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  • 1995-1999  (41,340)
  • 1980-1984  (36,763)
  • Chemistry  (74,105)
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  • 101
    Electronic Resource
    Electronic Resource
    A New Cytotoxic Diterpene with the Dolabellane Skeleton from the Marine Sponge Sigmosceptrella quadrilobata (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 227-230 
    Details
    ISSN: 1434-193X
    Keywords: Diterpene ; Dolabellane ; Sigmosceptrella quadrilobata ; Sponge ; Antitumor agents ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -A new diterpene with the dolabellane skeleton 1 has been isolated from the sponge of the Indian Ocean, Sigmosceptrellaquadrilobata. Its structure elucidation, including absolute stereochemistry, was performed with the aid of MS, NMR, and CD data and by molecular modeling. Compound 1 is moderately cytotoxic against four tumor cell lines.
    Additional Material: 2 Ill.
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  • 102
    Electronic Resource
    Electronic Resource
    Ectyoplasides A-B - Unique Triterpene Oligoglycosides from the Caribbean Sponge Ectyoplasia ferox (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 231-238 
    Details
    ISSN: 1434-193X
    Keywords: Saponins ; Terpenoids ; Sponges ; Ectyoplasiaferox ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Two novel norlanostane triglycosides, ectyoplasides A (1) and B (2), have been isolated from the methanolic extracts of the Caribbean sponge Ectyoplasiaferox (Raspaliidae, Axinellida). Their structures have been established by extensive application of high-resolution FABMS and two-dimensional NMR techniques, supported by chemical and enzymatic degradation methods. Ectyoplasides A and B have been shown to possess identical sugar chains (composed of two β-galactose units and one α-arabinose unit), but to differ in their aglycone moieties, which possess nortriterpene skeletons with an unprecedented substitution pattern on ring A. Ectyoplasides A and B show moderate cytotoxic activity when tested in vitro towards various cell lines
    Additional Material: 1 Ill.
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  • 103
    Electronic Resource
    Electronic Resource
    Reaction of Stabilised Phosphorus Ylides with Nitrogen Dioxide (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 251-254 
    Details
    ISSN: 1434-193X
    Keywords: Phosphonium ylides ; Nitrogen dioxide ; Oxidation ; Nitriles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The reaction of the stabilised ylides 14-23 with an excess of NO2 in CH2Cl2 at room temperature gives different results depending on the structure of the starting ylide. The monacyl ylides 14-16 give the corresponding α-oxo nitriles 4 together with Ph3PO · HNO3 (24) which has been fully characterised for the first time. Under the same conditions, the ylide 18 gives 2,4-dinitrobenzonitrile (26), Ph3PO, and benzoic acid. The other ylides examined all give 24 together with a variety of other products.
    Type of Medium: Electronic Resource
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  • 104
    Electronic Resource
    Electronic Resource
    Reactions of Diphenyl Ether with Chlorine and Bromine Atoms Around 750 K - Relevance for Gas-Phase “Dioxin” Formation (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 261-265 
    Details
    ISSN: 1434-193X
    Keywords: Dioxins ; Gas-phase reactions ; Diphenyl ether ; Hydrogen abstraction ; ipso Substitution ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The title reactions have been studied to scrutinize rate data recently inferred for the two reverse steps - reaction of phenoxy radicals with chlorobenzene and bromobenzene - which were at variance with commonly accepted model values. Both with chlorine and bromine atoms, splitting to halobenzene and phenoxy radical was found to occur in competition with abstraction of o-, m-, p-hydrogen atoms. On this basis, the displacements of Cl and Br from the benzene ring by phenoxy radicals must have activation energies above 20 kcal/mol, and are therefore slow. As a consequence, formation of “dioxins” from halogenated phenols, in (slow) combustion, should proceed by combination of two (halo)phenoxy radicals rather than by displacement of (ortho-)halogen in a halophenol molecule.
    Additional Material: 2 Ill.
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  • 105
    Electronic Resource
    Electronic Resource
    Deprotonation of Amides and Polyfunctional Imides Probed by Heteronuclear NMR and Quantum Chemical Calculations (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 287-295 
    Details
    ISSN: 1434-193X
    Keywords: Ab initio calculations ; Amides ; Imides ; Acidity ; NMR spectroscopy ; Deprotonation site ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The site of deprotonation of several types of amide acids (carboxylic amides and imides, sulfonamides, cyanamide, N-hydroxyurea) has been investigated by quantum chemical calculations and heteronuclear NMR measurements. Relative energies of tautomeric ions deriving from protonation at the various sites were determined both in the gas phase and in water (by the IPCM continuum solvation method). NMR properties (nuclear shielding and electric field gradient) of the involved heteronuclei were calculated and compared with experimental 14N, 17O and 33S chemical shifts and relaxation rates. It is shown that the combination of theoretical and experimental tools allows a reliable prediction of spectral parameters and ultimately of the deprotonation site of polyfunctional acids.Supporting information for this article is available on the WWW under -http://www.wileY-Vch.de/contents/jc_2046/1999/98401_s.pdf or from the author.
    Additional Material: 3 Tab.
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  • 106
    Electronic Resource
    Electronic Resource
    Silicon-Controlled Intramolecular Thermal H-Ene Reaction - A New Access to a Key Intermediate of (±)-cis-γ-Irone (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1825-1829 
    Details
    ISSN: 1434-193X
    Keywords: Ene reaction ; Allylsilane ; Annulation ; cis-γ-Irone ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The primordial influence of the silyl group in an intramolecular thermal H-ene reaction is reported. The silyl group lowered the energy of the transition state and allowed the reaction to proceed at a lower temperature than that of the simple all-carbon analog, thereby excluding the formation of undesirable by-products. This sequence has been applied for new access to a key intermediate in the diastereoselective synthesis of (±)-cis-γ-irone with high chemocontrol and significantly improved overall yield.
    Additional Material: 1 Ill.
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  • 107
    Electronic Resource
    Electronic Resource
    On the Unexpected Stereochemical Outcome of the Magnesium in Methanol - Conjugate Reduction of an Exocyclic α,β-Unsaturated Ester (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1803-1809 
    Details
    ISSN: 1434-193X
    Keywords: Magnesium-methanol reduction ; Reductions ; α,β-Unsaturated esters ; Stereoselectivity ; (R)-(-)-Carvone ; Analogs of 1α,25-(OH)2-vitamin D3 ; Steroids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conjugate reduction of the exocyclic α,β-unsaturated ester 9 with magnesium in methanol gives a mixture of diastereoisomers containing predominantly the less stable 10a. Eventual structural identification rests on the X-ray diffraction analysis of tosylate 18.
    Additional Material: 3 Ill.
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  • 108
    Electronic Resource
    Electronic Resource
    Observation of Stereotopic Group Recognition in Chiral Borate Complexes in Solution (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1775-1786 
    Details
    ISSN: 1434-193X
    Keywords: Borate esters ; Molecular recognition ; NMR spectroscopy ; Enantioselectivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The borate ester of 1a and 2e exhibits complete quaternization of the boron center. An equilibrium is observed between the two diastereomeric complexes 3 and 4 formed by coordination to the boron center of either amino group of 2e. At -78°C, this equilibrium is slow on the NMR timescale and a diastereomeric ratio of about 3:1 is observed. At room temperature, a rapid equilibration is observed resulting in two NMR resonances (both 1H and 13C) for the dimethylamino groups of 2e in 3/4. Comparison with the NMR characteristics of the analogous borate complexes of 1h and 1j and of 2c and 2d shows that these resonances each correspond to both methyl groups of a single dimethylamino group. The chemical shift difference occurs solely from the difference between the time fractions that the dimethylamino groups are coordinated to the boron center, and forms, therefore, a direct measure of the ratio 3/4. This ratio amounts to 63:37 at room temperature, which was confirmed by a protonation titration experiment. The elucidation of these NMR characteristics of B(1a)(2e) allowed the more rapid evaluation of the enantiotopic group recognition in many other borate complexes, which range from 57:43 to 67:33 at room temperature. The recognition of the (after complexation) diastereotopic dimethylamino groups of 2e in the borate complex B(1a)(2e) was utilized in a consecutive methylation reaction, which yielded enantiomerically enriched ammonium salt 7. The enantiomeric excess proved to be identical to the diastereomeric excess observed in the preceding recognition event. Strong evidence is presented that complex 3 is indeed the major diastereomer present in a solution of 3/4, as expected from molecular models.
    Additional Material: 6 Ill.
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  • 109
    Electronic Resource
    Electronic Resource
    On the Unusual Dienophilicity of Trimethylsilylphosphaacetylene (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 363-371 
    Details
    ISSN: 1434-193X
    Keywords: Phosphaalkynes ; Diels-Alder reactions ; Ene reactions ; Homo [4 + 1] cycloaddition reactions ; Ab initio calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The diphosphatricyclooctenes 8a, b are obtained from reactions of the phosphaacetylene 2 with the buta-1,3-dienes 6a, b through a reaction sequence involving Diels-Alder, ene, and intramolecular [4 + 2] cycloaddition reactions with the intermediates 7 and 9. An ab initio MO investigation at the MP2/6-31+G* level of theory has provided reasons why the postulated intermediates cannot be detected. The polycyclic products 8a, b were characterized in the form of the corresponding metal carbonyl complexes 10a, b, and 11. Reaction of the silylated phosphaalkyne 2 with cyclohexa-1,4-diene (12) furnishes the monophosphatricyclooctene 14; this reaction has also been studied by means of ab initio calculations. The result of the reaction of 2 with cyclopentadiene (15), however, is a complete surprise: the triphosphaspiropolycyclic system 17 is formed, apparently through Diels-Alder, homo-Diels-Alder, and homo [4 + 1] cycloaddition reactions with the intermediates 16 and 18.
    Additional Material: 4 Ill.
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  • 110
    Electronic Resource
    Electronic Resource
    Access to 4,5-Dinitro-1,3,5,7-octatetraene Systems (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 431-435 
    Details
    ISSN: 1434-193X
    Keywords: 3,4-Dinitrothiophene ring-opening ; Dinitrobutadienes ; Dinitrooctatetraenes ; Cycloisomerization ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The reactions between 1,4-bis(diethylamino)-2,3-dinitro-1,3-butadiene (2), easily obtainable by ring-opening of 3,4-dinitrothiophene with diethylamine, and 2-substituted vinylmagnesium bromides gave good yields of the 4,5-dinitro-1,3,5,7-octatetraene derivatives 3a-e. With 1-phenylmagnesium bromide, conversely, only 3,4-dinitro-1,4-diphenyl-1,3,5-cyclooctatriene (4f) was obtained. From the unsubstituted dinitrooctatetraene 3a both the octatriene cycloisomer 4a and the valence tautomer of the latter (6a) could be isolated and characterized. The results obtained evidence an unfavourable effect of the two nitro groups on the cycloisomerization process of the octatetraene system.
    Additional Material: 1 Tab.
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  • 111
    Electronic Resource
    Electronic Resource
    Building Blocks for the Construction of Large Chloro-Functionalized, Hexagonal Oligophenylene Cycles (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 451-458 
    Details
    ISSN: 1434-193X
    Keywords: Cross-coupling ; Modular chemistry ; Repetitive synthesis ; Macrocycles ; Shape persistence ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The synthesis of a variety of kinked oligophenylene building blocks is described. The synthetic strategy is repetitive, utilizes the Suzuki cross-coupling protocol, and involves potentially bifunctional building blocks (modules) which carry one functional group for coupling and one group which serves as place holder. After the coupling, the placeholder is converted into a coupling functionality for the next growth step. The building blocks also carry chloro-substituents which are unaffected by all transformations. For one of the large building blocks it is shown that it actually can be cyclized into an oligophenylene macrocycle with a chloro-substituent at each corner and a pair of hexyl chains at each side.
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  • 112
    Electronic Resource
    Electronic Resource
    Synthesis of C-Oligosaccharides That Mimic Their Natural O-Analogous Immunodeterminants in Binding to Monoclonal Immunoglobulins (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 471-476 
    Details
    ISSN: 1434-193X
    Keywords: C-Disaccharide ; C-Glycosides ; Carbohydrates ; Glycosylation ; Sugar lactone ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -We have stereoselectively synthesized the analogues of the methyl β-glycosides of (1→6)-β-D-galacto-oligosaccharides (up to tetrasaccharide), in which the interglycosidic oxygen atoms are replaced by a methylene group.
    Additional Material: 1 Ill.
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  • 113
    Electronic Resource
    Electronic Resource
    Preparation of 1-Hydroxycyclopentanecarboxylic Acid Derivatives from a Chiral Equivalent of Glycolic Acid (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 477-483 
    Details
    ISSN: 1434-193X
    Keywords: Radical reactions ; Glycolic acid ; Asymmetric synthesis ; Annulation ; Selenium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The stereoselective preparation of 1-hydroxycyclopentanecarboxylic acid derivatives is reported. The key reaction is either a radical cyclization or a radical annulation mediated by the transfer of a phenylseleno group. The radical precursors are easily synthesized by two consecutive enolate alkylations. Excellent stereochemical control for the quaternary C(1) stereogenic center has been achieved.
    Additional Material: 2 Ill.
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  • 114
    Electronic Resource
    Electronic Resource
    Unprecedented Formation of a Benzo[d]azepine by Acid-Catalyzed Cyclization of a Camphor-Derived N-Formylenamine (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 519-522 
    Details
    ISSN: 1434-193X
    Keywords: Enamides ; Diformamides ; Cyclizations ; Nitrogen heterocycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -A convenient synthesis of (-)-N-styryl-N-[2-(1,7,7-trimethylbicyclo[2.2.1]hept-2-enyl)ethyl]formamide (2) was developed starting from natural (+)-camphor (3) via (-)-2-(bornen-2-yl)ethanol (6). Cyclization of the N-formylenamine 2 with the aid of 9-borabicyclo[3.3.1]non-9-yl triflate yielded a methanobridged octahydro-1H-benzo[d]azepine derivative.
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  • 115
    Electronic Resource
    Electronic Resource
    Synthesis of Chiral 4-(ω-Hydroxyalkyl)pyrazolidin-3-ones by Ring-Chain Transformation of α-Alkylidenelactones with Hydrazines (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 525-525 
    Details
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The configuration and/or formulae of the following compounds need to be corrected.(1′R,E)-3-(2-Benzyloxy-3-hydroxypropylidene)dihydrofuran-2-one(7c): Correct formula:(4S,5R,4′S)-5-(2,2-Dimethyl[1,3]dioxolan-4-yl)-4-(2-hydroxyethyl)pyrazolidin-3-one (11a): Original formula correct(4R,5S,1′R)-5-(1-Benzyloxy-2-hydroxymethyl)-4-(2-hydroxyethyl)pyrazolidin-3-one (11g): Correct formula:
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  • 116
    Electronic Resource
    Electronic Resource
    Chemical Modification of the Structures and Functions of Proteins by the Cofactor Reconstitution Method (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 539-549 
    Details
    ISSN: 1434-193X
    Keywords: Protein engineering ; Cofactor reconstitution ; Non-natural molecules ; Hemoproteins ; Flavoenzymes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -A new strategy for the semi-synthesis of proteins and enzymes bearing non-natural functional groups is described. In line with our proposal that the replacement of native cofactors by modified cofactors may represent a useful and general methodology for introducing non-natural molecules into cofactor-dependent proteins and enzymes, we have shown that semi-synthetic cofactors bearing non-natural functional units can be incorporated in the proximity of the active sites of enzymes by cofactor reconstitution. A variety of non-natural building units such as a hydrophobic alkyl chain, a polyanion cluster, metal complexes, electron donors or acceptors, a photochromic moiety, peptides, and artificial receptors have successfully been introduced into hemoproteins (myoglobin, hemoglobin, and cytochrome b562) and their effects on the holoproteins have been examined. Incorporation of a long alkyl chain, for example, greatly facilitates conversion of a water-soluble protein into a membrane-bound protein. Attachment of a photosensitizer has enabled us to switch the enzyme activity by means of visible light. Artificial receptors introduced in the proximity of the active site can modulate the structures and activities of native proteins in response to specific guest molecules. This concept has been expanded to include flavoenzymes such as glucose oxidase. An immense variety of non-natural molecules are available that are potentially useful for protein engineering. By combination of the present cofactor reconstitution method with other techniques such as site-directed mutagenesis, chemical modification, peptide semi-synthesis, and non-natural amino acid incorporation using suppressor t-RNA, it should be possible to artificially manipulate native protein molecules with a similar degree of flexibility as that with which organic chemists manipulate small molecules.
    Additional Material: 10 Ill.
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  • 117
    Electronic Resource
    Electronic Resource
    A COCON Analysis of Proton-Poor Heterocycles - Application ofCarbon Chemical Shift Predictions for the Evaluation of Structural Proposals (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 579-586 
    Details
    ISSN: 1434-193X
    Keywords: Structure elucidation ; Computational method ; Natural products ; HMBC ; Constitutional analysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The application of the new computer program COCON (Constitutions from Connectivities) to the 2D-NMR data sets of three different complex natural products is described. The investigated compounds are proton-poor and therefore underdetermined systems. For such molecules the number of possible constitutions and the computational speed of COCON are of interest. Our investigation is focused on how methods of 13C-NMR chemical shift prediction can assist chemists with regard to refining the selection among the constitutions proposed by COCON.
    Additional Material: 6 Ill.
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  • 118
    Electronic Resource
    Electronic Resource
    The Reduction of α-X-Acetophenones (X = PhO, Br, Cl) in Hydrogen-Donating Solvents at Elevated Temperatures (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 607-616 
    Details
    ISSN: 1434-193X
    Keywords: Phenacyl halides ; Hydrogen donor ; Bond dissociation enthalpies ; Lignin ; DFT ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The reduction of α-X-acetophenones (X = PhO, Br, Cl), as model compounds for lignin liquefaction studies, has been investigated in the presence of a hydrogen-donating solvent such as 9,10-dihydroanthracene (AnH2) or 2-propanol, between 373 and 573 K. With α-phenoxyacetophenone (PAP) in AnH2, acetophenone and phenol have been obtained with high selectivities. The mechanism involves the reverse radical disproportionation (RRD) with AnH2. Hydrodebromination of α-bromoacetophenone (BrAP) is quantitative at 423 K using AnH2 as a reducing agent. Now, the hydrogen transfer proceeds by an uninhibited radical chain mechanism with anthracenyl radicals as the chain carriers. For the kinetic analysis, the C-X (X = Br, Cl) bond dissociation enthalpies (BDEs) have been determined by means of very low pressure pyrolysis to give BDE(C-Br) = 271 kJ mol-1 and, as a lower limit, BDE(C-Cl) ≈ 309 kJ mol-1, at 298 K. The BDEs are quite at variance with recently published insights derived from an electrochemical study. For comparison, density functional theory calculations (DFT) have been performed.
    Additional Material: 1 Ill.
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  • 119
    Electronic Resource
    Electronic Resource
    Influence of Transition-Metal Ions on the Aggregation Behavior of a Single-Chained Ammonium Amphiphile Carrying an o-Hydroxyazobenzene Unit (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 627-630 
    Details
    ISSN: 1434-193X
    Keywords: Surfactants ; H aggregates ; Ion pairs ; Copper(II) binding ; Copper ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -A novel, single-chained ammonium amphiphile that carries an o-hydroxyazobenzene unit at the terminus of its hydrophobic chain has been synthesized. This compound forms ordered thread-like aggregates upon dispersion in water. These aggregates exhibit a phase transition at 56 °C. The o-hydroxyazobenzene unit binds several transition-metal ions in a 2:1 stoichiometry. Binding of these metal ions results in a lowering of the critical aggregation concentration. From the changes in the UV absorption spectrum it is concluded that the Cu2+ complex forms more tightly packed aggregates in water than the Zn2+ and Ni2+ complexes.
    Additional Material: 4 Ill.
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  • 120
    Electronic Resource
    Electronic Resource
    Synthesis of Ether- and Imino-Linked Octyl N-Acetyl-5a′-carba-β-lactosaminides and -isolactosaminides: Acceptor Substrates for α-(1→3/4)-Fucosyltransferase, and Enzymatic Synthesis of 5a′-Carbatrisaccharides (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 631-642 
    Details
    ISSN: 1434-193X
    Keywords: Carbohydrates ; Cyclitols ; Carbohydrate mimetics ; Disaccharides, 5a′-carba- ; N-Acetyl-5a′-carbalactosaminides ; Fucosyltransferase ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Synthesis of ether-linked octyl 5a′-carba-β-lactosaminide 3 and -isolactosaminide 5 was carried out in seven steps, starting from the coupling products of 1,2-anhydro-5a-carba-β-D-mannopyranose derivative 7, and the oxide anions generated from the octyl N-acetyl-β-D-glucosaminide derivatives 13 and 16, respectively, under basic conditions. The 5a-carba-α-D-mannopyranose residues of the coupling products 17 and 26 were transformed into the β-D-gluco configuration through epimerization of the respective 2′-oxo derivatives 19 and 28, and selective reduction, and then into the β-D-galacto configuration by direct nucleophilic substitution of their 4′,6′-dimesylates 23 and 31 with an acetate ion. Biological assay has shown that 3 is an acceptor substrate for human-milk α-(1→3/4)-fucosyltransferase and, interestingly, 5 is not. In addition, the imino-linked congeners 4 and 6 have been synthesized by coupling of the 4-amino-4-deoxy- and 3-amino-3-deoxy derivatives 37 and 41 of octyl N-acetyl-β-D-glucosaminide, and the carba-sugar epoxide 8, respectively, and subsequent deprotection. Compound 4 is a substrate while 6 is neither a substrate nor an inhibitor for fucosyltransferase. Small-scale enzymatic synthesis was carried out by treatment of 3 and 4 with GDP-fucose and milk fucosyltransferase, which resulted in conversion into the corresponding trisaccharides 47 and 48, respectively.
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  • 121
    Electronic Resource
    Electronic Resource
    Synthesis of rac-(E)-Opposita-4(15),7(11)-dien-12-al (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 675-678 
    Details
    ISSN: 1434-193X
    Keywords: rac-(E)-Opposita-4(15),7(11)-dien-12-al ; Natural products ; Vetiver oil ; Octahydroindene derivatives ; Fragrances ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -rac-(E)-Opposita-4(15),7(11)-dien-12-al (1) was synthesized in 12 steps via the aldehyde 11 as the key compound. The spectroscopic data of rac-1 are identical with those of the corresponding natural product, previously isolated from Vetiver oil. Synthetic rac-1 has a strong, green woody odor with soft-floral undertones, which confirms that 1 is an olfactorily important constituent of Vetiver oil.
    Additional Material: 1 Tab.
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    Erinnerungen an Karl Johann Freudenberg (1886-1983) (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 707-736 
    Details
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Erinnerungen an Karl Johann Freudenberg (1886-1983)
    Additional Material: 23 Ill.
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  • 123
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    4-Methylpyrimidinium Ylides, 1 (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 703-706 
    Details
    ISSN: 1434-193X
    Keywords: 4-Methylpyrimidine ; Ylides ; Quantum chemical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -For the first time in the pyrimidinium ylides series we present a theoretical and experimental study concerning the structure, stability and reactivity of pyrimidinium ylides and interesting correlations between these properties. 4-Methylpyrimidinium ylides are relatively stable compounds, their stability varying with the nature of the ylide carbanion substituent. The more electron-withdrawing these substituents, the more the anionic charge is delocalized and therefore the higher the stability. The data from experiments and quantum chemical calculations are in agreement with each other. The quantum chemical calculations show the possibility of using pyrimidinium ylides as nucleophilic reagents as well as 1,3-dipoles in reaction with appropriate reagents. Eight new pyrimidine compounds (4 salts and 4 ylides) were obtained.
    Additional Material: 3 Ill.
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  • 124
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    Isolation and Structure of a GM3-Type Ganglioside Molecular Species (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 765-769 
    Details
    ISSN: 1434-193X
    Keywords: Glycosphingolipids ; Ganglioside ; GM3 ; Starfish ; Luidia maculata ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -A ganglioside molecular species, LMG-2 (1), has been obtained from the water-soluble lipid fraction of the chloroform/methanol extract of the starfish Luidia maculata. On the basis of chemical and spectroscopic findings, the structure of 1 has been elucidated. 1 is a ganglioside molecular species of starfish resembling the mammalian ganglioside GM3. In addition, 1 shows neuritogenic activity toward the rat pheochromocytoma cell line PC-12 cell.
    Additional Material: 1 Tab.
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  • 125
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    Synthesis of 17α-(Iodovinyl)estradiol and Analogous Derivatives by Iododestannylation of Insoluble Polymer-Supported Organotin Precursors (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 781-783 
    Details
    ISSN: 1434-193X
    Keywords: Polymer-supported organotin hydride ; Steroids ; Iodinated estrogens ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Iodoestrogen derivatives were prepared by iododestannylation of insoluble polymer-supported organotin precursors.
    Additional Material: 2 Tab.
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  • 126
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    Phenylethynyl- and Phenylethenylmetacyclophanes with π,π Interactions (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 791-795 
    Details
    ISSN: 1434-193X
    Keywords: Cyclophanes ; Sulfur heterocycles ; Cyclisations ; Cesium effect ; ππ* transitions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Two series of [8]metacyclophanes with cis or trans configurated double bonds or triple bonds in the bridges were prepared; the first series 6, 8, and 10 contains phenylethynyl, the second series 13, 14, and 15 phenylethenyl substituents so that ππ interactions between the π centres in the bridge and the π centres of the basic chromophores tolan and trans-stilbene, respectively, can be studied.
    Additional Material: 1 Ill.
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  • 127
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    Thermal Isomerisation of Substituted Semibullvalenes and Cyclooctatetraenes - A Kinetic Study (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 813-822 
    Details
    ISSN: 1434-193X
    Keywords: Cyclizations ; Hydrocarbons ; Isomerizations ; Kinetics ; Polycycles ; Rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Whereas 1,5-dimethylsemibullvalene (1b) equilibrates with 1,5-dimethylcyclooctatetraene (2b), the semibullvalene-2,6-dicarbonitriles 13 rearrange irreversibly to afford mixtures of the isomeric cyclooctatetraene-1,5-dicarbonitriles 14 and 15. Arrhenius and Eyring parameters of these thermal isomerisation reactions have been determined for the gas phase (1b → 2b) and [D6]benzene solutions (1b → 2b, 13 → 14 + 15). Furthermore, the activation parameters of the known rearrangement of octamethylcyclooctatetraene (3) to octamethylsemibullvalene (4) have been determined. - The data for these compounds, together with those for related compounds previously reported in the literature, show that substituents not only influence the relative stabilities of the semibullvalene and cyclooctatetraene systems but also the height of the energy barrier for their interconversion. Those substituents that lower the barrier toward the degenerate Cope rearrangement of semibullvalenes simultaneously accelerate their rearrangement to cyclooctatetraenes thus limiting the thermal stability of the former.
    Additional Material: 2 Ill.
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  • 128
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    Synthesis of the Enantiomers of 2-sec-Butyl-4,5-dihydrothiazole and (1R,5S,7R)-3,4-Dehydro-exo-brevicomin, Pheromone Components of the Male Mouse, Mus musculus (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2167-2173 
    Details
    ISSN: 1434-193X
    Keywords: Acetals ; Heterocycles ; Mouse ; Mus musculus/ Pheromones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two components [2-sec-butyl-4,5-dihydrothiazole (1) and 3,4-dehydro-exo-brevicomin (2)] of a male-produced pheromone of the mouse Musmusculus have been synthesized in optically active forms. The enantiomers of 1 were obtained with an enantiomeric purity of ca. 92% ee and were found to be readily racemizable. Asymmetric dihydroxylation was employed as the key reaction (15→16) allowing the preparation of (1R,5S,7R)-2 with ca. 94% ee.
    Additional Material: 1 Tab.
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  • 129
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    Syntheses of (±)- and (+)-Sorgolactone, the Germination Stimulant from Sorghum bicolor (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2183-2194 
    Details
    ISSN: 1434-193X
    Keywords: Ecology ; Lactones ; Phytochemistry ; Radical reactions ; Sorgolactone ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Syntheses of (±)-2a, the racemate of the structure proposed for sorgolactone, and its three racemic stereoisomers have been accomplished with confirmation of the stereostructures of the intermediate (±)-10 and the final product (±)-2a by X-ray analysis. Its optically active form, (3aR,8S,8bS,2′R)-(+)-2a, has also been prepared from (S)-(-)-citronellal by employing radical cyclization of 18 to 19 as the key step. Spectroscopic properties of the synthetic products are compared with those reported for natural sorgolactone. Bioassays using clover broomrape (Orobanche minor) seeds have revealed that all the stereoisomers strongly stimulate their germination.
    Additional Material: 1 Ill.
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  • 130
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    The Prooligonucleotide Approach: Synthesis of Mixed Phosphodiester and SATE Phosphotriester Prooligonucleotides Using H-Phosphonate and Phosphoramidite Chemistries (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2353-2358 
    Details
    ISSN: 1434-193X
    Keywords: Prooligonucleotides ; Solid-phase synthesis ; Phosphoramidite chemistry ; H-Phosphonate chemistry ; Nucleotides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of mixed SATE-phosphotriester and phosphodiester prooligonucleotides using both phosphoramidite and H-phosphonate chemistries is described. The key step is the selective oxidation of phosphite triester and H-phosphonate linkages to yield thiono- or oxo-triester SATE and thiono- or oxo-diester linkages. This approach allows the synthesis of any desired mixed prooligos bearing either tBu- or Me-SATE enzymolabile protecting groups.
    Additional Material: 5 Ill.
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  • 131
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    Photochemical pKa-Modulation and Gated Photochromic Properties of a Novel Diarylethene Switch (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2359-2366 
    Details
    ISSN: 1434-193X
    Keywords: Photochromism ; Diarylethenes ; Molecular devices ; Proton switches ; Merocyanines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The unsymmetrical dithienylethene photochrome 1a exhibits proton-gated photochromic properties as well as novel chemical reactivity. The dimethoxylated derivative 9a represents a light-triggered proton switch whose dissociation constant is increased by a factor of 16 upon photochemical cyclization to 9b. Such compounds are of potential interest for multiplexing photochemical systems.
    Additional Material: 6 Ill.
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  • 132
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    Synthesis and Structures of Novel Pyridine-Bridged Phanes of 4,4′-Bipyridine (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2373-2381 
    Details
    ISSN: 1434-193X
    Keywords: 4,4′-Bipyridine ; NMR spectroscopy ; Pyridinophanes ; X-ray crystal structure determination ; Macrocycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Syntheses of nine novel phanes derived from 4,4′-bipyridine incorporating new spacer elements are described together with their structural and spectroscopic properties. Starting from the previously unknown pyridine-bridged bis(4,4′-bipyridinium) dication 2 and various bis(bromomethyl)arenes, macrocycles 3-7 were prepared in a simple heterogeneous liquid-liquid reaction in satisfactory yields. In complementary experiments starting from bis(4,4′-bipyridinium) compounds 12-14 with substituted xylenes as spacers, ring closure was performed with 2,6-bis(bromomethyl)pyridine (1) to furnish compounds 16-18 in low yield. Temperature-dependent NMR-spectroscopic studies of the macrocycles were performed to investigate their conformational behaviour. The X-ray crystal structures for phanes 3,4 and 6 illustrate the preferred anti conformation of the macrocycles in the solid state. Consecutive reactions at the pyridine bridge were not successful; however, anisolophane 7 was converted into nitroanisolophane 11 by simple nitration of the anisolo spacer in excellent yield.
    Additional Material: 3 Ill.
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  • 133
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    1-Acyl-4-benzylpyridinium Tetrafluoroborates: Stability, Structural Properties, and Utilization for the Synthesis of Acyl Fluorides (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2383-2390 
    Details
    ISSN: 1434-193X
    Keywords: 4-Alkylidene-1,4-dihydropyridines ; 1-Acyl-4-benzylpyridinium tetrafluoroborates ; Carboxylic acid fluorides ; X-ray structures ; Ab initio calculations ; Density functional calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1-Acyl-4-benzylpyridinium salts 4 containing nonnucleophilic anions X- such as CF3SO3-, FSO3-, and BF4- can be generated quantitatively and in situ from 1-acyl-4-alkylidene-1,4-dihydropyridines 1a-f and the corresponding acid, HX. The BF4- salts reveal an interesting and unexpected thermal instability which allows the convenient synthesis of carboxylic acid fluorides 5b-f. This procedure offers advantages over known methods: All operations can be performed in a standard glass apparatus and do not require high pressures. The formation of RCOF 5 is assisted by the pyridine moiety of 4, which splits off and functions as a Lewis base to intercept the BF3 acid. The structural and electronic relationships as well as dominating differences between the very reactive cations of 4 and their almost “inert” uncharged precursors, the dihydropyridines 1, are discussed both on the fundament of experimental evidence (X-ray structures of 1f and the extremely reactive and very labile 4f) and theoretical investigations (ab initio and DFT MO calculations).
    Additional Material: 2 Ill.
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  • 134
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    Preparation and NMR Investigation of 1,2-Distannyl Aromatics and Heteroaromatics, 1,2-Distannylcycloalkenes and 1,2-Distannylcycloalkanes (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2413-2417 
    Details
    ISSN: 1434-193X
    Keywords: Organotin compounds ; Tin ; Diimine ; Reductions ; Aromatic compounds ; Aromatic compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of 1,2-distannylated aromatics, heteroaromatics and cycloalkenes was prepared by treating the dibromo precursors with Me3SnNa in tetraglyme. The latter were converted into the corresponding cycloalkanes by diimine reduction. Multinuclear NMR data are reported and discussed.
    Additional Material: 1 Ill.
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  • 135
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    Diastereoselective Generation of Quaternary Stereocenters by Ligand-Controlled Palladium-Catalyzed Allylations (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2327-2335 
    Details
    ISSN: 1434-193X
    Keywords: Quaternary carbon atoms ; Palladium ; Homogeneous catalysis ; Configuration determination ; NOE ; Taft parameter ; Diastereoselectivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 1,2-asymmetric induction in the formation of new quaternary centers by palladium-catalyzed allylation of substituted cyanoacetates 1a-c was controlled by using 2,9-disubstituted 1,10-phenanthrolines as ligands, giving the allylated products 3a-c with like/unlike diastereoselectivities up to 94:6. The increased selectivities were caused by a tight wrapping of the allyl unit in the intermediate complexes. Differences in stereoselectivity for varying substrates can be understood when modified Taft parameters, the homo-Es values, are considered. The like/unlike isomers were assigned by using a combination of molecular modeling, NOE measurements and analyses of chemical shifts. An X-ray analysis of the acid 14 confirmed the structural assignment.
    Additional Material: 7 Ill.
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  • 136
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    Improved Synthesis, Structure, and Cycloaddition Reaction of [34](1,2,4,5)Cyclophane (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1223-1231 
    Details
    ISSN: 1434-193X
    Keywords: Cyclophanes ; Barrelenophanes ; Semibullvalenophanes ; Cycloadditions ; Cyanoacetylenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The modified synthetic route to [34](1,2,4,5)cyclophane (3) provides gram quantities of this compound in fewer steps than the conventional routes. The cycloaddition of 3 with dicyanoacetylene (12) gave barrelenophane 13, which was transformed into semibullvalenophane 15 on photoirradiation. This transformation (13 → 15) is in sharp contrast to the case of Boekelheide's [24]barrelenophane, where cyclooctatetraenophane is obtained. The X-ray structural analyses of 3, 13, and 15 demonstrates their unique structures.
    Additional Material: 11 Ill.
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    A New Type of π-Electron Donors with One Dithiole Unit: Substituted 7-(1,3-Dithiol-2-ylidene)-7-hydrobenz[d,e]anthracenes (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1239-1247 
    Details
    ISSN: 1434-193X
    Keywords: Ab initio calculations ; Charge-transfer complexes ; Cyclic voltammetry ; π-Electron donors ; Density functional calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -New electron donors 11a-c bearing only one 1,3-dithiole ring conjugated with an acene backbone have been synthesized by Wittig-Horner reactions from 7H-benz[d,e]anthracen-7-one (10) and differently substituted phosphonate esters 9a-c. The donor properties of 11 have been characterized by means of both experimental techniques and quantum chemical calculations. Cyclic voltammetry measurements on the novel compounds predict donor abilities comparable to those of TTF and BEDT-TTF. The voltammograms show a multi-stage redox behaviour with oxidation up to the trication, where the first two oxidation processes take place at very close potentials. These processes have been assigned to a simultaneous oxidation of the dithiole and acene moieties on the basis of DFT/B3-P86/6-31G* theoretical calculations. Whereas the neutral compound and the monocation exhibit butterfly-shaped non-planar structures, the dication is fully aromatic and consists of a closed-shell, singly charged, planar polyacenic unit and a singly charged dithiole ring. The third redox process can be assigned to a second oxidation of the acene unit. The high aromaticity of the dication, coupled with the particular donor ability of the acene unit are shown to be the key factors in accounting for the electrochemical behaviour and the enhanced donor properties of the novel compounds. Theoretical calculations also help to rationalize the UV/Vis data in that they predict the appearance of a low-energy, intramolecular charge-transfer absorption band for the neutral compounds. Stable charge-transfer complexes with a 2:3 (D/A) stoichiometry have been prepared by reaction with the strong acceptors TCNQF4 and DDQ.
    Additional Material: 6 Ill.
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  • 138
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    The Reduction of α-X-Acetophenones (X = PhO, Br, Cl) in Hydrogen-Donating Solvents at Elevated Temperatures (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1267-1267 
    Details
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -No abstract
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    An Easy Access to Bicyclic Peptides with an Octahydro[2H]pyrazino[1,2-a]pyrazine Skeleton (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1345-1356 
    Details
    ISSN: 1434-193X
    Keywords: Peptidomimetics ; Octahydro[2H]pyrazino[1,2-a]pyrazine ; Diastereoselectivity ; Tripeptides ; Crystal structures ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -A new route to octahydrospiro(cyclopropane-1,1′-[2H]-pyrazino[1,2-a]pyrazine)-3′,6′,9′-triones 12-15 has been developed. Michael additions of primary amines onto methyl 2-Me or tert-butyl 2-tBu 2-chloro-2-cyclopropylideneacetates, followed by DCC- or EDC-induced coupling with Boc- or FmocGlyOH, deprotection and cyclization led to α-amino esters 4a-c and chlorohexahydrodiazepinediones 5a-c, or in the case of 2-tBu to the α-amino ester 7 exclusively. This reaction sequence with (S)-BocPheOH and (S)-BocTrpOH diastereoselectively gave (3′R,5′S)-9a,b and (2′S,6′R)-11a,b as the main products. Further peptide coupling, deprotection and cyclization with 4a-c yielded octahydrospiro(cyclopropane-1,1′-[2H]pyrazino[1,2-a]pyrazine)-3′,6′,9′-triones (7′S,9a′S)-12a-d, (6a′S,11a′S)-12e, (7′S,9a′R)-13a-d and (6a′S,11a′R)-13e which were easily separated. The α-amino esters 9a,b yielded (4′S,9a′R)-14a (≡15a) and (4′S,9a′R)-14b (≡15b), (4′S,7′S,9a′R)-14c and (4′R*,7′S*,9a′S*)-15c. The formation of compounds with three stereogenic centers 14c and 15c was accompanied by partial racemization. The versatility of the reported reaction sequence is limited by the steric availability of the secondary amino group in the intermediates 4, 9 and 10, as well as in the Michael adducts formed from primary amines and 2-Me.
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    Variable Stereocontrol in Cycloadditions of 1,7,9-Decatrien-3-ones by Different Lewis Acidic Promoters - Application to a Short Synthesis of α-Eudesmol (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1385-1393 
    Details
    ISSN: 1434-193X
    Keywords: Intramolecular Diels-Alder reaction ; 1,7,9-Decatrien-3-ones ; α-Eudesmol ; Lewis acids ; Chelates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Trienones 1-3 were subjected to Lewis acid-promoted intramolecular Diels-Alder reactions. It was shown that with monocoordinating Lewis acids the endo selectivity of the cycloaddition was generally high. The preference for either of the two possible endo products cis-a and cis-b, however, was shown to be highly dependent on the nature of the Lewis acid, and on the substitution pattern of the trienone substrates. Lewis acids with two coordination sites furnished predominantly the exo product trans-b via seven-membered ring chelate intermediates. The chelate-controlled intramolecular Diels-Alder reaction was then utilised as the key step in a stereocontrolled synthesis of the sesquiterpene α-eudesmol. This reaction mode, performed on a model trienone, also paved the way for the synthesis of the pharmaceutically important natural product dihydromevinolin.
    Additional Material: 6 Tab.
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    Vinyl- and Divinylcyclopentadienes by Rhodium-Catalyzed Condensation of Alkynes with Cross-Conjugated Amino Metallahexatrienes [= (1-Amino-1,3-butadien-2-yl)carbene Complexes] (M = Cr, W) (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2545-2561 
    Details
    ISSN: 1434-193X
    Keywords: Carbene complexes ; Catalysis ; Metallahexatrienes ; Vinyl cyclopentadienes ; Transmetallation ; Rhodium ; Tungsten ; Chromium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the first example of a transition metal-catalyzed cyclization reaction of a Fischer carbene complex. It comprises the generation of vinyl- and divinyl cyclopentadienes under exceedingly mild conditions at 20 °C by condensation of (1-amino-1,3-butadien-2-yl)carbene complexes (= cross-conjugated metallahexatrienes) (CO)5M=C(OEt)C(=CHNR2)CR1=CHR23 (M = Cr, W) with alkynes R3C≡CH 4 (R3 = Ph, cyclohex-1-enyl, isopropenyl, methoxymethyl, 1-trimethylsiloxycyclohex-1-yl) in the presence of catalytic amounts of [(COD)RhCl]2. The starting compounds 3 are accessible in high yields by addition of enamines (E)-R2NCH=CHR22 to (1-alkynyl)carbene complexes (CO)5M=C(OEt)C≡CR11 (M = Cr, W; R1 = Ph, cyclohex-1-enyl).
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    Kinetic Resolution of Racemic Cyclic Sulfoxides Using Hydrolytic Enzymes (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2573-2578 
    Details
    ISSN: 1434-193X
    Keywords: Enzymes ; Kinetic resolution ; Sulfoxides ; Chiral resolution ; Hydrolyses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Selected racemic trans-2-alkoxycarbonyl-3,6-dihydro-2H-thiopyran S-oxides 2a-h have been subjected to enzyme-assisted hydrolysis under conditions of kinetic resolution to give the corresponding acids and recovered esters with moderate to high enantiomeric purities (up to 95%). The enantioselectivity of the reaction was found to be strongly dependent on the structures of the substrates used.
    Additional Material: 3 Ill.
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    Approach Toward a Generic Treatment of Gram-Negative Infections: Synthesis of Haptens for Catalytic Antibody Mediated Cleavage of the Interglycosidic Bond in Lipid A (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2593-2600 
    Details
    ISSN: 1434-193X
    Keywords: Endotoxin ; Immunotherapy ; Catalytic antibodies ; Glycosidases ; Azasugars ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In order to develop a generic treatment for infections with Gram-negative bacteria, we developed a synthesis of 2-acylamino-deoxynojirimycin derivatives (17, 18, 19 and 20), which will be used as haptens for raising catalytic antibodies capable of hydrolyzing the interglycosidic bond in the lipid A moiety of endotoxins. A key intermediate in the preparation of compounds 17, 18, 19 and 20 is 3,4,6-tri-O-benzyl-2-[(benzyloxycarbonyl)amino]-2-deoxy-D-glucono-δ-lactam (6), which was prepared from known 3,4,6-tri-O-benzyl-2-[(benzyloxycarbonyl)amino]-2-deoxy-D-glucosamine (1) in four steps in 47% overall yield. Antibodies were generated against 2-[(6-aminohexanoyl)amino]-2-deoxy-D-glucono-δ-lactam (17) coupled to the carrier protein bovine serum albumin.
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  • 144
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    Synthesis of Isotopically Labelled L-Phenylalanine and L-Tyrosine (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2609-2621 
    Details
    ISSN: 1434-193X
    Keywords: Amino acids ; Isotopic labelling ; Ethyl benzoate ; Benzonitrile ; Sodium phenylpyruvate ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A synthetic route to stable-isotope-substituted L-phenylalanine is presented, which allows the introduction of 13C, 15N, and deuterium labels at any position or combination of positions. For labelling of the aromatic ring, a synthetic route to ethyl benzoate (or benzonitrile) has been developed, based on the electrocyclic ring-closure of a 1,6-disubstituted hexatriene system, with in situ aromatization by elimination of one (amino) substituent. Several important (highly isotopically enriched) synthons have been prepared, namely benzonitrile, benzaldehyde, ethyl benzoate, and ethyl diphenyloxyacetate. Labelled L-phenylalanines have been synthesized from both aromatic precursors by initial conversion into sodium phenylpyruvate and subsequent transformation of this intermediate into the L-α-amino acid by an enzymatic reductive amination reaction. In this manner, highly enriched phenylalanines are obtained on the 10-gram scale and with high enantiomeric purities (≥ 99% ee). The method has been validated by the synthesis of [1′-13C]-L-Phe and [2-D]-L-Phe. In addition, two methods are described for the introduction of isotopes into L-tyrosine starting from the isotopically enriched precursors benzonitrile and ethyl benzoate.
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  • 145
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    Stereoselective Amination of 5-Substituted γ-Lactones and γ-Lactams - A Convenient Route for the Preparation of 5-Substituted (3S,5S)-3-Acetylaminotetrahydrofuran-2-ones and (3S,5S)-3-Acetylaminopyrrolidin-2-ones (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1581-1584 
    Details
    ISSN: 1434-193X
    Keywords: Tetrahydrofuran-2-ones ; Pyrrolidin-2-ones ; Aminations ; Nitrosations ; Asymmetric synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5-Substituted (S)-tetrahydrofuran-2-ones (1a,b) and (S)-pyrrolidin-2-ones (1c,d) were transformed in three steps, by treatment with tert-butoxybis(dimethylamino)methane (Bredereck's reagent), followed by nitrosation and stereoselective catalytic hydrogenation, into the corresponding 5-substituted (3S,5S)-3-acetylaminotetrahydrofuran-2-ones (4a,b) and (3S,5S)-3-acetylaminopyrrolidin-2-ones (4c,d).
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  • 146
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    The Synthesis of Novel Cyclic β-Amino Acids as Intermediates for the Preparation of Bicyclic β-Lactams (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2433-2441 
    Details
    ISSN: 1434-193X
    Keywords: Amino acids ; Lactams ; Schiff bases ; Nitrogen heterocycles ; Heterocycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several derivatives of homopipecolic acid are prepared by α-amino alkylation of malonic acid with cyclic imines 6 and 7. These are prepared on a large scale and with different substitution patterns. The β-amino acids 8 and 9 were formed in high yield and with remarkable diastereoselectivity if chiral imines are used as starting materials. The diastereoselectivity of the amino alkylation leading to homopipecolic acid analogues is compared to those of thiazolidineacetic acids by epimerisation experiments. A method for resolution of the obtained racemic β-amino acids by diastereomeric salt formation is described. The β-amino acids 9 and 15 were converted into their corresponding carbacepham analogues 14 and isopenam 16. The isopenam endo-16 was selectively epimerised by mild basic treatment of the N/S-acetal to give an exo-configured precursor of isopenicillin G.
    Additional Material: 2 Tab.
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  • 147
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    Synthesis of Trifluoromethyl-Porphyrins and -Chlorins (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2471-2473 
    Details
    ISSN: 1434-193X
    Keywords: Trifluoromethylation ; Porphyrin ; Chlorophyll ; Fluorine ; Zinc ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of zinc 5,15-bis(3,5-di-tert-butylphenyl)porphyrin with S-(trifluoromethyl)-3,7-dinitrobenzothiophene trifluoromethanesulfonate in tetrahydrofuran gave the 10-trifluoromethylated compound as the major product and 2-CF3- and 10,20-di-CF3-porphyrins as the minor products. The direct trifluoromethylation is effective for preparation of longer-wavelength absorbing meso-trifluoromethylated porphyrins and chlorins.
    Additional Material: 2 Ill.
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  • 148
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    Molecular Recognition of Azobenzene Dicarboxylates by Acridine-Based Receptor Molecules; Crystal Structure of the Supramolecular Inclusion Complex of trans-3,3′-Azobenzene Dicarboxylate with a Cyclo-bis-intercaland Receptor (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2479-2484 
    Details
    ISSN: 1434-193X
    Keywords: Cyclo-bis-intercaland ; Acridine units ; cis/trans-Azobenzene dicarboxylic acids ; Inclusion compounds ; X-ray structure ; Molecular recognition ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The water soluble acyclic 1 and macrocyclic 2 receptor molecules, based on acridine units, form 1:1 complexes with the cis- or trans-2,2′ and 3,3′-azobenzene dicarboxylate substrates. The stability constants of these complexes, determined by 1H NMR spectroscopy, cover a wide range from 30 to 105M-1, thus displaying very pronounced structure selectivity with respect to both substitution pattern and cis, transconfiguration. The complexes of the cyclo-bis-intercaland receptor 2 are two or three orders of magnitude more stable than those of 1. The inclusion complex of cyclo-bis-intercaland 2 with trans-3,3′- azobenzene dicarboxylate has been isolated and its structure has been determined by X-ray crystallography using synchrotron radiation, confirming the intercalation of the substrate between the acridine residues in the species formed.
    Additional Material: 3 Ill.
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  • 149
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    Addition Chemistry of Rod-Shaped 1,6-Dicyanohexatriyne: Regioselectivity Control by the Remote Cyano Function (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2491-2500 
    Details
    ISSN: 1434-193X
    Keywords: Addition reactions ; Cycloadditions ; Dicyanohexatriyne ; Enediyne ; Regioselectivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The addition of different alcohols, amines, thioalcohols, ethers, cycloalkanes, tin hydride, alkenes or alkadienes to 1,6-dicyano-1,3,5-hexatriyne (1) leads, with a pronounced regioselectivity, to a series of substituted 1-ene-3,5-diynes. The addition of alcohols, amines and thiols proceeds preferentially in a syn mode leading to diastereomers with an E configuration. The reaction with free radicals, on the other hand, proceeds in an anti addition mode. The [4+2] and [2+2] cycloadditions performed with 1 yielded unsaturated six- and four-membered ring adducts, respectively. The cyclobutene derivatives resulting from [2+2] cycloadditions underwent spontanous electrocyclic ring-opening reactions with the formation of butadiene derivatives. Nucleophilic additions to form 1 lead exclusively to products where the donor atom is bound at C-1. All the reactions, except the formation of 19, showed an excellent regioselectivity with respect to the preferred addition to a terminal C≡C triple bond.
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  • 150
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    Pd-Catalyzed Asymmetric Allylic Alkylation of 3-Acetoxy-N-(tert-butyloxycarbonyl)-1,2,3,6-tetrahydropyridine - Preparation of Key Intermediates for Natural Product Synthesis (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2515-2521 
    Details
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Heterocycles ; Palladium ; Alkylations ; Allyl complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A convenient synthesis of racemic tetrahydropyridine 1 was developed. Pd-catalyzed allylic alkylation of 1 with malonate and dimethylacetoxymalonate as nucleophiles with the phosphanylcarboxylic acid L1 and the dihydrooxazol L2 as ligands, were carried out and gave enantiomeric excesses of up to 98%. Absolute configurations were determined for all compounds described. From the alkylation products (+)- and (-)-2a, and (+)- and (-)-2b a variety of versatile, nonracemic chiral intermediates were prepared.
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  • 151
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    EPC Syntheses of Trifluorocitronellol and of Hexafluoropyrenophorin - A Comparison of Their Physiological Properties with the Nonfluorinated Analogs (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2533-2544 
    Details
    ISSN: 1434-193X
    Keywords: (S)-4,4,4-Trifluoro-3-hydroxybutanoic acid ; 2-Trifluoromethyl-3-hydroxypropanoic acid (F3-Roche acid) ; Chiral CF3-containing synthetic building blocks ; Natural products ; Cyclizations ; Olfactory comparison ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The natural products pyrenophorin (1a) and citronellol (2a), in which CH3 groups are replaced by CF3, were synthesized in enantiomerically pure form from simple four-carbon trifluorohydroxy acids (obtained by resolution). The cyclizations of analogous CH3 and CF3 seco acids (cf. 9) to give pyrenophorin derivatives require different methodologies; the F6 derivative 10a could be obtained in only very poor yield; in contrast to pyrenophorin. Most surprisingly, F6-pyrenophorin (1d) has an extremely poor solubility in common organic solvents, and has essentially no antimicrobial activity (see Table 2). The synthesis of F3-citronellol is the first application of an enantiopure F3-Roche acid (12) as a synthetic builiding block (see its derivatives 17-23). An olfactory comparison of F3-citronellol [(R)-(+)-2b] with citronellol and ent-citronellol (Scheme 6) shows that the fluorine derivative has a “very metallic, aggressive” character and lacks totally the “sweetness” of (R)-(+)- and (S)-(-)-2a. A number of generally useful, CF3-substituted electrophilic (iodides 4, 18, 37, tosylates 19, 33, aldehydes 5, 29, 39) and nucleophilic (Li dithiane precursor of 5, Li compounds 20, 38) reagents are described for the first time.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/1999/99137_s.pdf or from the author.
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  • 152
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    Oxidation of o-Methoxyphenols with a Hypervalent Iodine Reagent: Improved Synthesis of Asatone and Demethoxyasatone (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2579-2581 
    Details
    ISSN: 1434-193X
    Keywords: Hypervalent iodine ; Phenols ; Oxidation ; Cyclohexa-2,4-dienones ; Dimerization ; Asatone ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation of the 2-methoxyphenols 1c-f with phenyliodonium(III) diacetate (PIDA) in methanol resulted in the formation of the cyclohexa-2,4-dienones 4c-f, which dimerized spontaneously to a single product (5c-f) in each case. Using this method the synthesis of asatone (5a) and its demethoxy analogue (5b) was also accomplished starting from the readily available phenol derivatives 1a and 1b, respectively.
    Additional Material: 1 Ill.
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  • 153
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    A New Synthetic Approach Toward (+)-Ambruticin Analogs: Preparation of a C10-C11 cis-Isomer Fragment (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2885-2892 
    Details
    ISSN: 1434-193X
    Keywords: Ambruticin ; Asymmetric synthesis ; Pyranose ; Carbohydrates ; C-Glycosylation ; Alkylations ; Palladium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new methodology has been applied to synthesize an isomer of the west part of (+)-ambruticin based on an efficient asymmetric de novo access to the A unit and sequential stereospecific reactions catalyzed by Pd0 for the construction of the B unit.
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  • 154
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    Solvent Effect on the Protonation of Acetylene and Ethylene - Continuum Solvent Quantum Chemical Calculations (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2893-2897 
    Details
    ISSN: 1434-193X
    Keywords: Acetylene ; Ethylene ; Carbocations ; Vinyl cation ; Ethyl cation ; Basicity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The protonation of acetylene and ethylene (yielding the vinyl and ethyl cation, respectively) was investigated computationally by ab initio calculations [B3LYP/6-31G(d,p)], in the gas phase and in water, as modeled by the IPCM and SCIPCM continuum methods. The structures and NBO atomic charges were thus determined for the neutral bases and their protonated forms, while the comparison of gas-phase and aqueous basicities afforded the hydration energies of the protonated bases. It was found that the aqueous protonation of acetylene is more endothermic than that of ethylene by 5 kcal/mol, owing to the lower intrinsic basicity of the former (by 7.4 kcal/mol), which is only partly compensated for by the more exothermic hydration (by 3.8 kcal/mol) of the vinyl cation.
    Additional Material: 5 Ill.
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  • 155
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    Conformation Control in Open-Chain Compounds with up to Six Rotatable Bonds (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2929-2936 
    Details
    ISSN: 1434-193X
    Keywords: Local conformer populations ; syn-Pentane interactions ; Conformational isomerism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Substituent patterns on 1,3,5…-polyoxygenated-2,4,6…-polymethylated alkane chains, which preferentially adopt a fully extended backbone conformation, have been identified. This is demonstrated by analysis of the vicinal H,H-coupling constants along the backbones of compounds 19, 23, and 24.
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  • 156
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    New Heterocycles from the Reaction between Some Natural α-Amino Acid Hydrazides and Formaldehyde (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2943-2948 
    Details
    ISSN: 1434-193X
    Keywords: Amino acids ; α-Amino acid phenylhydrazides ; Heterocycles ; Cyclization reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ring forming condensation between some natural α-amino acid phenylhydrazides (1) and aqueous formaldehyde (2) has opened a novel synthetic route to hexahydro-1,2,4-triazin-6-one derivatives (3). Polycyclic systems were obtained from the same reaction carried out with L-aspartic acid 1,4-bis(2-phenylhydrazide) (1d), L-histidine phenylhydrazide (1e) and L-tryptophan phenylhydrazide (1f) which gave perhydro-4,6-dioxo-2,8-diphenyl[1,2,4]triazino[4,5-d][1,2,4]triazepine (5) perhydro-1-oxo-3-phenylimidazo[5,4-d][1,2,4]triazino[4,5-a]pyridine (7) and 1,2,3-H-3-(2-phenylcarbazoyl)-β-carboline (8), respectively. Substrates 1 were conveniently obtained by direct reaction of phenylhydrazine with L-α-amino acid esters retaining the original chirality.
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  • 157
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    Stereoselective Synthesis of (E)- and (Z)-Acetals of Pent-2-en-4-yn-1-al and Related Dienynes and Dienediynes (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2957-2963 
    Details
    ISSN: 1434-193X
    Keywords: Sonogashira-Linstrumelle cross-coupling ; Stille cross-coupling ; Protected enynals ; Transacetalisation ; Pent-2-en-4-yn-1-al ; Deca-2,8-diene-4,6-diyne-1,10-dial ; Dienediynes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (3E)-5,5-Diethoxypent-3-en-1-yne was stereospecifically prepared by Sonogashira-Linstrumelle cross-coupling between (E)-3-iodoacrolein diethylacetal and (trimethylsilyl)acetylene. The (Z) isomer was obtained by Stille cross-coupling between the corresponding (Z)-vinyltin and 1-bromo-2-(trimethylsilyl)acetylene. In the case of the (E) isomer, transacetalisation occurred without isomerization affording a large variety of (E)-enynals protected as acetals. It has also been shown to be possible to obtain the corresponding (E,E)-dienediyne through sp-sp homocoupling under appropriate experimental conditions.
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  • 158
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    Synthesis of Enantiomerically Pure Amino Acids Containing 2,5-Disubstituted THF Rings in the Molecular Backbone (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2977-2990 
    Details
    ISSN: 1434-193X
    Keywords: β-Turn mimetic ; Conformational analysis ; Synthesis design ; Tetrahydrofuran ; THF amino acid ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: N- and C-protected derivatives of 2,5-disubstituted trans- and cis-THF amino acids 6 and 7 were prepared in enantiomerically pure form from L-alanine. Felkin-Anh-controlled reduction of the ketone 9 was achieved with a 85:15 diastereoselectivity. Epoxidation of 10 and subsequent intramolecular epoxide opening gave the trans- and cis-THF alcohols 11 and 12, which were further transformed into the corresponding N- and C-protected 2,5-disubstituted trans- and cis-THF amino acids. Conformational studies show that the cis-THF diamide 34 is a β-turn mimetic in the solid state and in CDCl3 solution.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2046/1999/99165_s.pdf or from the author.
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  • 159
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    The Addition Patterns of C60 Trisadducts Involving the Positional Relationships e and trans-n (n = 2-4): Isolation, Properties, and Determination of the Absolute Configuration of Tris(malonates) and Tris[bis(oxazolines)] (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3027-3039 
    Details
    ISSN: 1434-193X
    Keywords: Chirality ; Circular dichroism ; Cycloadditions ; Fullerenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of regioisomeric trisadducts of C60 (5-11, 14-18) having the positional relationships e and trans-n (n = 2-4) resulting from successive additions of malonates or bis(oxazolines) to [6,6]-double bonds of the fullerene framework has been isolated and characterized. The new adducts 8-11 and 15-17 represent examples of unprecedented addition patterns. The addition patterns of the new trisadducts with C2, Cs or C1 symmetry have been assigned on the basis of the known positional relationships of the addends in their precursor bisadducts, and those of the C2- or Cs-symmetric representatives have been verified by analysis of their NMR spectra. The absolute configurations of the adducts with inherently chiral addition patterns could be determined either by comparison of the calculated and experimental CD spectra of the bis(oxazoline) adducts or with knowledge of the absolute configurations of the chiral bisadduct precursors containing bis(oxazoline) addends. The CD spectra of the pairs of diastereomers with an enantiomeric addition pattern fA -15/fC -15, fA -16/fC -16, and fA -18/fC -18 show mirror image behavior and pronounced Cotton effects.
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  • 160
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    The Carbene/Bridgehead-Olefin Rearrangement: DFT Calculations on 1-Bicyclo[2.1.1]hexylcarbene and 1-Bicyclo[3.1.1]heptylcarbene; Rearrangement of (5-Bromobicyclo[3.1.1]heptyl)bromocarbene (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3057-3066 
    Details
    ISSN: 1434-193X
    Keywords: Small ring systems ; Strained molecules ; [3.1.1]Propellane ; Alkene dimerization ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: B3LYP/6-31G(d) calculations on bicyclo[2.1.1]hexylcarbene 7ashowed a preferred propensity for enlargement of the four-membered ring to give 1-norbornene 8. Only at high temperature does a rearrangement of bridgehead olefin 8 to 2-norbornylidene 11 seem to be possible; the competing rearrangement to afford 2-bicyclo[3.1.1]heptylidene 10has a higher activation barrier by 20.2 kcal/mol. For 1-bicyclo[3.1.1]heptylcarbene 12, the preferred stabilization is again enlargement of the four-membered ring to give bicyclo[3.2.1]oct-1(7)-ene 13. The further rearrangement of this alkene to 7-bicyclo[3.2.1]octylidene 16is endothermic by 32.2 kcal/mol and should not take place under moderate conditions. (5-Bromobicyclo[3.1.1]heptyl)bromocarbene 32, a derivative of 12 generated from the tetrabromide 23 with MeLi, rearranged by enlargement of the four-membered ring to give alkene 31, which reacted to give the head-to-head 2π+2π dimer 26, whose structure was established by X-ray crystallography.
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  • 161
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    Diastereoselective Synthesis of a Precursor of Homocarbocyclic Nucleosides (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3085-3087 
    Details
    ISSN: 1434-193X
    Keywords: Nucleoside analogue ; Nucleosides ; Palladium ; Dihydroxylations ; Catalysts ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A convenient diastereoselective synthesis of an immediate precursor for a rapid access to a variety of homo-carbocyclic nucleosides, is described. The synthetic scheme involves a high yielding new type of Pd(0) catalyzed cyclization of a suitable hydrazine derivative together with a stereospecific OsO4 triggered dihydroxylation step.
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    Efficient Biomimetic-Type Synthesis of the Benzo[a]naphthacenequinone Antibiotics G-2A and G-2N (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3099-3103 
    Details
    ISSN: 1434-193X
    Keywords: Antibiotics ; Biomimetic-type synthesis ; Polyketides ; Quinones ; Aldol reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Trioxo ester 15 was prepared by attachment of two vicinal C-4 and C-6 ketide side chains on the anthraquinone core (6b). Mild base treatment of 15 initiated successive aldol condensations to produce the benzo[a]naphthacenequinone 16 regioselectively in one operation. Deprotection of 16 afforded the antibiotic G-2A (1) and decarboxylation of 1 lead to G-2N (2).
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  • 163
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    Convenient Syntheses of Novel α- and β-Amino Acids with Spiropentyl Groups (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3105-3115 
    Details
    ISSN: 1434-193X
    Keywords: Amino acids ; Spiro compounds ; Michael addition ; Copper reagents ; Curtius degradation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic spiropentylglycine (8) has been synthesized by sodium borohydride reduction of benzyl (E/Z)-2-chloro-2-spiropentylideneacetate (5-Bn), nucleophilic substitution of the chlorine in the product 6 with azide and hydrogenolytic deprotection of the resulting 7 (overall yield 15%). An alternative approach to 8 consisted of the coupling of the higher-order cuprate 10, generated by halogen-metal exchange from bromospiropentane (9), with the electrophilic glycine equivalent 11 followed by deprotection (overall yield 47%). Enantiomerically pure (1′-aminospiropentyl)acetic acid [(R)-16] (overall yield 16% from 5-Me) and 1-aminospiropentanecarboxylic acid [(R)-23] (29% from 5-Me) were obtained from the Michael adduct 14-Me of (4R,5S)-4,5-diphenyloxazolidin-2-one (13) and methyl (E/Z)-2-chloro-2-spiropentylideneacetate (5-Me). Racemic 1-aminospiropentanecarboxylic acid (R/S-23) was prepared by rhodium-catalyzed addition of dimethyl diazomalonate to methylenecyclopropane and subsequent Curtius degradation of the halfester 28 via the azide 29 (overall yield 14%). Upon standing in aqueous solution, 23 underwent complete rearrangement to the new 1-amino-2-methylenecyclobutanecarboxylic acid (24). The interesting derivative of azabicyclo[3.1.0]hexane-1-carboxylate 34 with an annelated spiropentane moiety and a β-amino acid fragment was incidentally obtained in a one-step intermolecular domino reaction starting with the addition of lithium benzylamide to methyl 2-chloro-2-cyclopropylideneacetate (32, 41% yield).
    Additional Material: 2 Ill.
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  • 164
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    Studies Directed Toward the Synthesis of Taxanes: Construction of the Tricyclo[5.3.1.01,7]undecane System (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3251-3256 
    Details
    ISSN: 1434-193X
    Keywords: Taxol ; Nitrile oxide ; Cycloaddition reactions ; Isoxazoline ; Cyclopropane ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The intramolecular nucleophilic substitution of an epoxide by the anion of a nitrile was used to construct the bicyclo[3.1.0]hexane system of 9, derived from 8, and of 14, derived from 13. The intramolecular 1,3-dipolar cycloaddition of nitrile oxide 4, obtained from 14, led to isoxazoline 3. However, it was not possible to convert 3 into the corresponding β-hydroxy ketone. Rearrangements of 3 were observed in the presence of dehydrating reagents leading to compounds 19-21, which are structurally related to 11(15→1)abeotaxanes.
    Additional Material: 1 Ill.
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  • 165
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    Synthesis of a Dinuclear ZnII-Calix[4]arene Enzyme Model with Additional General Base Groups - Catalytic Activity in Phosphate Diester Transesterification (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3269-3275 
    Details
    ISSN: 1434-193X
    Keywords: Calixarenes ; Enzyme mimetics ; Phosphatases ; Transesterification ; Zinc ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Calix[4]arenes modified at the upper rim with two pairs of functional groups were synthesized and were used as starting materials for the synthesis of an enzyme model 3-Zn2 that has two ZnII centers and two dimethylamino groups. Under neutral conditions, this dinuclear metallo-enzyme model is catalytically active in the cleavage of the phosphate diester bond of the RNA model substrate 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP). The kinetics of 3-Zn2 were compared with a mononuclear complex 1-Zn and a dinuclear complex 2-Zn2 that lack the dimethylamino groups. A catalysis mechanism is proposed where the ZnII centers of 3-Zn2 cooperate in the activation of the phosphoryl group while one of the dimethylamino groups acts as a general base in the deprotonation of the hydroxyl group of HPNP.
    Additional Material: 4 Ill.
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  • 166
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    N-(α-Chloroalkyloxycarbonyl)pyrrolidines as a Source of Oxygenated d1-Reagents (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3005-3012 
    Details
    ISSN: 1434-193X
    Keywords: Organolithium compounds ; d1-Reagents ; Lithiation ; Carbamates ; 1,2-Diols ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of the N-(α-chloroalkyloxycarbonyl)pyrrolidines 1 with lithium powder and a catalytic amount of 4,4′-di-tert-butylbiphenyl (DTBB, 2.5 mol-%) in the presence of different electrophiles [iBuCHO, tBuCHO, PhCHO, Et2CO, (CH2)5CO, PhCOMe, Ph2CO, Me3SiCl], in THF at temperatures ranging between -78 and -60°C leads, after hydrolysis with water, to the expected functionalized carbamates 2. Deprotection of compounds 2, derived from carbonyl compounds, with lithium hydroxide in a mixture of ethanol and water at 80°C affords the correponding 1,2-diols 3.
    Additional Material: 2 Tab.
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  • 167
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    Synthesis of Reactive Sulfines by Flash Vacuum Thermolysis: Thioformyl Cyanide S-Oxide and Thioxoethanal S-Oxide (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3041-3045 
    Details
    ISSN: 1434-193X
    Keywords: Thiocarbonyl compounds ; Sulfines ; Flash vacuum thermolysis ; Cyanides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thioformyl cyanide S-oxide and thioxoethanal S-oxide were synthesized by retro Diels-Alder cleavage of their dimethylanthracene adducts under flash vacuum thermolysis conditions. They were characterized by low-temperature IR and NMR spectroscopy. The formation of thioformyl cyanide S-oxide was also confirmed by mass spectrometry.
    Additional Material: 3 Ill.
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  • 168
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    A Novel Approach to the Synthesis of Precursors of Tricyclic β-Lactam Antibiotics (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3067-3072 
    Details
    ISSN: 1434-193X
    Keywords: Thioester ; Enolate ; Imine ; Lactams ; Asymmetric synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The stereoselective synthesis of two precursors of tricyclic β-lactam antibiotics (trinems) has been attempted by a novel approach that involves a highly stereoselective azetidinone ring-forming reaction followed by the reduction of a functionalized aromatic substituent at C-4 of the β-lactam nucleus.
    Additional Material: 2 Ill.
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  • 169
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    Studies Relevant to Ellagitannin Chemistry: Strain-Energy Induced Opening of the D-Glucono-δ-lactone Ring of an Ellagitannin Derivative and the Synthesis of Ellagitannin Natural Products with a D-Gluconic Acid Moiety (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3079-3083 
    Details
    ISSN: 1434-193X
    Keywords: Natural product ; Ellagitannins ; Gluconic acid derivatives ; Atropisomerism ; Lactones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The opening of the D-glucono-δ-lactone ring of the ellagitannin derivative 8, which contains a hexabenzyloxydiphenoyl moiety, to the corresponding gluconic acid derivative 9 under mild acidic conditions was investigated. This transformation was found to occur quantitatively upon treatment of the D-glucono-δ-lactone derivative 8 with silica gel.
    Additional Material: 2 Ill.
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  • 170
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    The Effect of Lewis Acids on the Pinacol Homocoupling Reaction of Aldehydes Promoted by Samarium Diiodide (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3369-3374 
    Details
    ISSN: 1434-193X
    Keywords: Pinacol reaction ; Lewis acid ; Samarium diiodide ; 1,2-diols ; Carbonyl compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effect of various Lewis acids on the samarium diiodide promoted pinacol homocoupling of aldehydes was investigated. The reaction of benzaldehyde proceeded with fair to good 1,2-anti-stereoselectivity, while in the case of other aromatic and aliphatic aldehydes syn-stereoselectivity was generally observed. Chiral α-alkylaldehydes allowed for an almost complete stereocontrol favoring syn-1,2-diols.
    Additional Material: 5 Ill.
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  • 171
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    Consequences of the Preeminent Reactivity of 2-OH in Sucrose: Cyclic Acetalation at 2-OH and 3-OH under Basic Conditions (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3393-3398 
    Details
    ISSN: 1434-193X
    Keywords: Sucrose ; Acetalation ; Etherification ; Carbohydrate ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of unprotected sucrose with tert-butyl chloromethyl ketone in dimethylformamide was investigated as a model for the study of the relative reactivity of the various hydroxy groups of sucrose. Besides the monoethers arising from the substitution of the chlorine atom by 2-OH and 1′-OH of sucrose, the major product is a tert-butyl hydroxymethyl 5-membered ring acetal involving 2-OH and 3-OH. The formation of this product illustrates the preeminent reactivity of 2-OH towards the carbonyl group of the α-chloromethyl ketone, leading to an intermediate hemiacetalic anion, which can enter by intramolecular acetal formation through an intermediate epoxide.
    Additional Material: 2 Tab.
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  • 172
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    Cycloisomerization of 1-Phenyl-1-buten-3-yne to Naphthalene via Cinnamylidene Carbenes - A Complex Reaction Involving 1,2-C Switches and 1,2-Styryl Migrations (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3407-3412 
    Details
    ISSN: 1434-193X
    Keywords: Enyne pyrolysis ; 13C,D labelling ; Alkenylidene carbenes ; 1,2-Styryl migration ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermal conversion of [4-13C,4-D]- (1) and [4-13C]-1-phenyl-1-buten-3-yne (7) has been studied in a quartz tubular reactor at 650 °C (1, in the presence of N2 and N2/toluene, respectively) and at 600 and 620 °C (mixture of 1 and 7, in N2 only) at a reaction time of approximately 0.3 s. The liquid pyrolyzates were analyzed spectroscopically. By means of a special calculation method reported recently, the naphthalene isotopomers formed by reaction pathways other than those proceeding via cinnamylidene carbenes were arithmetically eliminated and the reaction events proceeding via carbene intermediates were mechanistically analyzed. The result of this analysis undoubtedly suggests a complex reaction in which the rates of the partial reactions may be placed in the following order: 1,2-D(H) 〉〉 1,2-styryl 〉 1,6-C,H.
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  • 173
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    One-Pot and Sequential Asymmetric Hydrogenation of β,δ-Diketoesters into Functionalized 1,3-Diols: From anti- to syn-Stereoselectivity (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3421-3427 
    Details
    ISSN: 1434-193X
    Keywords: Asymmetric hydrogenation ; 3,5-Dioxoester ; Diols ; Lactone ; Ruthenium catalyst ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The asymmetric hydrogenation of methyl 3,5-dioxohexanoate (1) into mixtures of 3,5-dihydroxyesters (2) and 3-hydroxylactones (3) has been reinvestigated with a variety of ruthenium catalysts. Catalysts bearing diphosphanes which possess axial chirality such as (S)-MeO-Biphep give predominantly (3R,5S)-anti-2 in up to 78% de and 95% ee, affording an efficient synthesis of (S)-6-methyl-5,6-dihydro-2-pyrone [(S)-5] in up to 95% enantiomeric excess. On the contrary, some Ru-{amidophosphane-phosphinite} complexes catalyze sluggishly the formation of syn-2 in up to 92% de but with poor enantiomeric excesses. In all cases, methyl (R)-3-hydroxy-5-oxohexanoate [(R)-11] is the exclusive primary hydrogenation product, which can be isolated in high yields and enantiomeric excesses up to 78%. Further hydrogenation of enantiomerically enriched (R)-11 in a separate experiment affords (3R,5R)-syn-2 in high diastereomeric and enantiomeric excesses (up to 80% and 98%, respectively), provided a Ru-(R)-Binap-type catalyst is used.
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  • 174
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    The Highly Selective Oxidation of a Tricarbonyliron Complex (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3455-3458 
    Details
    ISSN: 1434-193X
    Keywords: Stereoselectivity ; Carbonyliron Complex ; Oxidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The tricarbonyliron complex 2a underwent selective and diastereoselective oxidation with tert-butyl hydroperoxide at room temperature to provide, after decomplexation, the peroxyether 5 which, on reduction, yielded the corres-ponding secondary alcohol 7.
    Additional Material: 1 Ill.
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  • 175
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    Stereoselective Inter- and Intramolecular Pauson-Khand Reactions of N-(2-Alkynoyl) Derivatives of Chiral Oxazolidin-2-ones (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3459-3478 
    Details
    ISSN: 1434-193X
    Keywords: Alkyne complexes ; Asymmetric synthesis ; Chiral auxiliaries ; Cyclopentenones ; Pauson-Khand reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A complete account of the intermolecular and intramolecular Pauson-Khand reactions of N-(2-alkynoyl) derivatives of chiral 2-oxazolidinones is presented. The intermolecular Pauson-Khand reactions with norbornene or norbornadiene take place under mild conditions and in high yields. Phenyl- or trimethylsilylpropiolate derivatives lead to the exclusive formation of 1,4-dicarbonyl regioisomers, while mixtures of 1,3- and 1,4-regioisomers are obtained with tetrolate derivatives. The diastereoselectivity of the reaction, determined by the substitution pattern of the oxazolidinone moieties, can be very high (up to 17.5:1 dr) for the formation of 1,4-dicarbonyl regioisomers, and the diastereomeric products can often be separated by column chromatography. Under the appropriate conditions, the intramolecular Pauson-Khand reactions of oxazolidinone-derived enynes can also take place with very good yields, but with low diastereoselectivities. The absolute configurations of several adducts have been determined, and the stereochemical outcome of the reaction has been rationalized.
    Additional Material: 6 Ill.
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  • 176
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    Rearrangement of 2-(4-Hydroxyalkyl)-1,3-dioxolanes to 2-Hydroxyethyl Alkanoic Esters by 1,5-Hydride Shift - An Unprecedented Intramolecular Redox Reaction (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3495-3499 
    Details
    ISSN: 1434-193X
    Keywords: Hydride abstraction ; Carbenium ions ; Rearrangement ; Intramolecular redox reaction ; Deuterium label ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2-(4-Hydroxyalkyl)-1,3-dioxolanes (3a/3b, 5-7) undergo an acid-catalyzed rearrangement to give 2-hydroxyethyl alkanoic esters 8-11. The postulated mechanism, proceeding via the benzylic carbenium ion B and the 1,3-dioxolan-2-ylium cation C, is supported by the stereochemistry of the reaction and the results of deuterium labeling experiments.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2046/1999/99248_s.pdf or from the author.
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  • 177
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    Configurational Stability and Stannylation of Dipole-Stabilised Cyclic Tertiary Benzylic α-Oxycarbanions, which Occurs with Retention or Inversion of Configuration Depending on R and X of R3SnX Used (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3511-3518 
    Details
    ISSN: 1434-193X
    Keywords: 2,4,6-Triisopropylbenzoates, 1-indanyl, 1,2,3,4-tetrahydro-1-naphthyl ; N,N-Diisopropylcarbamate, 1-indanyl ; Carbanions, configurational sStability ; Stereochemistry, stannylation ; Liquid chromatography, determination of ee ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2,4,6-Triisopropylbenzoates of 1-indanol and 1-tetralol (ee 98%) are deprotonated at -78 °C in hexane by sBuLi/TMEDA to give partly configurationally labile organolithium intermediates 5a and 5b, which are deuterated by MeOD with retention of configuration. These intermediates are stannylated by trimethyltin chloride with inversion as determined by lithiodestannylation followed by addition of MeOD to produce deuterated esters of low ee. Stannylation of (S)-5b with (-)-menthyldimethyltin bromide affords stannane 7 (de ≥ 95%) with inversion. The carbanion derived from (S)-1-indanyl N,N-diisopropylcarbamate (9) is configurationally stable. It reacts with trimethyltin chloride favouring inversion of configuration (ee up to 17%). Tributyltin chloride and tributyltin triflate yield stannanes 11b of opposite stereochemistry, the latter giving retention of configuration. Tributyltin bromide behaves similarly to the chloride, but the ee of the reaction product is only about 30%.
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  • 178
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    Palladium-Catalyzed Hydrovinylation of Vinyl Triflates with Alkynes An Approach to the Synthesis of 3-Vinylfuran-2(5H)-ones (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3305-3313 
    Details
    ISSN: 1434-193X
    Keywords: Palladium ; Hydrovinylation ; Cyclizations ; Furan-2(5H)-ones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of alkynes with vinyl triflates in the presence of catalytic amounts of Pd(OAc)2 and HCOOK, omitting phosphane ligands, affords hydrovinylation products usually in good to high yields. The reaction has been employed to develop a regioselective synthesis of 3-vinylfuran-2(5H)-ones from methyl 4-hydroxy-2-butynoates.
    Additional Material: 2 Tab.
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  • 179
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    New Chiral Catalysts Containing N,O-Heterocycles Derived from Chiral Amino Alcohols (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3323-3333 
    Details
    ISSN: 1434-193X
    Keywords: Chirality ; Catalysis ; Amino acids ; Crown ethers ; Heterocyclic compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The enantioselectivity exerted by a new series of chiral catalysts containing N,O-heterocycles of different sizes has been checked in the addition of diethylzinc to benzaldehyde, which was used as a model reaction. The catalysts were derived from natural amino acids, following a relatively simple procedure, and in several cases excellent ee values were obtained. The results were complementary since ee's ranged from 98% (R) to 94% (S) excesses of the final 1-phenylpropan-1-ol. Molecular mechanics calculations suggested that the production of the R alcohol may be explained by a mechanism similar to that described by Noyori, in which ZnEt2 interacts solely with the N-C-C-OH fragment, whereas the formation of the S enantiomer needed the direct participation of the lateral chain of the parent amino acid and the N,O-heterocycle.
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  • 180
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    Tetraacylethenes as Dienophiles and Hetero Dienes in Two-Step Diels-Alder Reactions (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3343-3351 
    Details
    ISSN: 1434-193X
    Keywords: C-C coupling ; Cycloadditions ; Dehydrogenations ; Electron deficiency ; Isomerisations ; Oxidations ; Zwitterionic intermediates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation with barium manganate of the enol of tetraacetylethane (10) affords tetraacetylethylene (7a) in good yield. Treatment of the 1,3-diketones 15a and b with iodosobenzene in the presence of boron trifluoride does not result in oxidative coupling yielding 10 and diacetyldibenzoylethane 17, respectively, as has been reported by Moriarty et al. Instead the known difluoroborylenolates 16 are formed. Oxidative coupling of the sodium enolate of 15b with iodine affords the meso-tetraketone meso-17 and diacetyldibenzoylethylene (E)-7b besides small amounts of rac-17 and the bisenol 19. In the presence of 1,4-diazabicyclo[2.2.2]octane, meso-17 and 19 tautomerise yielding identical equilibrium mixtures of meso-17, rac-17, and 19 (26:29:45). Dehydrogenation with 5,6-dichloro-2,3-dicyanobenzoquinone of meso-17 yields (E)-7b and an isomer (77:23) which was assigned structure (Z)-7b on the basis of spectroscopic evidence. - Tetraacetylethylene (7a) reacts with 1,2-dimethylenecyclohexane to afford two different [4+2]cycloadducts, viz. the spirocyclic dihydropyran 25 (30 %) and the tetraacetyloctalin 27. Whereas the latter is stable, the former isomerises to the latter on heating. Formation of both [4+2]cycloadducts and the isomerisation 25 → 27 are interpreted by invoking the zwitterion 26 as common intermediate. Monoolefines that cannot generate stable carbenium ions don't give [4+2]cycloadducts with 7a. In contrast, α-methylstyrene yields the oxa Diels-Alder product 30. The structures of (E)-7b, 19, 25, and 27 are established by X-ray diffraction analyses.
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  • 181
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    Regio- and Stereoselective Ene and Tandem “Ene-Cycloaddition” Reactions of 2,4-Dioxopentane-3-thione (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3375-3379 
    Details
    ISSN: 1434-193X
    Keywords: α,α′-Dioxothiones ; Ene reactions ; Cyclo additions ; Sulfur heterocycles ; Stereoselectivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2,4-Dioxopentane-3-thione 1a reacts as an enophile with various allyl derivatives affording thiophilic ene adducts as single regioisomers with high (E) stereoselectivity. Using allyl ethers or allenes as ene counterparts, the formation of the thiophilic adduct is followed by a fast cycloaddition reaction in which 1a behaves as a diene or a dienophile, respectively.
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  • 182
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    Transketolase-Mediated Synthesis of 4-Deoxy-D-fructose 6-Phosphate by Epoxide Hydrolase-Catalysed Resolution of 1,1-Diethoxy-3,4-epoxybutane (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3399-3402 
    Details
    ISSN: 1434-193X
    Keywords: Carbohydrates ; Enzyme catalysis ; Epoxide hydrolase ; Transketolase ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 4-Deoxy-D-fructose 6-phosphate is synthesised from nonnatural sources in four steps, including two enzymatic reactions. (3S)-1,1-Diethoxy-3,4-epoxybutane is first obtained by epoxide hydrolase-catalysed resolution. Opening of this epoxide by inorganic phosphate leads to 2-deoxy-D-erythrose 4-phosphate. In the last step, transketolase transfers a hydroxyacetyl group from L-erythrulose onto this aldehyde, controlling the second asymmetric center.
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  • 183
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    Synthesis of Modified Ingenol Esters (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3413-3420 
    Details
    ISSN: 1434-193X
    Keywords: Natural products ; Diterpenes ; Ingenol ; Antitumor agents ; Isosters ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthetic protocols for the manipulation of the polyhydroxylated southern region of ingenol (1a) were developed, and a series of isosteres of the anticancer compound ingenol 3,20-dibenzoate (1b) was prepared. The biological evaluation of these compounds showed that cytotoxicity was relatively tolerant to changes at C-20, while PKC activation was markedly affected by these modifications. These data suggest that chemical manipulation can effectively dissect cytotoxicity and tumour-promoting activity (or potential) of ingenoids, affording more optimal candidates for development, like 20-deoxy-20-fluoroingenol 3,20-dibenzoate (5b). In mild acidic medium, an unexpected vinylogous retro-pinacol rearrangement of ingenol to a tigliane derivative was observed.
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  • 184
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    Synthesis of Novel Functionalised Zinc Phthalocyanines Applicable in Photodynamic Therapy (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3441-3453 
    Details
    ISSN: 1434-193X
    Keywords: Zinc phthalocyanines ; Tetra- and octa(carboxyalkyl)phthalocyaninatozinc compounds ; Water soluble ; Photodynamic therapy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of several new phthalonitriles 3, 9, 14, 25, 33, and 36, functionalised with carboxyl groups, including two examples of amino acid derivatives is described. All new phthalonitriles were converted into their corresponding phthalocyaninatozinc compounds. The phthalocyanines, 2,3,9,10,16,17,23,24-octa(1-carboxyethyloxy)phthalocyaninatozinc (5), 2,9,16,23-tetra(2-amino-2-carboxyethyl)phthalocyaninatozinc (11), 2,9,16,23-tetra(1-carboxy-2-hydroxyethylaminocarbonyl)phthalocyaninatozinc (16), 1,8,15,22-tetra(carboxybutyl)phthalocyaninatozinc (27), 2,3,9,10,16,17,23,24-octa(carboxyalkyl)phthalocyaninatozinc (39), and the nonidentically substituted 9,10,16,17,23,24-hexa(carboxyalkyl)-2-[4-(N-succinimidyloxycarbonyl)butyl]phthaloyaninatozinc (41) are all sufficiently soluble in water. The nonidentically-substituted compounds are important due to their selective binding to tumor-selective antibodies. UV/Vis-spectroscopy was used to investigate the effect of more or less sterically-demanding substituents in the periphery of the phthalocyanines towards aggregation. The phototoxicity towards cancer cells of some of the new compounds was investigated in several in-vitro experiments.
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  • 185
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    Regio- and Stereoselective Opening of Oxiranes through Neighbouring Group Participation: Stereocontrolled Synthesis of Enantiopure Hydroxylated Oxazolidin-2-ones (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3483-3488 
    Details
    ISSN: 1434-193X
    Keywords: Epoxides ; Epoxide ring opening ; Neighbouring group assistance ; Oxazolidinones ; Aminodiols ; Pyrrolidinones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The regio- and stereo-selective ring opening of (S)-pyroglutaminol derived epoxides provides an effective route to protected syn,syn-aminodiol units. The procedure involves the chemoselective aminolysis or alcoholysis of (3R,4R,5R)-N-(tert-butoxycarbonyl)-3,4-epoxy-5-[(1-ethoxy)ethoxymethyl]pyrrolidin-2-one (10), followed by the formation in quantitative yield of oxazolidinone intermediates, through the mediation of neighbouring N-Boc groups. The practical synthetic interest of this route is illustrated by the example of (3R,4S,5R)-3,4-diacetoxy-5-(acetoxymethyl)pyrrolidin-2-one which should serve as useful building block in further syntheses.
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  • 186
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    The Regiochemistry of the Cycloaddition of 4-R-Phenacylpyridazinium Ylides to Nonsymmetrical Substituted Olefins (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3501-3504 
    Details
    ISSN: 1434-193X
    Keywords: Pyridazine ; Ylides ; Tetrahydropyrrolopyridazine ; Regiospecificity ; Structure ; Semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The regiochemistry of the [3+2] cycloaddition reactions between 4-R-phenacylpyridazinium ylides and acrylonitrile has been studied. Theoretical and experimental studies have been performed, both of which show that the reaction is regiospecific. Four new tetrahydropyrrolopyridazine heterocycles have been obtained. The structures of the new compounds were established by elemental (C,H,N) and spectral analyses (IR, 1H and 13C NMR, MS).
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  • 187
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    Efficient Stereocontrolled Access to 15- and 16-Hydroxy Steroids (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3505-3510 
    Details
    ISSN: 1434-193X
    Keywords: Polyhydroxy steroids ; Marine natural products ; D ring hydroxylation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Four epimeric 15- and 16-hydroxy steroids have been stereoselectively synthesized from epi-androsterone. The key intermediate is the 3β-[(tert-butyldimethylsilyl)oxy]-5α-23,24-bisnorchol-16-en-22-ol (10), which allows both efficient D-ring functionalization and the possibility of facile side-chain construction. In the course of this synthesis, we have found that the stereochemical outcome of the C-15 carbonyl reduction is strongly dependent on the C-16 and C-17 hybridization.
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  • 188
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    Intramolecular Donor-Assisted Cyclization of Organotin Compounds (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 887-898 
    Details
    ISSN: 1434-1948
    Keywords: Tin ; Phosphorus ; Intramolecular coordination ; O ligands ; Heterocycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New intramolecularly coordinated organotin compounds containing the monoanionic O,C,O-coordinating ligand {4-tert-Bu-2,6-[P(O)(OEt)2]2C6H2}- have been synthesized by substitution reactions starting from organotin halides. In view of the enhanced reactivity of the intramolecularly coordinated compounds {4-tert-Bu-2,6-[P(O)(OEt)2]2C6H2}SnR2R′ (2, R = Ph, R′ = CH2SiMe3; 3, R = R′ = Ph; 6, R = R′ = Cl), cationic tin species are suggested to occur as intermediates in the formation of the heterocyclic compounds [1(Sn),3(P)-Ph2SnOP(O)(OEt)-5-tert-Bu-7-P(O)(OEt)2]C6H2 (8), [1(Sn), 3(P)-Ph(Me3SiCH2)SnOP(O)(OEt)-5-tert-Bu-7-P(O)(OEt)2]C6H2 (15), and {[1(Sn),3(P)-Cl2SnOP(O)(OEt)-5-tert-Bu-7-P(O)(OEt)]C6H2}2 (16). The latter compounds are formed by intramolecular cyclizations of pentacoordinate cationic tin species under elimination of ethyl halide. Furthermore, the synthesis of [1(Sn),3(P)-Ph2SnOP(O)(OH)-5-tert-Bu-7-P(O)(OH)2]C6H2 (13) is described. Reaction of 8 with an excess of Me3SiBr leads to the unexpected formation of {2-[P(O)(OEt)(OSiMe3)]-4-tert-Bu-6-[P(O)(OEt)2]C6H2}SnPhBr2 (9) as a result of an O-Sn bond cleavage initiated by Me3SiBr and subsequent reaction of the intermediate with further Me3SiBr under Sn-C bond cleavage. The high donor capacity and the rigidity of the new ligand {4-tert-Bu-2,6-[P(O)(OEt)2]2C6H2}- are demonstrated by X-ray diffraction analyses of the tetraorganotin compound 2 and the monoorganotin trichloride 6. Furthermore, the molecular structures of the 2,3,1-oxaphosphastannoles 8 and 16 are discussed.
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  • 189
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    Carbynehydrido- and Vinylidenehydridoosmium Complexes with Os(PCy3)2 as a Molecular Unit (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 951-957 
    Details
    ISSN: 1434-1948
    Keywords: Osmium ; Alkynes ; Carbyne complexes ; Hydrido complexes ; Vinylidene complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of OsCl3·2H2O with PCy3 leads under reducing conditions either to [OsH6(PCy3)2] (1) or [OsH2Cl2(PCy3)2] (2). Treatment of 2 with terminal alkynes HC≡CR (R = Ph, SiMe3) affords the carbynehydrido complexes [OsHCl2(≡CCH2R)(PCy3)2] (4, 5), of which 5 (R = SiMe3) is easily converted with traces of water into [OsHCl2(≡CCH3)(PCy3)2] (6). Compound 4 (R = Ph) reacts with NaOMe to yield the vinylidenehydridoosmium(II) complex [OsHCl(=C=CHPh)(PCy3)2] (7) which upon treatment with HBF4/OEt2 gives the five-coordinate cationic species [OsHCl(≡CCH2Ph)(PCy3)2]-BF4 (8). The reaction of [OsH3Cl(PCy3)2] (9) with HC≡CC(CH3)2Cl affords a mixture of [OsHCl2(≡CCH=CMe2)(PCy3)2] (10) and [OsCl2(H2)(=CHCH=CMe2)(PCy3)2] (11). Compound 11 is quite labile and by elimination of H2 gives 10. The molecular structure of 10 has been determined by X-ray crystallography.
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  • 190
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    Synthesis, Structural Characteristics and Biological Activities of Complexes of ZnII, CdII, HgII, PdII, and PtII with 2-Acetylpyridine 4-Methylthiosemicarbazone (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 965-973 
    Details
    ISSN: 1434-1948
    Keywords: 2-Acetylpyridine 4-methylthiosemicarbazone ; Palladium ; Platinum ; Group-12 metal(II) complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of 2-acetylpyridine 4-methylthiosemicarbazone (H4ML) with halides of zinc(II), cadmium(II), and mercury(II) afforded complexes of the form [M(H4ML)X2] [M = ZnII (1-3), CdII (4-6) or HgII (7-9); × = Cl, Br, or I]. Reaction of H4ML with K2PdCl4 and K2PtCl4 gave compounds of the form [M(4ML)Cl] [M = PdII (10) or PtII (11)]. In all the new compounds, which were characterized by elemental analyses, conductance measurements, and electronic, IR and 1H- and 13C-NMR spectroscopy, and by 113Cd-, 195Pt-, or 199Hg-NMR spectroscopy when relevant, the ligand is N,N,S-tridentate, coordinating to the metal centre through its pyridine and azomethine nitrogen atoms and its thiocarbonyl sulfur atom, as was confirmed by X-ray diffraction studies in the cases of 4· 2 DMSO, 5· 2 DMSO, 6· 2 DMSO, 7· 2 DMSO, 10, and 11. In in-vitro assays, only [Zn(H4ML)Cl2] and [Zn(H4ML)Br2] showed some sign of antifungal activity against Aspergillus niger or Paecilomyces variotii.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2005/1999/98381_s.pdf or from the author.
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  • 191
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    A Molecular Mechanics Force Field for Alkylcobaloximes, a Model of Vitamin B12 Coenzyme - Implications of Steric and Electronic Factors in the Co-C Bond Cleavage (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 981-992 
    Details
    ISSN: 1434-1948
    Keywords: Cobaloximes ; Vitamin B12 ; Semiempirical calculations ; Force field ; Conformation analysis ; Cone angles ; Crystal structure ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular mechanics force field constants have been derived for alkylcobaloximes, (alkyl)Co(DH)2L (DH = monoanion of dimethylglyoxime and L = planar N-donor ligand), implementing the AMBER force field. Atomic charges have been calculated by the semiempirical ZINDO/1 method. Stretching and bending constants have been calculated by the Badger and Halgren equations, with the introduction of simple functions for the description of the electronic influence of the axial ligands on the coordination geometry. 26 parameters have been optimized, by the Simplex method, to fit 4523 bond lengths and angles of 52 alkylcobaloxime accurate crystal structures. In spite of the oversimplification of the adopted method for the description of the electronic effects, this approach provides a good description of the metal coordination geometries. The root-mean-square deviations of the calculated bond lengths and angles from the experimental values average to 0.023 Å and 1.4°, respectively. The molecular mechanics results are discussed in terms of the steric properties of the axial ligands and correlated with their calculated cone angles. The force field has been used to analyse some conformational features of these compounds, such as the influence of the rotation of the axial ligands on their coordination geometry. The calculated energy for the Co-N rotational barrier agrees well with the experimetal ones derived from dynamic and saturation 1H-NMR spectroscopy in (alkyl)Co(DH)2(2-NH2-py). In order to have more experimental data for this analysis, the crystal structures of two new cobaloximes, (CH2Cl)Co(DH)2L with L = py, 1, and 1-Me-Im, 2) have been determined. The results of a conformational analysis on ribosyl-imidazole derivatives, taken as a suitable vitamin B12 coenzyme model, suggest that the 1-Me-imidazole-like ligands have a significantly greater rotation freedom with respect to the benzimidazole-like ones, but cause similar stretchings of the Co-C bond, and a significantly less stretching of the Co-N one. Implications for the recent findings on the binding mode of the coenzyme B12 in the enzyme active site are discussed.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2005/1999/98399_s.pdf or from the author.
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  • 192
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    Thermochemical Study of Reduction and Oxidation Reactions of Molybdovanadophosphoric Heteropolyacids in Aqueous Solutions (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1009-1014 
    Details
    ISSN: 1434-1948
    Keywords: Heteropolyacids ; Molybdovanadophosphates ; Hydrazine hydrate ; Reduction ; Oxidations ; Thermochemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Differential enthalpies of reduction, -ΔrH4j, have been found at 343 K for 0.2 m aqueous heteropolyacids H3+xPVxMo12-xO40 (HPA-x, where 1 ≤ x ≤ 6) and their salts by hydrazine hydrate (HH). The -ΔrH4j values depend on x and m, where m = [VIV]/[HPA-x]. At m ≈ 0, the initial -ΔrH4j value is 436 ± 14 kJ mol-1 HH for all HPA-x. For HPA-x with 3 ≤ x ≤ 6, the curves -ΔrH4j = f(mj) have maxima associated with formation and reduction of the outer-sphere cation VVVIVO33+. A similar maximum is observed on reduction of 0.1 M VO2ClO4 in 4 M HClO4. No maxima are observed for the reduction of HPA-1, HPA-2, and salts of HPA-3. For HPA-3 salts, -ΔrH4j = f(mj) depends primarily on the pH of the salt solution, rather than on the nature of the cation. Reduction and oxidation of HPA-x and their salts are reversible reactions. In HPA-x solutions, VO2+ in the outer sphere is reduced first, and VV in the inner coordination sphere of the heteropoly anion is reduced next. Differential enthalpies for the oxidation of the reduced HPA-x by dioxygen (-ΔrH2j) have been calculated. The sequence of the oxidation of different VIV forms in the reduced HmHPA-x solutions is the reverse of that for the reduction of the VV forms in the initial HPA-x solutions.
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  • 193
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    Syntheses and Characterization by NMR Spectroscopy and X-ray Diffraction of Complexes Derived from Metals of Groups 2 and 13 and the Ligand Bis(3,5-di-tert-butyl-1-hydroxy-2-phenyl)amine (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1021-1027 
    Details
    ISSN: 1434-1948
    Keywords: Alkaline earth metals ; Group-13 metals ; Coordination chemistry ; Bis(3,5-di-tert-butyl-1-hydroxy-2-phenyl)amine ligand ; Nonacoordinated strontium compound ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of 3,5-di-tert-butylcatechol and aqueous ammonia with boron, aluminum, gallium, or strontium chloride, or with calcium or barium acetate produce, under oxidizing conditions, neutral complexes of stoichiometric formulae ML (M = BCl), ML2, [M = Al (2), Ga (3), Ca (4), and Ba (5)], and ML3H [M = Sr (6)] [L = different oxidation states of bis(3,5-di-tert-butyl-1-hydroxy-2-phenyl)amine]. The structures of 1 and 4-6 were established by 1H- and 13C-NMR spectroscopy and of 2, 3, and 6 by X-ray diffraction. The coordination compound ClBL (1) was obtained by transmetallation of ZnL2 with BCl3. It is a tetracyclic compound with tetracoordinated boron and nitrogen atoms, the ligand being in the reduced form, bis(hydroxyphenyl)amine. The paramagnetic complexes 2 and 3 crystallized in the triclinic system and are hexacoordinated with two orthogonal planar ligands bonded to the metal atoms. The ligands appear in two different oxidation states: one as the monoanion and another as a radical dianion. Based on their X-ray diffraction molecular structures, it is deduced that both ligands have similar structures, confirming electronic delocalization and fast interconversion of their oxidation states. Ca and Ba complexes are diamagnetic and hexacoordinated, both ligands being monoanions The strontium complex crystallized in the monoclinic system, the metal atom is nonacoordinated, having two ligands as monoanions, and a third one is a neutral protonated diquinone. Its structure is a distorted helix with three paddles in a C3 geometry and its unexpected diamagnetic behaviour is due to the presence of a reduced protonated ligand.
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  • 194
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    Synthesis, Structure and Reactivity of 3,4-Dihydro-2H-1,2,4,3-triazaboroles (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1193-1198 
    Details
    ISSN: 1434-1948
    Keywords: Boron ; Triazaboroles ; Heterocycles ; Halogens ; Cyanides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 3,4-Dihydro-2,4-diphenyl-2H-1,2,4,3-triazaboroles 3a, 3b, and 4 were synthesized by cyclocondensation of N1,N3-diphenylformamidrazone (1) with dibromophenylborane, dibromomethylborane, and boron trichloride. 3-Chloro-3,4-dihydro-2,4-diphenyl-2H-1,2,4,3-triazaborole (4) was converted into 3,4-dihydro-2,4-diphenyl-2H-1,2,4,3-triazaborole (5) by treatment with LiAlH4. The corresponding 3-cyanato and 3-cyano derivatives 6 and 7 resulted from the reaction of 4 with AgOCN and AgCN, respectively. Compound 7 was transformed into the bis(1,2,4,3-triazaborolyl)oxane 8 by silver oxide. Compounds 1-8 were characterized by elemental analyses and spectroscopic methods (1H, 11B, and 13C NMR; IR; MS). The molecular structure of 8 was established by single-crystal X-ray diffraction analysis.
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  • 195
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    Combined Quantum Chemical and Mass Spectrometry Study of [Ge,C,H]+ and Its Neutral Counterpart (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1203-1210 
    Details
    ISSN: 1434-1948
    Keywords: Organogermanium ions ; Mass spectrometry ; Ab initio theory ; NBO ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cation [Ge,C,H]+ has been generated by the electron ionisation of trichloromethylgermane. Collisional activation experiments were used to establish a Ge-C-H connectivity in this species, for which a significant fraction of the ion population was found to survive neutralisation-reionisation mass spectrometry (NRMS) experiments. Thus, the neutral counterpart [Ge,C,H]0 is stable on a microsecond timescale. Becke's 3 parameter hybrid density functional (B3LYP) was used to map the ion and neutral potential-energy surfaces, in conjunction with double-zeta and triple-zeta basis sets. The computational results obtained using the triple-zeta basis sets suggest that, for the cation, the global minimum is the high spin 3Σ GeCH+, with the first Ge-C-H excited state, 1Σ GeCH+, approximately 39 kcal mol-1 less stable. The lowest energy ion structure with H-Ge-C connectivity is bent (3A′′HGeC+,∠H-Ge-C = 126.3) and 69 kcal mol-1 less stable than the global minimum. For the neutral, a doublet (2π) with Ge-C-H connectivity is predicted to be the global minimum. The classical barrier for the neutral 1,2-hydrogen shift reaction on the doublet surface is negligible (0.1 kcal mol-1), while the smallest barrier for the cation is 13.0 kcal mol-1, corresponding to (3A′′) HGeC+ → (3Σ) GeCH+. Natural bond order analysis has been used to establish the order of the metal-carbon bond for selected states of both the neutral and the ion. Neutral and cationic isomers with Ge-C triple bonds were found to be high-energy excited states, with the metal-carbon bonds in the cation and neutral ground states of order 2.0 and 2.5, respectively. The instability of Ge-C triple bonded species is attributed to the energy required for electronic promotion in the metal in order to achieve a hybrid configuration suitable for the formation of such a bond.
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    Linear Five-Centred Chromium Multiple Bonds Bridged by Four tpda2- Ligands [tpda2- = tripyridyldiamido dianion] - Synthesis and Structural Studies (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1243-1251 
    Details
    ISSN: 1434-1948
    Keywords: Metal string complexes ; Multicentered metal-metal multiple bond ; Quadruple bonds ; Metal-metal interactions ; Chromium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The linear pentanuclear chromium complexes [CrII5(μ5-tpda)4Cl2] (1), [CrII5(μ5-tpda)4(NCS)2] (2), [CrIIICrII4(μ5-tpda)4F2](BF4) (3), and [CrIIICrII4(μ5-tpda)4F(OTf)](OTf) (4), with four all-syn tri(α-pyridyl)diamido dianion (tpda2-) ligands, have been prepared and structurally characterized. Compounds 1 and 2 possess a delocalized Cr(II)-Cr(II)-Cr(II)-Cr(II)-Cr(II) five-centred metal-metal bond of order 1.5. In both 1 and 2 two values for CrII-CrII bond lengths are found both; the outer ones connected with axial ligands are 2.284(1) and 2.285(2) Å, and the inner ones are 2.2405(8) and 2.246(1) Å, for 1 and 2, respectively. When compound 1 reacts with 2 equiv. of AgBF4 or Ag(OTf), a oxidation reaction takes place and one of the terminal chromium(II) ions is oxidized to produce [CrIIICrII4(μ5-tpda)4F2]BF4 (3) or [CrIIICrII4(μ5-tpda)4F(OTf)](OTf) (4). Two short Cr-Cr distances [1.969(2) and 2.138(2) Å for 3, 1.846(1) and 1.922(1) Å for 4] are found, with the presence of two quadruple bonds among four adjacent CrII ions. The fifth CrIII ion, which is separated from the neighboring CrII ion by 2.487(2) Å for 3 and 2.610(1) Å for 4, is simply a square pyramidal unit with no metal-metal bonding interaction.
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    Synthesis and Characterization of the Heterometallic Aggregate Pb2Al5(μ3-O)(μ4-O)(μ-OiPr)9(OiPr)3(μ-OAc)3 (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1291-1293 
    Details
    ISSN: 1434-1948
    Keywords: Aluminum ; Lead ; O ligands ; Alkoxy carboxylates ; Structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel heterometallic aggregate Pb2Al5(μ3-O)(μ4-O)(μ-OiPr)9(OiPr)3(μ-OAc)3 obtained from the interaction of Pb(OAc)2 and Al(O-iPr)3 is the first structurally characterized example based on lead and aluminum. This compound has been isolated in high yield and examined by 1H-, 13C-, and 27Al NMR, and in the solid state by X-ray crystallography.
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    Pyrazolato Metal Complexes - Synthesis, Characterization, and X-ray Crystal Structures of Heterotrimetallic Re-M2 Derivatives (M = Cu, Ag, Au) (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1301-1307 
    Details
    ISSN: 1434-1948
    Keywords: Pyrazolato ligands ; Rhenium ; Group-11 metals ; Heterometallic complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: By treating [(CO)3Re(Hpz)2(pz)] (Hpz = pyrazole) with [Cu(CH3CN)4]BF4, AgNO3, or [Au(tht)Cl] (tht = tetrahydrothiophene) in acetonitrile in the presence of Et3N, the corresponding trinuclear complexes {(CO)3Re(pz)3[M(CH3CN)]2} (1a-c) have been obtained. Treatment of complexes 1a-c with cyclohexyl isocyanide (c-C6H11NC) afforded the derivatives {(CO)3Re(pz)3[M(c-C6H11NC)]2} (2a-c). Complexes 2a-c have been characterized in solution by 1H-NMR spectroscopy and in the solid state by single-crystal X-ray diffraction analysis.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 199
    Electronic Resource
    Electronic Resource
    Synthesis and Investigations on Nitrido(phthalocyaninato)rhenium Complexes (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1309-1313 
    Details
    ISSN: 1434-1948
    Keywords: Phthalocyanines ; Rhenium ; Nitrido(octa-n-alkylphthalocyaninato)rhenium compounds ; Nucleophilic additions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactions of nitrido(tetra-tert-butylphthalocyaninato)rhenium (1) with boron tribromide leading to (tBu4Pc)ReNBBr3 (4) and with acetone to give the imido complex (tBu4Pc)Re[NC(CH3)2CH2C(O)CH3]OH (2b) and its μ-oxo dimer 3 are reported. Starting from the corresponding 4,5-di-n-alkylphthalonitriles and ammonium perrhenate four soluble nitrido(octa-n-alkylphthalocyaninato)rhenium complexes 5-8 were synthesized. Nitrido(octa-n-pentylphthalocyaninato)rhenium (6) was treated with boron tribromide to afford [(C5H11)8Pc]ReNBBr3 (9).
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 200
    Electronic Resource
    Electronic Resource
    Synthesis, Chemistry, and Characterization of Perfluoroaromatic Selenium Derivatives (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1359-1366 
    Details
    ISSN: 1434-1948
    Keywords: Selenium ; Fluorine ; Multinuclear NMR ; Raman spectroscopy ; X-ray crystallography ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of perfluoroaryllithium RC6F4Li, where R = F or 4-CF3C6F4O, respectively, with selenium gives the known diselanes (RC6F4Se)2 (1a, 1b). Redox reactions of 1 with hydrogen peroxide result in the formation of the seleninic acids RC6F4SeOOH which crystallize as hydrates (2a, 2b); with mercury give the bis(arylseleno)mercuries (RC6F4Se)2Hg (3a, 3b); with sulfuryl chloride or bromine give the selenenyl chlorides (4a, 4b) or selenenyl bromides (5a, 5b). Selenenyl chlorides (4a, 4b) react with a variety of trimethylsilyl reagents Me3SiX (X = Br, CN, NMe2, NEt2) to form 5a, 5b; selenocyanates RC6F4SeCN (6a, 6b); selenenyl amides RC6F4SeNMe2 (7a, 7b) and RC6F4SeNEt2 (8a, 8b). A new synthetic route to diorgano selanes is developed by reaction of 4a, 4b with perfluoroaryllithium to give the symmetric (RC6F4)2Se (9a, 9b). All derivatives are thoroughly characterized and in addition the molecular structures of 2a, 6a, and 9a are established by X-ray crystallography.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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