity in oxidation reactions involving H2O2. Hydrolysis yields “perrhenic acid” Re2O7.2H2O, the diglyme adduct of which compound was also characterized by means of an X-ray diffraction analysis.
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    URL: http://dx.doi.org/10.1002/chem.19960020208
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    The Effect of Microsolvation on E2 and SN2 Reactions: Theoretical Study of the Model System F- + C2H5F + nHF (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 196-207 
    Details
    ISSN: 0947-6539
    Keywords: eliminations ; orbital interactions ; solvent effects ; substitutions ; theoretical chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The model reaction system F- +C2H5F + nHF (n = 0-4) has been investigated by use of a density-functional method, in order to achieve a qualitative understanding of the effect of solvation on the E2 and SN2 reactions. Two characteristic effects already occur upon monosolvation: a) the activation energies of the E2 and SN2 pathways increase significantly and even become positive, because reactants are more strongly solvated than transition states; b) the SN2 transition state is stabilized much more and becomes lower in energy than the anti-E2 transition state. This agrees with general experience from gas- and condensed-phase experiments. The solvation is analyzed from two complementary viewpoints: a) as an interaction between solvent molecules and the F-/C2H5F reaction system; b) as an interaction between the [F-, nHF] solvated base and the C2H5F substrate. The extent to which condensed-phase characteristics can be modeled by this microsolvation approach is discussed.
    Additional Material: 11 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020212
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    1,2,3-Triphosphetenes, First Examples of a New Class of Phosphorus HeterocyclesReactive E=C(p-p)π Systems, Part 41. Part 40: see ref. [1]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 208-213 
    Details
    ISSN: 0947-6539
    Keywords: heterocycles ; insertion reactions ; phosphaalkynes ; phosphorus ylides ; triphosphetenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hitherto unknown 1,2,3-triphosphetenes RC=P—PCF3—PCF3 (3a-d) are formed as main products in reactions of phosphaalkynes R—C≡P [R = iPr2N (1a), tBu (1b), Me2EtC (1c), 1-methylcyclohexyl (1d)] with the cyclotetraphosphane (PCF3)4 (2). According to NMR results the CF3 groups in 3a-d have a trans disposition; an X-ray diffraction study of 3a confirms this structure. The P—P bond lengths in 3a are equal [2.201 (2) and 2.204 (2) Å] and correspond to single bonds. A considerable shortening is observed for the sp2-C-N bond (1.336 Å) which, together with the elongation of the P=C bond (1.746 Å), indicates effective π donation of the lone pair on nitrogen. Surprisingly, 3a can be prepared in quantitative yields by reaction of the PP ylide Me3P=PCF3 (6) with 1 a (molar ratio: 2:1). In contrast, the corresponding reactions of 6 with the alkyl-substituted phosphaalkynes 1b-d lead to the novel phosphorus ylides Me3P=C(R)—P—PCF3—PCF3PCF3 [R = tBu (10a), Me2EtC (10b), 1-methylcyclohexyl (10c)] in good yields. In their molecular groundstate structures, determined by X-ray diffraction, the lone pair on the phosphano P atom prefers the syn position with respect to the ylidic P=C bond. An unusual lengthening of the sp2-C-C bond [1.553 (4) (10a), 1.543 (6) (10b), 1.551 (4) Å (10c)] to values typical for sp3-C/sp3-C distances is observed.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020213
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020302
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    Zintl Anions as Starting Compounds for the Synthesis of Polynuclear Transition Metal ComplexesDedicated to Professor Kurt Dehnicke on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 238-244 
    Details
    ISSN: 0947-6539
    Keywords: crystal structure ; density functional calculations ; polyarsenido complexes ; polyphosphido complexes ; Zintl anions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Zintl anion P3-7 reacts with complexes of transition metal halides to form multinuclear metal phosphorus clusters. Reaction of Li3P7·3 DME with [FeCp(CO)2Br] or [NiCl2(PBu3)2] leads to the formation of [P7{FeCp(CO)2}3] (1) and [{Ni(PBu3)2}4P14] (2), respectively. X-ray structure determinations show that in 1 the P7 cage of Li3P7 remains intact but in 2 a P14 framework is formed by linkage of two norbornadiene-like P7 units. The P14 skeleton coordinates to four Ni(PBu3)2 groups. LiCp* and CoCl2 react with P7(SiMe3)3 to give [(Cp*Co)3-(P2)3] (3) with three Cp*Co groups bridged by P2 units. Reaction of FeCl2 and LiCp* with P7(SiMe3)3 yields [(Cp*Fe)3P6][FeCl3(thf)] (4) or [(Cp*Fe)3{n3-P3)Fe}P6] (5), depending on the reaction conditions. In 4 the structure of the Fe3P6 core of the [(Cp*Fe)3P6]+ cation is comparable to closo-B9H92-, while in 5 the Fe4P6 core does not obey the Wade rules. The unusual Co-As cluster [Co6As12(PEt2Ph)6] (6), prepared from K, As, and [CoCl2(PEt2-Ph)2], can be described as a Co6As6 heteroicosahedron linked to two Co3As3 octahedra by common Co3 faces. A theoretical treatment within the density functional approximation reproduces the experimental structures of 2 and 6 and allows an interpretation of molecular electronic structures. In 2 one finds P-P double bonds that are delocalized to some extent into vacant Ni AOs. For the cage compound 6 the Co 3 d AOs participate in cage bonding and 38 electrons can be assigned to cage bonds; this is in accord with the (2N + 2) rule for 18-atom cages but not with the usual electron counting rules.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020217
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  • 138
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    Heterosupramolecular Chemistry: An Approach to Modulating Function in Molecular Devices (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 420-428 
    Details
    ISSN: 0947-6539
    Keywords: anthraquinones ; molecular devices ; supramolecular chemistry ; titanium dioxide ; viologens ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Linked viologen-anthraquinone molecules are attached to TiO2 nanocrystallites supported on a conducting glass substrate. The resulting assembly is incorporated as the working electrode in an electrochemical cell. Under opencircuit conditions, band-gap excitation of the semiconductor nanocrystallite results in viologen-mediated electron transfer to anthraquinone. Prior application of a negative potential step, which results in 2e-/2H+ reduction of anthraquinone, permits electron transfer only to viologen. At positive applied potentials, electron transfer following band-gap excitation is largely suppressed. Some implications of these findings for modulation of function in molecular devices are considered.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020411
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  • 139
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    Pulled Molecular Strings and Stacked Molecular Decks: Chelate-Ring Formation vs. Metal-Metal Bridging in Dicopper(I) Complexes of 2,2′-Bipyrimidine with Diphosphine Ligands of Variable Polymethylene Chain Length (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 446-451 
    Details
    ISSN: 0947-6539
    Keywords: bridging ligands ; chelate ligands ; copper complexes ; self-assembly ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The “self-assembly” reaction between bischelating 2,2′-bipyrimidine, metallic copper, Cu2+, and two equivalents of bidentate Ph2P(CH2)nPPh2 produces crystallographically characterized dinuclear complexes with a [Cu1(μ-bpym)-Cu1]2+ core and chelating (n = 5; type I) or bridging diphosphine ligands (n = 6, 8; type II). Structures II may be designated as “inverse cryptate” arrangements. The structural data of the solids and the spectroscopic results for the dissolved species indicate a strained configuration in the case of complex [(μ-bpym)Cu2{μ-Ph2P-(CH2)n]2+ (6) with n = 6. Once the polymethylene chain length is reduced one notch further to n = 5 (5), the diphosphines can no longer bridge but help to form eight-membered chelate rings (structure I). In this complex there is a distortion towards a trigonal-pyramidal coordination geometry at the copper(I) centers, which is supported by the formation of an “organic sandwich” arrangement between the central bpym acceptor and two phosphino phenyl rings.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020414
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020501
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  • 141
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    A Novel Route to Racemic and Nonracemic Products of the Ugi Reaction: Synthesis of Ugi′s Labile α-Adducts from Iminoaziridines and Carboxylic Acids, and their TransformationsIminoaziridines, Part 7 [1]. Part 6: ref. [37].Dedicated to Professor Ivar Ugi on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 462-469 
    Details
    ISSN: 0947-6539
    Keywords: aziridines ; imides ; isoimides ; rearrangements ; Ugi reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Iminoaziridines (11) are highly reactive synthetic equivalents for three of the four components in the Ugi four-component condensation. Thus, iminoaziridines react rapidly with carboxylic acids at temperatures as low as -20°C to afford α-amino isoimides (14), which are identical to the elusive α-adducts of isocyanides in the Ugi reaction. 1,4-Migration of the acyl group (O → α-N) in 14 furnishes the α-acylamino amides 15. Very little, if any, racemisation is observed when carboxylic acids react with nonracemic iminoaziridines [(R)-11 a,c], which are readily available. Mumm rearrangement by O → N-acyl 1,3-migration to afford α-amino imides (16 e, f) competes if the O → α-N 1,4-shift is slowed down by steric hindrance. The latter acyl shift is catalysed by carboxylic acids while the former is not. The iminoaziridines (R)-11 a,c react quantitatively and without racemisation with hydrazoic acid to produce the 5-aminoalkyltetrazoles (R)-21 a,c.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020417
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    Electron-poor 2,3-Dihydro-1,3-Diborolyl Complexes of Iron and Ruthenium: Synthesis, Reactivity, and Crystal and Electronic Structures of an Iron Sandwich ComplexDedicated to Professor Herbert Schumann on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 487-494 
    Details
    ISSN: 0947-6539
    Keywords: boron compounds ; diboroles ; Group 8 complexes ; sandwich complexes ; semi-empirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The addition product of sodium hydride and the 2,3-dihydro-1,3-diborole (CiPr)2(BEt)2CHMe (3c) reacted with [{(C5Me5)FeCl}x] to produce the green sandwich complex [(C5Me5)Fe{n5-(CiPr)2(BEt)2CMe}] (2 c), which formally contains 16 valence electrons (VE). Complex 2c has unexpected structural properties in the solid state: the 1,3-diborolyl ring is extremely folded (41°), and the Fe-C2 distance is short (1.90 Å). Analogously, violet Ru complexes 4a,c,d were obtained from 3a,c, NaH or tBuLi, and [{(C5Me5)RuCl}4]. With the less bulky heterocycles 3 b,e the new 30 VE triple-decker complexes [(C5Me5)Ru{μ,n5-(CR1)2(BR2)2CMe}RuH(C5Me5)] (5b,e) were formed, which contain a Ru-H bond. Cyclic voltammetric studies revealed the existence of stable anions 2c- and 4d- formed by reversible one-electron reduction at -1.26 and -1.40 V, respectively (vs. SCE). The red-brown anions were further characterized by ESR spectroscopy following stepwise reduction of the neutral species with potassium in THF. Addition of CO to 4a and 4d led to formation of the monocarbonyl complexes [(C5Me5)Ru(CO){n5-(CR1)2-(BR2)2CMe}] (6a,d), and 6d was characterized by X-ray structure analysis. The heterocycle in 6d is less folded (19°) than in 2 c. Its CO ligand causes a 28.5° tilt of the cyclic ligands. Reaction of CO with 2 c yielded a red product of unknown structure. The electronic structure of 2 was studied by EH-MO theory, which revealed a unique bonding in the sandwich. The s̰ electron density of the B-C bonds participates in the bonding to the iron atom; this demonstrates that the number of bonding electrons is the same as in ferrocene. Thus, the complexes 2 actually have 18 VE.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020506
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    Separation of Diastereomeric and Enantiomeric Alkyl Nitrates-Systematic Approach to Chiral Discrimination on Cyclodextrin LIPODEX-D (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 539-544 
    Details
    ISSN: 0947-6539
    Keywords: analytical methods ; alkyl nitrates ; chiral resolution ; cyclodextrins ; gas chromatography ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: High-resolution gas chromatographic separation of all diastereomeric monomethyl-substituted cyclohexyl nitrates is shown on a nonpolar methylpolysiloxane stationary phase, and the first application of this procedure to the environmental diastereomeric analysis of alkyl nitrates is presented. Two characteristic signals in the achiral analysis of atmospheric samples could be assigned to the smallest alkyl nitrate containing two asymmetric carbon atoms, 3-methyl-2-pentyl nitrate. Retention indices in the temperature-programmed separation based on the n-alkanes were determined. The homologous series of 1-alkyl nitrates were found to be useful as ECD-visible n-alkanes. Enantiomeric separation of alkyl nitrates was achieved on heptakis(3-O-acetyl,-2,6-di-O-pentyl)-β-cyclodextrin (LIPODEX-D). The influence of the nitrooxy group and the alkyl chain length on the chiral discrimination on LIPODEX-D is discussed for 25 chiral alkyl nitrates. The absolute configurations of some alkyl nitrates were assigned by asymmetric synthesis of enantiomerically pure references. The complexity of the alkyl nitrate mixtures present in air samples does not allow a direct chiral separation as the alkyl nitrates partly coelute on the LIPODEX-D column. Column coupling of LIPODEX-D with a polar achiral stationary phase like polyalkylenglycol (PAG) was successfully applied to solve this problem, and the chiral alkyl nitrates present in a typical air sample were separated. A systematic nomenclature for alkyl nitrates is introduced to handle the steadily growing number of branched and long-chain alkyl nitrates detected in environmental analysis.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020513
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  • 144
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    The Pagodane → Dodecahedrane Concept-Shorter Routes, Higher YieldsDedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 570-579 
    Details
    ISSN: 0947-6539
    Keywords: cage effects ; dodecahedranes ; organic synthesis ; pagodanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two variants of the “SN2 route” from pagodanes (A, B) to functionalized dodecahedranes (D, F) and particularly dodecahedradienes (E) offer considerable improvements in the number of operations (from nine to five to three) and yields (e.g., for diester F from 55-65 to 70-75 to 85-91%). Key steps are the regio- and stereospecific introduction of four to six bromine substituents into dimethyl pagodane-4-syn,9-syn-dicarboxylate (1 b) and a highly complex (thirteen bond-breaking/bond-forming events in four participating structures). yet very convenient (one-pot operation) and extremely efficient (nearly quantitative) transformation of secopagodane to bissecododecahedradiene with complete stereocontrol in transannular CH2 functionalizations. The prohibitively low kinetic acidity of “caged” hydrogens has so far only been overcome with the recently reported P2F reagent (Schwesinger). Further improvement of the overall economy of the pagodane → dodecahedrane scheme has been achieved by efficiently channeling a byproduct of the pagodane synthesis (ca. 10%) back into the SN2 track.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020517
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    O-Atom Transfer to Fe+n Clusters (n = 2-10) from O2, N2O and CO2: “Microoxides of Iron”This work is part of O. Gehret's dissertation (Darmstadt, D17). (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 598-603 
    Details
    ISSN: 0947-6539
    Keywords: clusters ; gas-phase chemistry ; ionmolecule reactions ; iron complexes ; mass spectrometry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the gas phase reactions of small Fe+n clusters (n = 2-10) with O2, N2O and CO2 in an FT-ICR mass spectrometer. Under our experimental conditions, clusters of all sizes reacted readily with O2 and all but the dimer reacted with N2O. Only the smallest Fe+n clusters (n = 2-4) appeared to activate CO2. For all X-O molecules (X = O, N2, CO), reaction pathways were observed that include the transfer of O atoms. In addition, the reactions with O2 and N2O were accompanied by the loss of one or two Fe atoms. Thermochemical considerations based upon the well-known X-O bond energies were used to calculate Fen-O+ bond dissociation energies (BDEs) for sizes n = 2-6; these amount to roughly 550 kJ mol-1 and thus are considerably higher than the BDE of the Fe-O+ ion. All oxidation reactions of the Fe+n clusters (n = 2-6) studied in more detail were terminated by products of FexOx+ stoichiometry (x = 1-4). These “microoxides of iron” are not able to activate any further X-O bonds. Secondary reactions of FexOx+ clusters with C6H6, C2H4 and NH3 were investigated for two selected sizes (x = 2, 3) and compared with reactions of the naked Fe+n clusters.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020520
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    A [3]rotaxane of the amide type (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 640-643 
    Details
    ISSN: 0947-6539
    Keywords: lactam ; macrocycles ; rotaxanes ; self-assembly ; template syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two new rotaxane types, the [2]rotaxanes 12a,b and the [3]rotaxane 13, have been synthesised by treating the “axle” 7 with the stopper component 9 in the presence of the “wheel” 8. A nonionic template intermediate of type 10 is proposed. The [3]rotaxane 13 was only obtained when the chain of the “axle” had a certain length: with n = 1 only [2]rotaxane 12 a was isolated, whereas with n = 2 the [3]rotaxane 13 was formed besides the [2]rotaxane 12 b. This suggests that more extended rotaxanes and polyrotaxanes can be synthesised by the template strategy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020605
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    Bis(ZnII-cyclen) complex as a novel receptor of barbiturates in aqueous solution (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 617-623 
    Details
    ISSN: 0947-6539
    Keywords: barbiturates ; macrocycles ; molecular recognition ; receptors ; zinc complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new bis-zinc(II) receptor (Zn2L), which has two macrocyclic 12-membered tetraamine (cyclen) ZnII complexes connected through a p-xylene bridge, has been synthesized as a novel host molecule to recognize barbiturates (such as barbital (bar)) in aqueous solution. Each of the zinc(II) ions in the biszinc(II) receptor was originally intended to match the dianionic barbital anion (bar2-) with supplementary hydrogen bonds between the cyclen NH's and the three carbonyl oxygens in complementary positions to yield a 1:1 complex, Zn2L-bar2-. From an aqueous solution of equimolar Zn2L and barbital at pH 8, however, a cyclic 2:2 complex, (Zn2L-bar2-)2, was isolated and characterized by X-ray crystal analysis. The NMR study in 10% (v/v) D2O/H2O has revealed dissociation of (Zn2L-bar2-)2 solely into the original target 1:1 complex Zn2L-bar2- and established the dimerization constant for 2Zn2L-bar2- → (Zn2L-bar2-)2, Kd ( = [(Zn2L-bar2-)2]/[Zn2L-bar2-]2) to be 103.4 M-1. The thermodynamic parameters were evaluated from the NMR measurements at 25, 35, 45, and 55°C: ΔG = -1.9 × 104 J mol-1, ΔH = - 3.3 × 104 J mol-1, ΔS = - 49 J mol-1 K-1 at 25°C. Potentiometric pH titration of Zn2L (1 mM) and barbital (1 mM) disclosed extremely facile deprotonation of the two imido groups of barbital at pH less than 7 to form the dianionic barbital-bound ZnII complexes Zn2L-bar2- and (Zn2L-bar2-)2, where-by the barbital binding affinity for Zn2L was estimated to be Kbar ( = [Zn2L-bar2-]/[uncomplexed Zn2L][uncomplexed barbital]) = 105.8 M-1 at pH 8 and 25°C with I = 0.10 (NaNO3). The significance of the bis-zinc(II) receptor in stabilizing the dianionic barbital is evident by comparison with the interaction of ZnII-cyclen complex (ZnL) with barbital, which yields only a 1:1 monoanionic barbital complex, ZnL-bar- (Kbar = [ZnL-bar-]/[uncomplexed ZnL][uncomplexed barbital] = 104.2M-1 at pH 8 and 25°C with I = 0.10 (NaNO3)).
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020602
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    Dimensional reduction in II-VI materials: A2Cd3Q4 (A = K, Q = S, Se, Te; A = Rb, Q = S, Se), novel ternary low-dimensional cadmium chalcogenides produced by incorporation of A2Q in CdQ (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 656-666 
    Details
    ISSN: 0947-6539
    Keywords: cadmium compounds ; chalcogen compounds ; crystal structure ; dimensionality ; photoluminescence ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the isomorphous, layered chalcogenides K2Cd3S4 (I), Rb2Cd3S4 (II), K2Cd3Se4 (III), Rb2Cd3Se4 (IV), and K2Cd3Te4 (V) in molten A2Qx fluxes in reported (A = K, Rb; Q = S, Se, Te; x = 2 to 3). The compounds form as (Cd3Q4)n2n- layers interspersed with A + cations; the layers are composed of Cd3Q2-4 units shaped as truncated cubes. The compounds have room-temperature band gaps of 2.75, 2.92, 2.36, 2.37, and 2.26 eV for I, II, III, IV, and V, respectively, and also display strong photoluminescence. The thermal analysis data for all compounds are reported. The properties of these compounds are compared with those of the three-dimensional compounds CdS, CdSe, and CdTe, as well as those of the nanometer-sized CdQ clusters. A conceptual context is presented to connect all these different types of compounds.
    Additional Material: 15 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020607
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  • 149
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    A tricyclic dehydrorubanone and new isomers of the major quinidine metabolite (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 680-684 
    Details
    ISSN: 0947-6539
    Keywords: amino alcohols ; asymmetric syntheses ; dihydroxylations ; diol cleavage Horner-Wittig reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spiroepoxide 1 was prepared from quinidine and converted into β-amino alcohol 3 (86% over two steps). Dihydroxylation of enantiopure oxazatricylic olefin (E)-4 provided diastereomeric diols 5a and 5b. Stereospecific conversion of 1,2-secondary, tertiary diol 5b into tetracyclic spiroepoxide 6 was accomplished in high yield by a one-pot tosylation-cyclization procedure. 1,2-Diol cleavage with NaIO4 in 80% acetic acid afforded the new tricyclic dehydrorubanone 7, containing the 4-oxa-7-azatricyclo[4.3.1.03,7]-decan-2-one core structure. Similarly, acetylated rubanone 9 was prepared on a 20 g scale. Reduction with NaBH4 in the presence of CeCl3 provided rubanols 10a and 10b (1:1.1). Horner-Wittig reaction of 9 with diethyl cyanomethylphosphonate was (Z)-selective, furnishing unsaturated nitrile (Z)-13. Conversion into the α,β-unsaturated aldehyde (Z)-14 and reduction afforded enartiopure allylic alcohol (Z)-12, which is a new isomer of the key quinidine metabolite 15.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020610
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  • 150
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    Bismuth(III) complexes of the tripeptide glutathione (γ-L-Glu-L-Cys-Gly) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 701-708 
    Details
    ISSN: 0947-6539
    Keywords: bismuth complexes ; drugs ; glutathione ; NMR spectroscopy ; red blood cells ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The tripeptide glutathione (γ-L-Glu-L-Cys-Gly, GSH) is thought to play an important role in the pharmacology of bismuth drugs, but to our knowledge no chemical studies of bismuth glutathione complexes have been reported. We report here studies of interactions of the antiulcer compound ranitidine bismuth citrate (1) and [Bi(edta)]- with glutathione in aqueous solution and in intact red blood cells by NMR spectroscopy. The deprotonated thiol group is shown to be the strongest binding site for BiIII, and a complex with the stoichiometry [Bi(GS)3] is formed, as determined by 13C NMR titrations. A remarkably large lowfield shift of approximately 1.37 ppm for the β-CH2 1H NMR resonances of GSH was observed on binding to BiIII. The complex [Bi(GS)3] is stable over the pH* range 2-10 (pH* = pH meter reading in D2O solution). A formation constant log K of 29.6±0.4 (I = 0.1 M, 298 K) for [Bi(GS)3] was determined by displacement of edta by GSH. The rate of exchange of GSH between free and bound forms is pH-dependent, ranging from slow exchange (on the 1H NMR timescale) at low pH (ca. 3 s-1 at pH 4.0) to intermediate exchange at biological pH (ca. 1500 s-1). Such facile exchange may be important in the transport and delivery of BiIII in vivo. Spin-echo 1H NMR showed that 1 reacts with GSH in red cells both in vivo and in vitro. A first-order reaction of 1 with red blood cells was observed in vitro (k = 0.20±0.04 h-1, t1/2 = 3 h, 310 K), and the rate-determining step appeared to involve the passage of BiIII through the cell membrane.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020613
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  • 151
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    Reply to “on the definition of chirality” further comments on the chirality of large random objects (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 744-746 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020618
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  • 152
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    Concepts in chemistry (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 751-751 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020702
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  • 153
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020701
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  • 154
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    Biomimetic ion-binding monolayers on gold and their characterization by AC-impedance spectroscopy (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 759-766 
    Details
    ISSN: 0947-6539
    Keywords: ionophores ; impedance spectroscopy ; membrane models ; monolayers ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Novel ion-binding monolayers on gold surfaces are presented where the molecular design is based upon the natural ion binder ferrichrome. The new ion binders possess hydroxamate coordinating groups arranged in C2 symmetry (bishydroxamate binder, BHB) or C3 symmetry (trishydroxamate binder, THB), and a separate dialkyl sulfide moiety, which serves as an anchor to the gold substrate. The separation between the ion-binding cavity and the attachment site to the gold allows each parameter to be controlled separately, namely, cavity size, its symmetry and external envelope, as well as the functional group used for immobilization. The monolayers were characterized with respect to ellipsometric thickness, wettability (advancing and receding contact angles (CAs) for water), and surface coverage; the latter is determined by metal underpotential deposition (UPD). It is shown that the introduction of hydrophobic side chains (i-butyl) improves the CAs, thickness, and surface coverage of the monolayers. A detailed analysis of the alternating-current (AC) impedance spectra is presented for THB monolayers on gold electrodes, where the impedance data are fitted to an equivalent circuit model. It is shown that the AC response in a wide frequency range can be used to probe ion binding and release in monolayer systems on electrodes.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020704
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  • 155
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    Synthesis, crystal structure, electronic structure, and properties of Hf2In5, a metallic hafnide with one-dimensional Hf-Hf and two-dimensional in-in bonding (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 800-804 
    Details
    ISSN: 0947-6539
    Keywords: crystal structure ; electronic structure ; hafnium compounds ; indium compounds ; metallic conductivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hf2In5 - previously reported with the tentative composition “Hf3In4” - was prepared from the elements in a tantalum tube at 970 K. The X-ray diffractional characterization by means of single-crystal refinement reveals the presence of a tetragonal structure (a = 1024.71 (9), c = 305.66 (3) pm, P4/mbm, Z = 2) of Mn2Hg5 type. Hf2In5 is Pauli-paramagnetic and a good metallic conductor. Quasi-relativistic semiempirical and scalar-relativistic ab initio band structure calculations reveal Hf2In5 to be a hafnide, composed of a two-dimensional indium network threaded by infinite hafnium chains. The amount of In-In bonding scales counterintuitively with the interatomic distances.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020709
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    Enantioselective synthesis of highly functionalized 4-piperidones by the asymmetric imino-diels-alder reaction of chiral 2-amino-1,3-butadienesAbbreviations used in this article: NaHMDS: sodium bis(trimethylsilyl)-amide; TMS: trimethylsilyl; MOM: methoxymethyl; TBDMS: tert-butyldimethylsilyl; 3-Fu: 3-furyl; p-MeOPh: 4-methoxyphenyl; o-BrPh: 2-bromophenyl; p-BrC6H4CO: 4-bromobenzoyl; (R)-MPTA: (R)-α-methoxy-α-Phenyl-α-trifluoromethylacetyl. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 805-811 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric Diels-Alder reactions ; butadienes ; Diels-Alder reactions ; piperidones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chiral 2-amino-1,3-butadienes 1 derived from commercially available (S)-2-methoxymethylpyrrolidine react with aromatic N-trimethylsilylaldimines and N-phenylaldimines in the presence of ZnCl2 to give, after the reaction workup, 4-piperidones 4 and 6, respectively, with moderate to very high enantiomeric excesses. In addition, the absolute configurations of derivatives of 4 a and 4 g were determined by circular dichroism and NMR spectroscopy on the Mosher ester, respectively.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020710
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  • 157
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    The β-lactone route to a totally stereoselective synthesis of carnitine derivatives (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 826-831 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric ring-opening ; carnitine ; cyclizations ; β-lactones ; nucleophilic substitutions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of the enantiomerically pure, carnitine-related β-lactones 10 and 12 starting from various carnitine precursors of opposite configuration (or carnitine itself) are described. (R)-3-Chlorocarnitine (20) has also been directly prepared from (S)-carnitine (14) and has been cyclized to 12 by a second inversion of configuration of the stereogenic centre. By nucleophilic attack at the carbonyl carbon, the β-lactone carnitine derivatives have been converted into esters, amides and guanidino congeners. Following this route, it is possible to obtain the biologically active isomer (R)-carnitine (1) starting from the otherwise useless industrial by-product (S)-carnitine (14). Nucleophilic attack by selected ambidient nucleophiles at the β-carbon of the same β-lactone derivatives results in a second inversion of configuration of the stereogenic centre. Besides aminocarnitine (3), chiral acetylcarnitine (2) and acetylthiocarnitine (5) have been synthesized in homochiral forms following this latter procedure.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020713
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  • 158
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    Total synthesis of Swinholide A, Preswinholide A, and Hemiswinholide A (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 847-868 
    Details
    ISSN: 0947-6539
    Keywords: natural products ; swinholide A ; total syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The total synthesis of synthesis of swinholide A (1) has been accomplished via key intermediate aldehyde 12 (Fig. 3), whose construction started from L-rhamnose (18), epoxide 21, and phenylsulfone orthoester 22, and proceeded through an Enders asymmetric alkylation (16 + 17 → 15), a Ghosez cyclization (21 + 22 → 20), and a Corey-Sharpless coupling reaction (13 + 14 → 12). Elaboration of compound 12 along slightly different pathways culminated in the synthesis of carboxylic acid 10 and hydroxy compound 11, whose union by an esterification reaction, followed by ring closure of the subsequently derived hydroxy acid under Yamaguchi conditions, led to swinholide A (1) upon deprotection. The chemistry developed also allowed the total synthesis of prewinholide A methyl ester (7), preswinholide A (8), and hemiswinholide A (78).
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020716
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
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    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020721
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  • 160
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020801
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  • 161
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    Observing a homogeneous ziegler catalyst precursor at work: Insertion reactions into the zirconium - carbon bond of the (butadiene)ZrCp2/B(C6F5)3 addition product (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 919-924 
    Details
    ISSN: 0947-6539
    Keywords: homogeneous catalysis ; metallocenes ; polymerization ; Ziegler catalysts ; zirconium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (Butadiene)zirconocene adds to B(C6F5)3 to form the metallacyclic metallocene-boron betaine system 1, which contains a C-F → Zr interaction. Addition of tert-butylisocyanide or tert-butyl-cyanide leads to cleavage of the zirconium - fluorine linkage and formation of the adducts 4 and 7, respectively. With additional tert-butylisocyanide, 4 reacts further to yield the corresponding η2-imino-acyl betaines 5 and 6 (2 stereoisomers), whereas 7 inserts additional tert-butyl-cyanide into the Zr-C bond to give a chiral metallacyclic N-zirconaketimine complex that is probably stabilized by an internal borate → Zr interaction. Ethene and propene insertion reactions into the Zr - C bond of 1 follow a similar course: the mono-insertion products 9 and 10, respectively, generated in situ and characterized by NMR spectroscopy at - 35°C in [D8]toluene solution, are chiral and are also stabilized by internal borate → Zr coordination. Complexes 9 and 10 are likely to be intermediates in the formation of active homogeneous Ziegler polymerization catalysts.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020803
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  • 162
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    Oxypyriporphyrin, the first fully aromatic porphyrinoid macrocycle with a pyridine subunitConjugated Macrocycles Related to the Porphyrins, Part 6. Part 5: ref. [1]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 944-948 
    Details
    ISSN: 0947-6539
    Keywords: aromaticity ; porphyrins ; pyridones ; MacDonald condensation ; tautomerizations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the first example of an aromatic pyridine-containing porphyrinoid 10 has been accomplished in excellent yields by the acid-catalyzed “3 + 1” condensation of 3-hydroxy-pyridinedicarboxaldehyde (8) with tri-pyrrane 9. The key intermediate 8 was obtained by the selenium dioxide oxidation of the known biscarbinol 7. The aromaticity of “oxypyriporphyrin” 10 has been confirmed by MS, NMR, IR, and UV/Vis spectroscopy. This system afforded a monocation in 0.2% TFA-chloroform, and a dication was observed in 2% TFA-chloroform; these species also retained macrocyclic aromaticity. Oxypyriporphyrin readily formed the corresponding metal chelates 14a-c by reaction with zinc, copper(II), or nickel(II) acetate, and this observation suggests that there are extensive possibilities for the use of 10 in coordination chemistry. Oxypyriporphyrin and the related semiquinone system oxybenziporphyrin represent the first two members of a new class of aromatic porphyrinoids.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020806
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  • 163
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    Oxidative addition of aryl halides to transient anionic à-aryl-palladium(0) intermediates - application to palladium-catalyzed reductive coupling of aryl halides (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 957-966 
    Details
    ISSN: 0947-6539
    Keywords: catalysis ; C-C coupling ; mechanistic studies ; oxidative addition ; palladium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rates and mechanism of the reactions of a series of aryl-ligated, anionic palladium(0) complexes Ar-Pd0-(PPh3)-2 with para-substituted iodobenzenes were investigated by means of transient electrochemistry. The reaction was found to be first order in each reactant and to proceed similarly to oxidative addition of aryl halides to the halide-ligated species X-Pd0(PPh3)-2, although much faster and less sensitive to electronic factors. Owing to the short lifetime (t1/2 ≍ 1 - 5 ms) of the product of this reaction, it could not be characterized in detail. However, based on kinetic results, this transient species is thought to be an anionic pentacoordinated bisarylpalladium(II) complex, which undergoes rapid loss of halide ligand to yield, most probably, a bisarylpalladium(II) neutral species. Based on the study of this reaction and on previously reported results, we propose a mechanism for the palladium-catalyzed homocoupling of aryl halides consisting of a catalytic cycle initiated by oxidative addition of an aryl halide to a zerovalent tris-ligated palladium center. Two-electron reduction of the pentacoordinated arylpalladium(II) anionic species thus formed gives a tris-ligated anionic arylpalladium(0) center, which undergoes oxidative addition with a second aryl halide molecule to eventually lead to a bisaryl-palladium(II) neutral species. Reductive elimination of a bisaryl molecule from this center closes the catalytic cycle by regenerating the initial zerovalent palladium complex. The application of this sequence to the catalytic heterocoupling of aryl halides is discussed, and it is concluded, on the basis of Hammett correlations, that statistical yields should be observed, in agreement with the results obtained for preparative reactions in DMF.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020808
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  • 164
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    Structural motifs of the dimeric lewis glycolipids as determined by NMR spectroscopy and molecular dynamics simulations (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 981-988 
    Details
    ISSN: 0947-6539
    Keywords: computer simulations ; conformations ; Lewis glycolipids ; molecular dynamics ; thioglycosides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several monomeric and dimeric Lewis glycolipids have been investigated by NMR spectroscopy, and structural aspects were modelled by computer. From the pseudo-C2-symmetric tetrasaccharide unit that forms the recognition domain of the Lewis Y and Lewis b antigens, a totally C2-symmetric tetrasaccharide was designed that contains the structural element common to all Lewis antigens. Finally, a model for the presentation of dimeric Lewis antigens at membrane surfaces was derived. The overall shapes of the dimeric Lewis oligosaccharides are defined by the connectivity of the sugar residues within rigid tri- and tetrasaccharide building blocks.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020811
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    Synthesis of a conformationally constrained heparin-like pentasaccharide (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1007-1013 
    Details
    ISSN: 0947-6539
    Keywords: antithrombotics ; conformation ; molecular recognition ; oligosaccharides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An octasulfated pentasaccharide 1 having an L-iduronic acid moiety in a fixed 1C4 conformation was synthesized by the coupling of a triosyl donor 3 with a disaccharide acceptor 4 followed by deprotection and O-sulfation. The acceptor 4 was prepared from the fully acetylated 5-C-allyl-β-D-glucose building block 7 by means of a TMSOTf-promoted glycosylation, intramolecular substitution and ozonolysis of the olefinic bond as the key reactions. Compound 1 showed very low activity in an antithrombin 111-mediated anti-Xa assay; this reflects the importance of the presence of a flexible L-iduronic acid moiety in heparin-like antithrombotics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020815
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020901
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  • 167
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    The aminium salt and photoinduced electron transfer initiated diels-alder cycloaddition of electron-rich allenes: Evidence for a stepwise mechanism and the importance of steric and electronic effects for the reactivity of distonic radical cation intermediatesRadical-Cation-Catalyzed Reactions, Part 8. Part 7: see ref. [1].Dedicated to Prof. Dr. S. Hünig on the occasion of his 75th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1031-1040 
    Details
    ISSN: 0947-6539
    Keywords: allenes ; cycloadditions ; Diels-Alder reactions ; mechanistic studies ; radical cations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several novel electron-rich aryl-allenes have been synthesized, characterized, and used as dienophiles in the radical-cation-catalyzed cycloaddition with 1,2,3,4,5-pentamethylcyclopentadiene, which affords, in most cases, the Diels-Alder products in 5 min at 0°C with a high peri-, chemo-, facial, and stereoselectivity. In line with oxidation-potential considerations it is concluded that the electron-transfer-induced reaction proceeds along a [3 + 2] pathway by cycloaddition of the diene radical cation to a neutral allene with a rather short chain length. The low cycloaddition yields from some of the allenes are interpreted as evidence for a stepwise mechanism involving distonic radical cations as key intermediates. We discuss the tendency of the distonic radical cations to undergo ring closure to the Diels-Alder-product radical cations in terms of enthalpy considerations, which, we suggest, offer a novel criterion for the design of stepwise radical cation reactions.
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    URL: http://dx.doi.org/10.1002/chem.1460020818
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    Oxidation products of organic trisulfanes: Molecular structures and energies of various isomers of the dimethyltrisulfane oxides Me2S3O and Me2S3O and of 1,3-tBu2S3O2Sulfur Compounds, Part 193; for Part 192 see R. Steudel, Ind. Eng. Chem. Res. 1996, 35, 1417. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1060-1067 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; rotamers ; stereoisomers ; sulfane oxides ; torsional potentials ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio MO calculations (MP 2/6-311 G**//HF/6-311 G**) have been performed for several isomers (including rotamers) of Me2S3O and Me2S3O2. MeS(O)SSMe exists as five rotamers; the most stable form (1a) has a helical backbone CSSSC with S-S bond lengths of 206.0 (SIISII) and 212.4 pm (SIISII). The most stable rotamer of MeS-S(O)SMe (2a), is less stable than 1a by 10.7 kJ mol-1; it is of Cs, symmetry, while a rotamer of Cs symmetry (2b) is less stable than 2a by only 1.4 kJ mol-1. Both 2a and 2b are stabilized by O ⃛ H hydrogen bonds. The S-S bond lengths of 2a are 210.0 and 212.0 pm; the CSSSC chain is not helical (CSSS torsion angles 166.3 and -75.4°). The 1,3-dioxide MeS(O)-SS(O)Me (3) has two equivalent chiral centers and exists as diastereomers. The most stable isomer 3a (RR)/(SS) is of C2 symmetry with methyl groups trans to each other; the SO bonds form an angle of about 90°. The meso form 3b is less stable than 3a by 17.2 kJ mol-1 and the rotamers 3c and 3d are less stable by 25.6 kJ mol-1 and 28.4 kJ mol-1, respectively. The trisulfane-1.2-dioxide MeS(O)-S(O)SMe has two nonequivalent chiral centers and exists as five isomers. The most stable form, the (RS)/(SR) form 4a, is less stable than 3a by 21.4 kJ mol-1 and is characterized by SS bonds of 220.9 (SIIISIII) and 208.3 pm (SIIISII). The rotamer 4b is less stable by 5.9 kJ mol-1. The isomers 4c, 4d, and 4e are all of (SS)/(RR) configuration and are less stable than 4a by 6.3, 12.7, and 12.0 kJ mol-1. For comparison, ab initio MO calculations (HF/6-311 G*) for tBuS(O)SS(O)-tBu yielded two diastereomers of practically identically energy that both contain helical CSSSC backbones. The (RS) form is less stable than the (RR)/(SS) form by 1.8 kJ mol-1.
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    URL: http://dx.doi.org/10.1002/chem.1460020903
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    Protonation and alkylation of the thiolato donors in [Fe(CO)(“NHS4”)]: Effects on the structural, electronic, and redox properties of metal-sulfur complexesTransition Metal Complexes with Sulfur Ligands, Part 120. Part 119: D. Sellmann, T. Becker, F. Knoch, Chem. Ber. 1996, 119, 509-519. “NHS4”2- = 2,2′-Bis(2-mercaptophenylthio)diethylamine(2-).Dedicated to Professor Rudolf Taube on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1092-1098 
    Details
    ISSN: 0947-6539
    Keywords: cyclic voltammetry ; iron complexes ; redox systems ; structure elucidation ; sulfur ligands ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effects of protonation and alkylation at the sulfur donors in metal-sulfur complexes have been investigated by taking the specific example of [Fe(CO)(“NHS4”)] (1). The 18 valence electron (VE) complex 1 consists of a low-spin FeII center and the dithioether thiolato amine ligand “NHS4”2-- ( = 2,2′-bis(2-mercaptophenylthio)diethylamine(2-)). Complex 1 can be reversibly protonated at the two thiolato donors; this results in an increase in ν(CO) of 35 cm-1 after the first protonation and 45 cm-1 after the second. Alkylation of 1 with one or two equivalents of the oxonium salts R3OBF4 (R = Me, Et) yields [Fe(CO)(“NHS4”-R)]BF4 (R = Me: 4, Et: 5), [Fe(CO)(“NHS4” -R2)](BF4)2 (R = Me: 6, Et: 7), and the methyl ethyl derivative [Fe(CO)(“NHS4” -Me-Et)](BF4)2 (8). An increase in ν(CO) of 31-32 cm-1 is observed for each successive alkylation. Due to the C1 symmetry of 1, complexes 4, 5, and 8 are formed as 1 : 1 mixtures of two diastereomers, whereas 6 and 7 are present as only one stereoisomer. Acidic hydrolysis of 4, 6, and 7 liberates the corresponding ligands [“NHS4” -Rn](BF4)n (9-11; n = 1,2), which were isolated as the ammonium tetrafluoroborate salts. The molecular structure of 8a has been elucidated by X-ray structure analysis. This shows that the Fe-N and Fe-S bonds in the [FeNS4] core do not change after alkylation of the thiolato donors despite the Δν(CO) of ca. 60 cm-1 between 1 and 8, which indicates a distinct decrease in electron density at the Fe center. This decrease can be rationalized by increased π-acceptor character of the sulfur donors upon protonation or alkylation. The change in electron density at the [Fe(“NS4”)] core is further corroborated by cyclic voltammetry. For each successive protonation or alkylation of 1, the redox couple potentials shift by 600-800 mV. As a consequence, the diethyl derivative, for example, gives rise to reduced species (19 or 20 VE) which are not observed for 1. The implications of these results for the reduction of N2 within the coordination sphere of the FeMo cofactor of nitrogenases are discussed.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020907
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  • 170
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    Peptide helices as rigid molecular rulers: A conformational study of isotactic homopeptides from α-methyl-α-isopropylglycine, [L-(αMe)Val]nLinear Oligopeptides, Part 360. Part 359: A. Bianco, M. Maggini, G. Scorrano, C. Toniolo, G. Marconi, C. Villani, M. Prato, J. Am. Chem. Soc. 1996, 118, 4072-4080. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1104-1111 
    Details
    ISSN: 0947-6539
    Keywords: amino acids ; conformation ; helices ; molecular rulers ; oligopeptides ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Terminally blocked, isotactic homopeptides from the sterically demanding α-methylvaline of general formula Y-[L-(αMe)Val]n-OtBu (Y = Z, pBrBz, Ac; n = 2-8) have been prepared step-by-step in solution and fully characterized. The conformations preferred in solution (β-turn and right-handed 310-helix) have been assessed by FT-IR, 1H NMR and CD spectroscopy. The molecular and crystal structures of the Z-protected trimer, hexamer, heptamer and octamer have been determined by X-ray diffraction. In the crystal state, while the trimer is folded in a type III β-turn conformation, the longest homopeptides form well-developed, regular, right-handed 310-helices. The screw sense in the helix of the pBrBz-blocked octamer has been confirmed to be right-handed by solid-state and solution CD spectroscopy. The possible exploitation of these peptide helices as rigid and precise molecular rulers is discussed.
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    URL: http://dx.doi.org/10.1002/chem.1460020909
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  • 171
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    Thermal and light-induced spin transition in [Fe(bpen)X2] (bpen = 1,6-Bis(2-pyridyl)-2,5-diazahexane, X = NCS-, NCSe-)Dedicated to Professor Max Herberhold on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1129-1133 
    Details
    ISSN: 0947-6539
    Keywords: iron complexes ; kinetics ; magnetic susceptibility ; Mössbauer spectroscopy ; spin crossover ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermal and light-induced spin transitions in [Fe(bpen)(NCS)2] and [Fe(bpen)(NCSe)2] have been investigated by means of magnetic susceptibility and Mössbauer measurements. The HS → LS transition in [Fe(bpen)(NCS)2] is detected at temperatures below 120 K (T1/2 [when the fraction of high-spin species γHS = 0.5] = 68 K) with a residual HS fraction of 8% at 20 K. The transition is fairly abrupt and no hysteresis is observed, in contrast to earlier studies (ref. [8]). The generation of metastable HS states in [Fe(bpen)(NCS)2] is achieved by rapid cooling of the sample and by irradiating the compound at low temperatures with light of appropriate wavelengths (LIESST). The HS → LS relaxation of the metastable spin states after rapid cooling is observed at temperatures between 35 and 42.5 K and shows strong deviations from single exponential behavior. The spin-transition behavior changes drastically on substitution of the thiocyanate ions by NCSe-. In [Fe(bpen)(NCSe)2] a gradual and complete spin transition between 140 and 250 K is observed (T1/2 = 181 K). Irradiation of the compound with green light at low temperatures does not result in formation of long-lived metastable HS states. The spintransition curves derived from magnetic susceptibility and Mössbauer studies show good agreement for each compound, and no hints of significantly different Lamb-Mössbauer factors for the HS and LS states are observed.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020912
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  • 172
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    Highly Selective Formation of tert-Butyl Hydroperoxide from the Reaction of Isobutane and O2 in a Zeolite under Visible Light (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 385-389 
    Details
    ISSN: 0947-6539
    Keywords: hydroperoxides ; isobutane ; oxygen ; photooxidations ; zeolites ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Isobutane and oxygen gas loaded into zeolite BaY react upon irradiation with green or blue light to yield tert-butyl hydroperoxide in a single-photon process. This was discovered when monitoring the reaction at room temperature in situ by FT-infrared spectroscopy. Selectivity was 98%, even upon conversion of more than half of the reactants loaded into the zeolite. Diffuse reflectance spectra revealed a visible absorption tail which originates from an isobutane · O2 collision complex. It is attributed to the isobutane · O2 contact charge-transfer absorption, whose onset is shifted from the UV into the visible region by the high electrostatic field of the zeolite.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020406
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  • 173
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    Structural Principles of the Coordination Number Eight: WF2-8, ReF2-8, and XeF2-8 (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 398-402 
    Details
    ISSN: 0947-6539
    Keywords: coordination geometry ; fluorides ; square antiprisms ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: WF6, ReFn (n = 6 and 7), and XeF6 combined with NOF to give (NO+2) WF2-8, (NO+)2ReF2-8, and (NO+)2XeF2-8, respectively. Also NO2F reacted with ReF6 to form (No2+)ReF2-8. Cs2XeF8·4BrF5 crystallized from a solution of Cs2XeF8 in BrF5. These five compounds were subjected to X-ray structure determinations. The structure of [NO(NOF)2]+IF-8 was used as a standard for comparison. All anions exhibit square-antiprismatic geometry, independent of their electronic configurations. Bond lenghts in ReF2-8 and XeF2-8 are larger than in WF2-8 and IF-8, owing to the presence of nonbonding electrons. Deviations from the ideal structure in XeF2-8 are attributed to cation-anion interactions.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020408
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  • 174
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    An Ab Initio Study of Structures and Energetics of Copper Sulfide Clusters (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 429-435 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; chalcogen compounds ; copper clusters ; ligand stabilisation ; thermodynamics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of ab initio calculations for the sulfur-bridged copper clusters [Cu2nSn(PR3)m] (n = 1-4, 6; m = 0, 2, 4, 6, 8; R=H, CH3) were compared with those of theoretical investigations of the selenium-containing analogues that have been recently reported. The theoretical work was carried out in order to find a possible explanation for the experimentally observed-different-properties of both cluster types. The structural principles turned out to be independent of the chalcogen, except the necessary shortening of the copper-chalcogen atomic distance that results from the formal substitution of sulfur for selenium. Comparison of the energetics of the sulfur- and selenium-containing compounds showed that it is necessary to discuss the influence of the tertiary phosphine ligands that protect the existing clusters from reaction to give the solids Cu2S and Cu2Se. However the results of the calculations suggest that the different thermodynamic data of the cluster core and the Cu—P bonds-at least if PH3 or PMe3 are taken into account-cannot be the only cause for the differences in the experimental behaviour. The responsibility rests rather with kinetic effects, such as the steric demand of the actual ligands or the activation energy of decomposition of the ligand shell during the cluster-forming reaction.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020412
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  • 175
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 476-479 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020503
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  • 176
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    Ionic Fluorination of Carbon Monoxide as a Route to Gasphase Carbonylation of Inert C—H and N—H Bonds (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 495-501 
    Details
    ISSN: 0947-6539
    Keywords: carbon monoxide ; carbonylations ; fluorine compounds ; gas-phase chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Gaseous FCO+ ions from the ionization of mixtures of nitrogen trifluoride and carbon monoxide execute selective and efficient CO-functionalization of the C-H bonds of benzene and toluene and of the N-H bond of ammonia. The occurrence of these carbonylation reactions has been unambiguously ascertained by Fourier-transform ion cyclotron resonance (FT-ICR) spectrometry, and the details of the structure and the mechanism of formation of the precursor FCO+ ions have been investigated. FT-ICR experiments show that these ions, structurally assigned as F-C-O+ by collisionally activated dissociation (CAD) spectrometry, arise from the reaction of CO.+ with NF3 and of NF+2 with CO. Combining the latter F+ transfer with the independently observed fluoride-ion abstraction by FCO+ from NF3 results in a catalytic cycle in which gaseous NF+2 ions promote the conversion of carbon monoxide into carbonic difluoride, F2CO, with nitrogen trifluoride as the source of F.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020507
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  • 177
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    Conformational Preference of 2-(Halomethyl)- and 2-(Oxymethyl)Pyridines: Microwave Spectrum, Ab Initio, and MM3 Studies of 2-(Fluoromethyl)Pyridine (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 516-522 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; conformation ; microwave spectroscopy ; pyridines ; stereoelectronic effect ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: One single conformer was assigned from the microwave spectrum of 2-(fluoromethyl)pyridine, investigated in the gas phase in the 26.0-39.0 GHz spectral region at about -10°C. Its Cα-F bond was found to be coplanar with the ring and anti to the N-C2 bond (syn to the C2-C3 bond). There was no indication in the microwave spectrum of the presence of other rotameric forms of the molecule.The results of the spectroscopic study were backed up by ab initio calculations at the MP2/6-31 G** (frozen core) level. These calculations predict that the assigned conformer is the only stable form of the molecule. The transition state was calculated to have the CH2F group 180° from the stable anti conformation. The energy of the transition state was computed to be 20.2 kJ mol-1 higher than the energy of the anti rotamer.The results are interpreted in terms of a stereoelectronic effect, and the orbital overlaps responsible for the observed effect are discussed. It is shown that 2-(fluoromethyl)pyridine serves as a good model for 2-(alkoxymethyl)pyridines, previously found to show the same conformational preference.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020510
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  • 178
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    Synthesis of Methyl 5′-Thio-α-isomaltoside via an Acyclic Monothioacetal and its Behavior toward Glucoamylase (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 556-560 
    Details
    ISSN: 0947-6539
    Keywords: binding studies ; glucoamylase ; isomaltoses ; oligosaccharides ; thiosugars ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methyl 5′-thio-α-isomaltoside (1), which contains the ring-sulfur analogue of the nonreducing glucoside of isomaltose, was synthesized from gentiobiose through a novel ring opening-recyclization approach. The nonreducing glucoside of per-O-benzylated phenyl 1-thio-β-gentiobioside underwent O-5′-C-1′ bond cleavage with dimethyl-boron bromide and thiolacetic acid to give the acyclic monothioacetal 4 with the 1-thioglucopyranoside at the reducing end intact. The HO-5′ group in 4 was inverted by a standard oxidation-reduction process with good efficiency. Recyclization under Mitsunobu condition allowed C-5′-S-1′ bond formation with inversion of configuration at C-5′, to give 1 after functional group interconversion. TLC analysis showed that 1, unlike isomaltose, was not hydrolyzed by glucoamylase from Rhizopus niveus. A fluorometric assay confirmed that the dissociation constant (Kd) for 1 with the enzyme was 39 mM at 20°C, which is comparable with that for isomaltose. A binding assay involving fluorescence titration of the enzyme-1 complex with gluconolactone indicated that the disaccharide 1 was bound to the catalytic and noncatalytic subsites. Since isomaltose is known to bind only to the noncatalytic subsites, this result indicates a relatively high affinity of the 5-thioglucose moiety for the catalytic subsite.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020515
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  • 179
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    Versatile Synthesis of Enantiomerically Pure 2-Alkoxy-1-Ethynylcyclopropanes and their Application in the Synthesis of Enantiomerically Pure Bicyclo-[3.3.0]oct-1-en-3-onesCyclopropyl Building Blocks for Organic Synthesis, Part 31. Part 30: T. Heiner, S. Michalski, K. Gerke, G. Kuchta, M. Buback, A. de Meijere, Synlett 1995, 335; Part 29: J. L. Coudret, K. Ernst, A. de Meijere, B. Waegell, Synthesis 1994, 920.Dedicated to Professor Manfred Regitz on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 545-555 
    Details
    ISSN: 0947-6539
    Keywords: alkynes ; asymmetric syntheses ; cyclopropanes ; spiro compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variety of chiral, nonracemic 2-alkoxy-1-alkynylcyclopropanes 7 were synthesized in good to very good yields from enantiomerically pure glycidol derivatives (glycidol tosylate, epichlorohydrin) by boron trifluoride promoted addition of lithium trimethylsilylacetylide followed by protection of the secondary hydroxyl group and finally a diastereoselective γ-elimination. The 2-ethoxy derivative (S,R)-7 b was deprotonated with n-butyllithium, and the resulting 1-lithio-2-ethoxy derivative (S,R)-20 functionalized by treatment with oxygen followed by tosyl chloride. Protodesilylation and catalytic hydrogenation smoothly furnished 1-ethenylcyclopropyl sulfonates, which underwent a clean Pd0-catalyzed SN2′-type substitution with dimethyl propargylsodiummalonate to give the (E)-configurated enyne (R,E)-26 with a methylenecyclopropane end group. A diastereoselective Pauson-Khand reaction completed the sequence to give the enantiomerically pure spirocyclopropaneannelated bicyclo[3.3.0]octane derivative 31.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19960020514
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  • 180
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    Emission from Zeolite-Occluded Manganese-Diimine Complexes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 592-597 
    Details
    ISSN: 0947-6539
    Keywords: charge transfer complexes ; diimine complexes ; emission spectroscopy ; manganese complexes ; zeolites ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Manganese complexes of 2,2′-bipyridine (bpy) and 1,10-phenantroline (phen) have been synthesised in the supercages of cubic NaX and NaY and in the hypercages of the hexagonal NaEMT faujasites. The coordination and redox chemistry were studied with ESCA, CA, FT-IR, FT-Raman, diffuse reflectance and emission techniques. FT-IR/FT-Raman shows cis coordination for all complexes and a high Mn-N stretching frequency in the phen complexes as a result of steric constraints imposed by the ligand. [Mn(bpy)2]2+ in the different zeolites shows metal-to-ligand charge transfer (MLCT; at 495 nm); for [Mn(phen)2]2+-NaY the MLCt is broadened owing to complex distortion. On MLCT excitation [Mn(bpy)2]2+ complexes show an ipsochromic shift in the emission and an increase in quantum yield with increasing steric restrictions imposed by the zeolite. The ipsochromic shift of the emission band of [Mn(phen)2]2+ in NaY results from the combined effect of the ligand field (this suggests emission from a CT state) and of coordinative distortion. The key factor influencing the emission properties is found to be the overall matrix-induced complex distortion. Cation stabilisation of the ligand anion affects emission indirectly. The decay times for [Mn-(bpy)2]2+-NaY are in the millisecond range (7.5-11.5 ms). A proposed model for excitation and emission properties of zeolite-occluded MnII complexes involves excitation of a quartet CT state, intersystem crossing and subsequent emission. The enhanced stability of the coordination sphere in the zeolite allows complexes to luminesce from a CT state, which is not detected in solution. The zeolite behaves as a supramolecular cryptating agent, protecting complexes from photodissociation.
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    URL: http://dx.doi.org/10.1002/chem.19960020519
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  • 181
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    [Lithium tert-butylperoxide]12: Crystal Structure of an Aggregated Oxenoid (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 604-607 
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    ISSN: 0947-6539
    Keywords: aggregates ; lithium compounds ; oxenoids ; peroxides ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The X-ray crystal structure of the dodecameric lithium tert-butylperoxide [2]12 is the first of an alkali or alkaline earth peroxide. It shows the lithium ion bridging the two oxygen atoms of the peroxide unit and a slight lenghtening of the O-O bond, in agreement with quantum-chemical calculations. A calculation for the model reaction of MeLi with LiOOH to give MeOLi and LiOH reveals the importance of Li bridging the O-O bond in the transition state of this reaction, as similarly discussed for many oxidation reactions of (transition-) metal peroxides. Preliminary theoretical studies of the O-O bond length (and thus of the oxenoid character) as a function of the aggregation of 2 disclose that increasing aggregation leads to stabilization of the charge at the anionic oxygen atom and thus to a reduction of the O-O bond length (oxenoid character). Related considerations of the effect of aggregation should also be valid for other lithium (organometallic) compounds and their structure and reactivity as well as other properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020521
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  • 182
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    Extended X-ray absorption fine structure (EXAFS) studies of hydroxo(oxo)iron aggregates and minerals, and a critique of their use as models for ferritin (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 634-639 
    Details
    ISSN: 0947-6539
    Keywords: biomimetic chemistry ; EXAFS spectroscopy ; ferritin ; iron homeostasis ; nanoparticles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structural models for the hydroxo(oxo)iron core of ferritin include extended mineral structures and cluster systems such as [Fe11O6(OH)6(O2CPh)15] ( = Fe11) and two clusters that crystallise in the same lattice (compound 1), namely, [Fe17(m̈3-O)4(m̈3-OH) 6(m̈2-OH)10(heidi)8(H2O)12]3+and[Fe19(m̈3-O)6(m̈3-OH)6(m̈2-OH)8(heidi)10(H2O)12]1+. The suitability of these systems as models for the structure of the core of ferritin has been tested by comparing their Fe K-edge EXAFS and X-ray crystallographic results with the Fe K-edge EXAFS data on horse spleen ferritin. The interpretative procedure for the EXAFS analysis was optimised by using the X-ray crystallographic data for compound 1 as a basis. This protocol was then used to interpret the Fe K-edge EXAFS spectra of α- and γ-Fe(O)OH and to reinterpret that previously recorded for horse spleen ferritin. The published Fe K-edge EXAFS data on Fe11 were also considered. The Fe⃛Fe distances provide a clear indication of the nature of the hydroxo(oxo)iron assembly. It was found that the iron-iron interactions are the most reliable guide. Clusters are shown to be more appropriate than infinite lattices as structural models for the core of ferritin.
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    URL: http://dx.doi.org/10.1002/chem.1460020604
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  • 183
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    Conformational studies of chiral vinylogous sulfonamidopeptides (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 644-655 
    Details
    ISSN: 0947-6539
    Keywords: conformation ; crystal structure ; molecular modeling ; NMR spectroscopy ; sulfonamido-pseudopeptides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformational preferences of chiral vinylogous aminosulfonic acids (vs-amino acids) and of the corresponding oligomers (vs-peptides) were investigated by a combination of X-ray crystallography, variable-temperature (VT) 1H NMR spectroscopy, FT-IR spectroscopy, and NOE experiments. The major source of conformational freedom in the monomers is the rotation around the C—C bond connecting the double bond with the allylic stereocenter (N—C*—C=C). The allylic conformational perferences can be altered in the oligomers by the formation of secondary structures enforced by hydrogen bonding. Twelve-membered-ring hydrogen bonding is detected in the crystal structure of vs-dipeptide 9, while fourteen-membered-ring bydrogen bonding is the most common folding pattern for the oligomers in chloroform solution. The experimental results are complemented by computer modeling: suitable force-field (FF) parameters for the unsaturated sulfonamide group nwere develiped from ab initio calculations. A Goodman-Still systematic pseudo-Monte-Carlo search was used for the conformational search. The conformers were minimized in chloroform with the GB/SA model. The calculations correctly predicted both the size of the hydrogen-bonded ring and its relative importance, in agreement with the experimental data in solution.
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    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020606
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  • 184
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    Microstructured peptide-functionalised surfaces by electrochemical polymerisationDedicated to Professor Ivar Ugi on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 667-672 
    Details
    ISSN: 0947-6539
    Keywords: binding assays ; immunosensors ; electrochemical polymerisations ; peptide derivatives ; peptide immobilisation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For the first time, antigenic peptides have been immobilised by electrochemical polymerisation after having been modified with a polymerisable functional group. 3-Hydroxyphenylacetic acid was chosen as the novel polymerisable group. The synthetic peptides represent epitopes of the bovine foot and mouth disease virus and of the sodium channel of the cardiac muscle. The polymerisation was performed by applying a constant anodic potential or by cyclic voltammetry. A combination of these two methods was also employed, that is, cyclic voltammetry with a delay at the anodic vertex potential. No additional free phenolic monomer was required for the polymerisation. The layers formed by the polymerisation were recognised by specific antibodies. The specific binding of the antibodies to the polymer film could be demonstrated by ELISA, an enzyme-linked amperometric immunoassay, and electrochemical impedance measurements, as well as by fluorescence-labelled antibodies. A peptide derived from laminine was also immobilised by electrochemical polymerisation. It could be shown that neuroblastoma cells adhere to this layer.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020608
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    N-heterocyclic carbenesCarbenes may be defined as “divalent” carbon compounds. More precisely, the carbene carbon is linked to two adjacent groups by covalent bonds and possesses two nonbonding electrons, which may have antiparallel spins (singlet state) or parallel spins (triplet state). In other compounds with formally “divalent” carbons (e.g., carbon monoxide, isocyanides), resonance stabilization of the “divalent” carbon by neighbouring groups is extremely strong. The term carbene, conceived by Doering, Winstein and Woodward, seems well suited to describe the class of compounds under discussion, but the carbene nomenclature conflicts with established rules of the International Union of Pure and Applied Chemistry (IUPAC). The nomenclature for metal-carbene complexes was introduced by E. O. Fischer [2a-d] and became common in the organometallic literature.: Generation under Mild Conditions and Formation of Group 8-10 Transition Metal Complexes Relevant to CatalysisHeterocyclic Carbenes, Part 5. Part 4: see ref. [20].Dedicated to Professor Max Herberhold on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 772-780 
    Details
    ISSN: 0947-6539
    Keywords: carbenes ; Group 8-10 complexes ; heterocycles ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stable mono- and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloroand acetato-bridged dinuclear metal complexes with the free carbenes (e.g., 1,3-dimethylimidazoline-2-ylidene). A general novel method for the deprotonation of N-substituted azolium salts in liquid ammonia-the liquid ammonia route-is described. This method yields not only the known free monimeric 1,3-dimethylimidazoline-2-ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example, imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted and even chiral hydrocarbon residues can be obtained. The nucleophilic behaviour of 1,3-dimethyl-imidazoline-2-ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of RuII, RhI PdII, OsII and IrI are presented. Reaction of the potentially chelating ligand 1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazoline-2,2′-diylidene with [(COD)-RhCl]2 yields a dinuclear complex, in which two rhodium centres are linked by the dicarbene bridge. Four single-crystal X-ray diffraction structures of new metalcarbene complexes (Rh, Os) are reported. N-Heterocyclic carbene complexes of Group 8-10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal-carbon double bonds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewis-basic imidazoline-2-ylidene ligand and the Lewis-acidic organometallic fragment.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020706
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  • 186
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    Dynamic behavior in solution of the trans-hydridodihydrogen complex [OsHCl(n2-H2)(CO)(PiPr3)2]: Ab Initio and NMR studies (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 815-825 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; dihydrogen complexes ; isotope effects ; NMR spectroscopy ; osmium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of complex [OsHCl-(CO)(PiPr3)2] (1) with hydrogen gives the trans-hydridodihydrogen complex [OsH-Cl(n2-H2)(CO)(PiPr3)2] (2). The H-H distance in the dihydrogen ligand, determined by variable-temperature 1H T1 measurements, is 0.8 Å. The fast-spinning nature of the dihydrogen ligand of 2 has been deduced by evaluating the deuterium quadrupole coupling constant for the n2-D2 ligand of [OsDCl(n2-D2)(CO)(PiPr3)2] ([D3]2). Measurements of the equilibrium constants for formation of 2 (K) give ΔH0 = - 14.1 ± 0.5 kcal mol-1 and ΔS0 = - 30 ±1 e.u. An equilibrium isotope effect Kd/K of 2.8 is found for this reaction. The activation parameters for the H2 loss from 2 are ΔH
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020712
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  • 187
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    Conformation and self-association of a hybrid peptide of cecropin a and melittin with improved antibiotic activity (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 838-846 
    Details
    ISSN: 0947-6539
    Keywords: aggregation ; antibiotics ; circular dichroism ; helices ; peptides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A 15-residue hybrid peptide containing residues 1-7 from cecropin A and residues 2-9 from melittin, CA-(1-7)M(2-9), is a potent antibiotic with broader activity than cecropin A, but without the cytotoxic character of melittin. The conformational behaviour of CA(1-7)M(2-9) including the formation of multimeric species in solution has been investigated by circular dichroism, ultracentrifugation, electrospray mass spectrometry, NMR and energy calculations. Addition of hexafluoroisopropanol or liposomes causes the appearance of a CD spectrum characteristic of a helical structure that changes with pH, buffer and peptide concentration. The concentration dependence is atypical, as the ellipticity at 222 nm decreases with peptide concentration and is not correlated with a correponding decrease in helix content as measured from the NMR spectra. The presence of aggregated structures is demonstrated by ultracentrifugation and ES-MS experiments, which also provide an indication of the stoichiometry. Longrange NOEs suggest a model of aggregation with neighbouring molecules packed antiparallel. Aggregation causes very slow proton-deuterium exchange in some amide protons in the C-terminal region and provides a method for estimating a very large association constant (ca. 106M-1) as well as the stoichiometry of the aggregates. The tendency to aggregate seems to be an inherited feature from melittin and may enhance the antibiotic activity either by faciliting the incorporation of the peptide into the membrane in large quantities or by promoting the disruption of the membrane.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020715
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 752-755 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020720
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  • 189
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    Vanadium(II)- and Niobium(III)-Induced, Diastereoselective Pinacol Coupling of Peptide Aldehydes to Give a C2-Symmetrical HIV Protease InhibitorDedicated to Professor Richard Neidlein on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 307-315 
    Details
    ISSN: 0947-6539
    Keywords: enzyme inhibitors ; niobium complexes ; peptide aldehydes ; pinacol coupling ; vanadium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Peptide aldehydes 15 a-c are prepared without epimerization from enantiomerically pure (S)-α-amino acids (Scheme 3). Reductive pinacol homocoupling of 15 a-c, induced by vanadium complex 11 or niobium complex 16 in refluxing THF, yields C2-symmetrical (S,R,R,S)-configurated 6a, 6b and 2, respectively, with moderate to high stereoselectivity (Scheme 4). In a novel protocol for the preparation and utilization of THF solutions of 11, the isolation of air-sensitive intermediates can be avoided and the potent HIV protease inhibitor 2 prepared in enantio- and diastereomerically pure form on a kilogram scale without chromatographic purification. The (S,R,R,S) selectivity of the pinacol homocouplings is confirmed by means of an independent, stereochemically unequivocal synthesis of 6 a and 2 from D-mannitol 4 (Scheme 1).
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19960020312
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  • 190
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    Unexpected Basicity of a Hexacoordinate Silicon Compound, {2,6-Bis[ (dimethylamino)methyl]phenyl}bis(1,2-benzenediolato)silicate (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 342-347 
    Details
    ISSN: 0947-6539
    Keywords: basicity ; hydrogen bonds ; organosilicon compounds ; zwitterions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The high basicity of {2,6-bis-[(dimethylamino) methyl]phenyl} bis (1,2-benzenediolato)silicate (1) was demonstrated by its quantitative protonation in methanol to give the zwitterion 2. It was found that 1 is much more basic (pKa in CH2Cl2 = 16.7) than the “proton sponge” 1,8-bis(dimethylamino)naphthalene (3) and also much more basic than {2,6-bis-[(dimethylamino) methyl]phenyl} bis (1,2-benzenediolato)phosphorane (4). The stability of the zwitterion 2, the geometry of which corresponds to a more perfect octahedron than 1, can explain the high basicity of 1. Dynamic NMR studies of 2 in solution show that at low temperature the hydrogen-bonding interaction with one oxygen atom observed in the solid state is maintained. On raising the temperature, firstly the hydrogen bond breaks, then dynamic coordination occurs, in which the NMe2 groups displace each other in conjunction with transprotonation.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020316
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020402
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  • 192
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    Self-Assembled Monolayers on Gold NanoparticlesPresented in part at the 78th Canadian Society for Chemistry Conference and Exhibition, May 28 - June 1, 1995, Guelph, Canada and at the 210th National Meeting of the American Chemical Society, Aug. 20 - 25, 1995, Chicago, Ill. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 359-363 
    Details
    ISSN: 0947-6539
    Keywords: alkylthiols ; gold ; nanoparticles ; phase transitions ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Self-assembled monolayers (SAMs) of n-alkanethiolates on gold, silver, and copper have been intensively studied both as model organic surfaces and as modulators of metal surface properties. Sensitivity restrictions imposed by monolayer coverage and the low surface area of planar metal substrates, however, limit the characterization of these films in molecular terms to surface enhancement techniques. As a result, key aspects such as film dynamics and alkyl chain ordering remain ill-defined. The characterization of the thermal behaviour of SAMs is important not only for the design of stable, well-ordered organic superlattices, but also for the fundamental understanding of the factors that drive molecular interactions in two dimensions. Phase properties in SAMs have been addressed here through the synthesis of gold nanoparticles of 20-30 Å in diameter and fully covered with alkylthiol chains. These thiolmodified gold nanoparticles with large surface areas have enabled the monolayer film structure to be uniquely characterized by transmission FT-IR spectroscopy, NMR spectroscopy, and differential scanning calorimetry. Our studies reveal that for long-chain thiols (≥ C16), the alkyl chains exist predominantly in an extended, all-trans ordered conformation at 25°C. Furthermore, calorimetry, variable temperature transmission FT-IR spectroscopy, and solid-state 13C NMR studies have established that a cooperative chain melting process occurs in these alkylated metal colloids. How this arises is not immediately evident, given the relation between the extended chain conformation and the geometry of the spherical nanoparticles. Transmission electron microscopy (TEM) reveals that adjacent gold particles are separated by approximately one chain length; this suggests that chain ordering arises from an interdigitation of chains on neighboring particles. The thermotropic behavior is sensitive to the alkyl chain length and chain packing density. The alkylated nanoparticles can thus serve as a highly dispersed analogue to the much-studied planar SAMs.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020318
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  • 193
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    A Modified Cyclodextrin with a Fully Encapsulated Dansyl Group: Self-Inclusion in the Solid State and in Solution (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 373-381 
    Details
    ISSN: 0947-6539
    Keywords: crystal structure ; cyclodextrins ; dansyl derivatives ; fluorescent sensors ; self-inclusion ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A monofunctionalized β-cyclodextrin containing a dansyl moiety, 6- deoxy- 6 - N - ( N′- (5- dimethylamino - 1 - naphthalenesulfonyl)diaminoethane) - β-cyclodextrin (CD-en-DNS, 2), was synthesized and its crystal structure determined. It was shown that the dansyl group is fully encapsulated within the cyclodextrin cavity, with the dimethylamino and sulfonyl groups emerging from opposite sides. The shape of the cavity is considerably flattened, since O(4)-O(4) distances parallel to the naphtalene ring were found to be longer than the others. The conformation of the diaminoethane linker was found to be determined by the inclusion of the dansyl group and by a hydrogen bond between the sulfonamide NH and one of the O(6)-H groups on the cyclodextrin rim. The self-inclusion features of the aromatic moiety were found to be consistent with the solution data: 1H NMR ROESY spectra suggested that the orientation of the dansyl moiety observed in the solid state was retained in aqueous solution; the circular dichroism spectrum was consistent with an axial complexation model. Fluorescence spectra showed that the inclusion of the dansyl group in the cyclodextrin cavity considerably increases the quantum yield: time-resolved fluorescence experiments showed the presence of a long-lifetime component (16.1 ns), which was attributed to the included fluorophore. The ability of 2 to act as a fluorescence sensor was evaluated by the addition of several guests of different shape: fluorescence intensity was lowered, especially upon addition of adamantanecarboxylic acid. All the data obtained were consistent with the model of the in-out movement of the dansyl group from the self-included conformation observed in the solid state to a position more exposed to the bulk solvent. Copper(II) was shown to enhance the difference in the fluorescence of 2 in the presence of guests by additional static quenching.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020404
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  • 194
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    A New Experimental Method to Determine the Mutual Orientation of Helices in Coiled-Coil Proteins: Structural Information about the Dimeric Interface of cJun, cFos, GCN4, and gp41Part of this paper was presented at the 13th American Peptide Symposium, Edmonton, Alberta (Canada), June 20-25, 1993. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 403-411 
    Details
    ISSN: 0947-6539
    Keywords: helices ; proteins ; structure elucidation ; viral proteins ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Disruption of protein dimers interacting by a leucine zipper motif represents a new potential pharmaceutical target. However, structural information concerning the exact nature of the interacting helices is usually not available. Towards this end, we have developed a disulfide-trapping approach capable of distinguishing between the ad and gd modes of dimerization (Fig. 1), thus providing information useful in the design of small molecules that interfere with helix-helix interactions. We designed and synthesized nine cysteine-substituted peptide fragments: GCN 4(g), GCN 4(a), GCN 4(d), cFos(g), cFos(a), cFos(d), cJun(g), cJun(a), and cJun(d), and evaluated the covalent crosslinking rates for them and their binary mixtures. Neither homogeneous cJun nor cFos dimerized and crosslinked, but their binary mixtures did with t1/2 of formation a 〉 d 〉 g, indicating to cFos-cJun heterodimerization according to ad mode (Fig. 1 a). Similarly, GCN 4 dimerized and crosslinked in the ad fashion; this result was in excellent agreement with the published X-ray structure. Next, we investigated the mode of gp 41 dimerization, which appears critical for HIV-1 replication The gp41 cysteinesubstituted fragments gp 41(g), gp 41(a), and gp 41(d) also dimerized and crosslinked, but with a different order of t1/2 of formation g 〉 d 〉 a, thus providing evidence that gp41 dimerizes in the gd mode (Fig. 1 b). Thus, the crosslinking experiments allow rapid elucidation of structural details of macromolecular interactions in aqueous media. These findings should prove useful in the design of compounds that inhibit macromolecular association.
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020409
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  • 195
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    1,3-Alternate Calix[4]arenecrown-5 Conformers: New Synthetic Ionophores with Better K+/Na+ Selectivity than Valinomycin (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 436-445 
    Details
    ISSN: 0947-6539
    Keywords: binding studies ; calixarenes ; crown ethers ; ionophores ; membranes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New 25,27-dialkoxycalix-[4]arenecrown-5 conformers 8, 10, and 11 have been synthesized and studied. The compounds 8a and 8b, fixed in 1,3-alternate structure, have been obtained in 57 and 40% yield, respectively, by reaction of the corresponding 25,27-dialkoxycalix[4]arenes 7a-b with tetraethylene glycol di-p-toluenesulfonate in the presence of Cs2CO3. The cone 10a and 10b and the partial cone 11 conformers were obtained by selective demethylation of the 25,27-dimethoxycalix[4]arenecrown-5 (6a) and subsequent dialkylation with NaH/DMF and KOtBu/THF, respectively. In the solid state (X-ray), compound 6a adopts a flattened cone conformation, which is also found to be most abundant in CD3CN and CD3OD solution. Upon complexation with potassium picrate compound 6a was converted quantitatively into the 1,3-alternate conformation. All new ligands synthesized were used in the extraction of alkali metal cations from H2O into CHCl3, and as active components in supported liquid membranes and in chemically modified field effect transistors. Results were compared to those obtained with with the natural antibiotic valinomycin 1. All ligands showed high selectivity for potassium. Ligand 8a, fixed in the 1,3-alternate conformation, is more selective than valinomycin and shows the highest K+/Na+ selectivity known so far.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020413
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020502
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  • 197
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    Supramolecular Motifs: Concerted Multiple Phenyl Embraces between Ph4P+ Cations Are Attractive and Ubiquitours (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 481-486 
    Details
    ISSN: 0947-6539
    Keywords: cations ; crystal structures ; phenyl rings ; supramolecular chemistry ; tetraphenylphosphonium ions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Examination of the Cambridge Structural Database reveals that Ph4P+ cations in crystals associate through phenyl-phenyl nonbonded interactions which are attractive, concerted, and widespread. Intermolecular phenyl-phenyl conformations, which are offset-face-to-face (off), edge-to-face (ef) or vertex-to face (vf), combine in five classes of supramolecular motifs for {Ph4P+}2 pairs, namely the sextuple phenyl embrace (SPE) with (ef)6 and offset sextuple phenyl embrace (OSPE) containing (off)1 (ef)2(ef/vf)2, the translational quadruple phenyl embrace (TQPE) with (ef)4, the parallel quadruple phenyl embrace (PQPE) with (off)1(vf)2, and the double phenyl embrace (DPE) with (off)1. Typical intermolecular attractive energies (kJ per mol of {Ph4P+}2) for these motifs are SPE 85, OSPE 57, TQPE 70, PQPE 41, DPE 34. There is strong interpenetration of the cations in these motifs: 489/770 structures in the CSD have P⃛P≤7 Å (spherical Ph4P+ has a van der Waals diameter of 13.6 Å). Of the 812 instances of P⃛P ≤7 Å, 86% are SPE, 10% are OSPE, 2% are TQPE, and only 2% are unclassified, Average P⃛P separations in the PQPE and DPE are 8.3 Å. Centrosymmetry is prevalent in all except the TQPE, which has implications for the engineering of noncentric crystals.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020505
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    Reactions of the double-stranded oligonucleotide d(TTGGCCAA)2 with cis-[Pt(NH3)2(H2O)2]2+ and [Pt(NH3)3(H2O)]2+Kinetic Analysis of the Reactions Between GG-Containing Oligonucleotides and Platinum Complexes, Part II. Part I: Reactions of Single-Stranded Oligonucleotides with cis-[Pt(NH3)2(H2O)2]2+ and [Pt(NH3)3(H2O)]2+, ref.[15]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1068-1076 
    Details
    ISSN: 0947-6539
    Keywords: antitumor agents ; DNA ; kinetics ; platinum complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetics of the reactions between the GG-containing double-stranded oligonucleotide d(TTGGCCAA)2 (II) and the platinum complexes cis-[Pt-(NH3)2(H2O)2]2+ (1) and [Pt(NH3)3-(H2O)]2+ (2) were studied and compared with those already determined for the reactions of the single-stranded octanucleotide d(CTGGCTCA) (I).[1] The results were as follows: i) Complex 1 reacted faster than 2 with both I and II. ii) Both complexes 1 and 2 reacted faster with II than with I. This acceleration was greater for 1 (x 13) than for 2 (x4) and only due to the increase of the platination rate of the 5′-G of the GG sequence. iii) For both I and II, the first platination by 1 and 2 was faster on the 5′-G than on the 3′-G. This difference was more significant for the platination of II (k5′/k3′ = 12 for 1 and 5 for 2) than of I (k5′/k3′ ≤ 2). iv) The cyclization reaction of the monoadducts (G*) of 1 to yield the GG cis-Pt(NH3)2+2 chelate (G*G*) was considerably slowed down in the duplex. This rate decrease was significantly larger for the chelation of the 5′-G* (factor of 16) than of the 3′-G* (factor of 4) monoadducts. v) The intrastrand chelation of the 3′-G* monoaducts (k3′c) was faster than that of the 5′-G* monoadducts (k5′c), both for I and II (k3′c/k5′c = 3 and 13, respectively). vi) In addition to the intrastrand G*G* crosslink, we also observed the interstrand crosslink d(GG*CC)-d(GG*CC) between the two 3′-Gs of the central tetranucleotide. The rate constant for the interstrand crosslinking (k3′i) was half that of the intrastrand chelation (k3′c). vii) The 5′ monoadduct, which was formed faster (k5′ 〈 k3′) and was chelated more slowly (k5′c 〉 k3′i 〈 k3′c), exhibited a half-life of 3.2 h under our experimental conditions.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020904
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    9,10-Dimetallatriptycenes of group 14: A novel synthetic approach from ortho-Phenylenemagnesium (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1139-1142 
    Details
    ISSN: 0947-6539
    Keywords: Grignard reagents ; Group 14 compunds ; metallatriptycenes ; phenylenemagnesium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 9,10-Dimetallatriptycenes of Group 14 are readily accessable by a novel synthetic approach: reaction of ortho-phenylenemagnesium (1) with RMCl3 afforded 9,10-dimethyl-9,10-dimetallatriptycenes (3; M = Si, Ge, Sn) or 9, 10-dimethyl-9-germa-10-stannatriptycene (11). The reaction can even be tuned to afford unsymmetrically substituted triptycenes such as 9-methyl-10-phenyl-9,10-digermatriptycene (8); in some of these reactions, strong indications were obtained for the intermediacy of tri-Grignard reagents RM(2-C6H4MgCl)3 (5, 10). The new triptycenes were characterised by their spectral data and in the case of 3c (M = Sn) by an X-ray crystal structure determination.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020914
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    Multi-mode switching based on dihydroazulene/vinylheptafulvene photochromism: Synergism of photochromism and redox switching in heteroaryl-functionalized systemsFor the preceeding publication. see ref. [1]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1150-1158 
    Details
    ISSN: 0947-6539
    Keywords: electrochromes ; cyclic voltammetry ; heterocycles ; optical memory ; photochromes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of the dihydroazulenes (DHAs) DHA-a-DHA-f containing covalently linked heteroaromatic subunits derived from dibenzodioxin, thianthrene, phenoxathiine, N-methylphenothiazine, N-methylphenoxazine, and N,N′-dimethylphenazine groups are described, and their spectroscopic and analytical data are reported. The dihydroazulene/vinylheptafulvene (DHA/VHF) photochromism (photochemical forward and thermal back reaction) depends with high sensitivity on the electronic properties of the functional groups. Whereas the dimethylphenazine derivative DHA-f is photochemically inactive towards rearrangement, all other DHAs (DHA-a-DHA-e) were found to isomerize to the corresponding vinylheptafulvene forms under irradiation. Cyclic voltammetry revealed that the DHA and the VHP forms have significantly different oxidation and reduction waves. The products of the-oxidative one-electron transfer are characterized by UV/Vis/NIR spectroelectrochemistry. Those DHAs having weaker donor substituents (DHA-a-DHA-c) undergo oxidative dimerization whereas DHAs with stronger donating heterocyclic subunits (DHA-d-DHA-f) form stable radical cations.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020916
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