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  • 1995-1999  (539)
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  • 1996  (539)
  • Organic Chemistry  (539)
  • Nuclear reactions
  • 101
    Electronic Resource
    Electronic Resource
    Detection of Noncovalent Complexes of Hydroxamic-Acid Derivatives by means of electrospray mass spectrometry (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1701-1709 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The complexes of isolated benzoxazinone derivatives with FeIII were investigated by means of electrospray ionization mass spectrometry. The hydroxamic-acid derivatives could be differentiated from the lactam or the methyl-hydroxamate derivatives by the formation of FeIII complexes with two or three ligands. These complexes or adducts of them have been identified in the mass spectra. Moreover, the mass-spectral behavior of these complexes was not markedly influenced by the presence of a β-D-glucopyranosyloxy substituent.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790620
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  • 102
    Electronic Resource
    Electronic Resource
    Polymer- and Dendrimer-Bound Ti-TADDOLates in Catalytic (and Stoichiometric) Enantioselective Reactions: Are pentacoordinate cationic Ti complexes the catalytically active species? (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1710-1740 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: α,α,α′,α′-Tetraaryl-1,3-dioxolane-4,5-dimethanols (TADDOLs), containing styryl groups either at C(2) of the heterocyclic ring or in the α-position, were prepared in the usual way (18-22, 24, 25). These compounds were copolymerized with styrene and divinylbenzene in a suspension, yielding polymers (33-40, Scheme 3) as beads with a rather uniform particle-size distribution (150-45 μm), swellable in common organic solvents. HOCH2- and BrCH2-substituted TADDOLs were also prepared and used for attachement to Merrifield resin or to dendritic molecules (23, 26-32). The TADDOL moieties in these materials are accessible to form Ti (and Al) complexes (Scheme 4) which can be used as polymer- or dendrimer-bound reagents (stoichiometric) or Lewis acids (catalytic). The reactions studied with these new chiral auxiliaries are: enantioselective nucleophilic additions to aldehydes (of R2Zn and RTi(OCHMe2)3; Scheme 5, Table 1) and to ketones (of LiAlH4, Table 2); enantioselective ring opening of meso-anhydrides (Scheme 6); [4+2] and [3+2] cycloadditions of 3-crotonyl-1,3-oxazolidin-2-one to cyclopentadiene and to (Z)-N-benzylidenephenylamine N-oxide ( → 48, 49, Scheme 7, Tables 3, 4, and Fig. 5). The enantioselectivities reached with most of the polymer-bound or dendritic TADDOL ligands were comparable or identical to those observed with the soluble analogs. The activity of the polymer-bound Lewis acids was only slightly reduced as compared with that encountered under homogeneous conditions. Multiple use of the beads (up to 10 times), without decreased performance, has been demonstrated (Figs. 3 and 4). The poorer selectivity in the Diels-Alder reaction (Scheme 7a), induced by the polymer-bound Cl2Ti-TADDOLate as compared to the soluble one, is taken as an opportunity to discuss the mechanism of this Lewis-acid catalysis, and to propose a cationic, trigonal-bipyramidal complex as the catalytically active species (Fig. 6). It is suggested that similar cations may be involved in other Ti-TADDOLate-mediated reactions as well.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790621
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  • 103
    Electronic Resource
    Electronic Resource
    Synthesis, Separation, and Characterization of Optically Pure C76 Mono-Adducts (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1741-1756 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nucleophilic Bingel cyclopropanation of D2-C76 with bis[(S)-1-phenylbutyl] 2-bromomalonate in toluene in the presence of base yielded three constitutionally isomeric pairs of diastereoisomeric mono-adducts together with one other constitutional isomer. All seven mono-adducts were isolated in optically pure form by prep. HPLC on a (S,S)-Whelk-O1 chiral stationary phase. They represent the first optically pure adducts of an inherently chiral fullerene. Characterization by UV/VIS, CD, 13C- and 1H-NMR spectroscopy allowed identification of pairs of stereoisomers and symmetry assignments: the two pairs of diastereoisomers which were isolated as the major product possess C1 symmetry, whereas the third pair of diastereoisomers, which is a minor product, is C2-symmetrical. The circular dichroism spectra of the optically active C76-adducts showed very pronounced Cotton effects resulting from strong chiroptical contributions of the chiral fullerene chromophore with the maximum observed Δε values being twice as high than those previously measured for optically active adducts of achiral fullerenes with a chiral addition pattern. Whereas the regioselectivity of mono-additions to C70 correlates with the degree of local bond curvature and the regioselectivity of multiple Bingel cyclopropanations of C60 with electronic parameters such as coefficients of the lowest unoccupied molecular orbital (LUMO), no such simple predictive correlations exist for the nucleophilic addition to C76. Despite full spectral characterization, an unambiguous structural assignment of the isolated compounds was not possible, except for the two C2-symmetrical isomers. Based on considerations of local bond curvature and the previous experiences with the chemistry of C70, the structures of the C2-symmetrical stereoisomers were assigned as (S,S,fC)-3 and (S,S,fA)-3, resulting from addition to the polar α-type C(1)—C(6) bond.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790702
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  • 104
    Electronic Resource
    Electronic Resource
    Synthesis and Structural Characterization of the Trimeric Furoxan (= Furazan 2-Oxide) System, a New Potent Vasodilating Moiety (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1803-1817 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis, structural characterization, and NO donor properties of a series of terfuroxan (= terfurazan trioxide) derivatives 1a-h and 2a-j are reported (Schemes 1 and 2). Structural assignments were confirmed principally by mass and 13C- and 1H-NMR spectroscopy. The extent and the initial rate of NO release in the presence of thiol cofactor was evaluated for each derivative. Vasodilator effects of all the terfuroxan derivatives were evaluated on endothelium-denuded strips of rat aorta precontracted with noradrenaline. Concentration-response curves were also evaluated in the presence of 10 m̈M oxyhemoglobin (HbO2), a well known NO scavenger. The whole series displays high vasodilating potency; the most active terfuroxans (1a, b, g and 2i) are 5-10 times as potent as glyceryl trinitrate taken as reference (see Table 3). The potency decrease observed in the presence of HbO2 agrees with the involvement of NO in the vasorelaxing action. The 4,3′:4′,4″ connection (series 1; furoxan numbering) gives rise to the most potent compounds. The conformational factors seem to play important roles in the activity. No clear relationship between physico-chemical properties of the substituents and potencies of derivative emerges.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790706
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  • 105
    Electronic Resource
    Electronic Resource
    Probing the Helical Secondary Structure of Short-Chain β-PeptidesDedicated to Professor Teruaki Mukaiyama, a dear friend and revered colleague on the occasion of his 70th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2043-2066 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structural prerequisites for the stability of the 31 helix of β-peptides can be defined from inspection of models (Figs. 1 and 2): lateral non-H-substituents in 2- and 3-position on the 3-amino-acid residues of the helix are allowed, axial ones are forbidden. To be able to test this prediction, we synthesized a series of heptapeptide derivatives Boc-(β-HVal-β-HAla-β-HLeu-Xaa-β-HVal-β-HAla-β-HLeu)-OMe 13-22 (Xaa = α- or β-amino-acid residue) and a β-depsipeptide 25 with a central (S)-3-hydroxybutanoic-acid residue (Xaa = -OCH(Me)CH2C(O)-) (Schemes 1 3). Detailed NMR analysis (DQF-COSY, HSQC, HMBC, ROESY, and TOCSY experiments) in methanol solution of the β-hexapeptide H(-β-HVal-β-HAla-β-HLeu)2-OH (1) and of the β-heptapeptide H-β-HVal-β-HAla-β-HLeu-(S,S)-β-HAla(αMe)-β-HVal-β-HAla- β-HLeu-OH (22), with a central (2S,3S)-3-amino-2-methylbutanoic-acid residue, confirm the helical structure of such β-peptides (previously discovered in pyridine solution) (Fig.3 and Tables 1-5). The CD spectra of helical β-peptides, the residues of which were prepared by (retentive) Arndt-Eistert homologation of the (S)- or L-α-amino acids, show a trough at 215 nm. Thus, this characteristic pattern of the CD spectra was taken as an indicator for the presence of a helix in methanol solutions of compounds 13-22 and 25 (including partially and fully deprotected forms) (Figs.4-6). The results fully confirm predicted structural effects: incorporation of a single ‘wrong’ residue ((R)-β-HAla, β-HAib, (R,S)-β-HAla(α Me), or N-Me-β-HAla) in the central position of the β-heptapeptide derivatives A (see 17, 18, 20, or 21, resp.) causes the CD minimum to disappear. Also, the β-heptadepsipetide 25 (missing H-bond) and the β-heptapeptide analogs with a single α-amino-acid moiety in the middle (13 and 14) are not helical, according to this analysis. An interesting case is the heptapeptide 15 with the central achiral, unsubstituted 3-aminopropanoic-acid moiety: helical conformation appears to depend upon the presence or absence of terminal protection and upon the solvent (MeOH vs. MeOH/H2O).
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790802
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  • 106
    Electronic Resource
    Electronic Resource
    Nucleosides. Part LXI.Part LX: [1]. A Simple Procedure for the Monomethylation of Protected and Unprotected Ribonucleosides in the 2′-O- and 3′-O-Position Using Diazomethane and the Catalyst Stannous Chloride (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2114-2136 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Intensive studies on the diazomethane methylation of the common ribonucleosides uridine, cytidine, adenosine, and guanosine and its derivatives were performed to obtain preferentially the 2′-O-methyl isomers. Methylation of 5′-O-(monomethoxytrityl)-N2-(4-nitrophenyl)ethoxycarbonyl-O6-[2-(4-nitrophenyl)ethyl]-guanosine (1) with diazomethane resulted in an almost quantitative yield of the 2′- and 3′-O-methyl isomers which could be separated by simple silica-gel flash chromatography (Scheme 1). Adenosine, cytidine, and uridine were methylated with diazomethane with and without protection of the 5′ -O-position by a mono- or dimethoxytrityl group and the aglycone moiety of adenosine and cytidine by the 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) group (Schemes 2-4). Attempts to increase the formation of the 2′-O-methyl isomer as much as possible were based upon various solvents, temperatures, catalysts, and concentration of the catalysts during the methylation reaction.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790808
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  • 107
    Electronic Resource
    Electronic Resource
    Rarisetenolide, Epoxyrarisetenolide, and Epirarisetenolide, New-Skeleton Sesquiterpene Lactones as Taxonomic Markers and Defensive Agents of the Marine Ciliated Morphospecies Euplotes rariseta (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2180-2189 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rarisetenolide (3) and epoxyrarisetenolide (4), new-skeleton α,β-conjugated sesquiterpene lactones, were isolated from various strains of the unicellular ciliated protist Euplotes rariseta collected from marine coasts widely far apart from each other: northern and southern Australia, southern Brazil, and Canary Islands. A strain of E. rariseta from New Zealand gave epirarisetenolide (5) instead, revealing a subtle variability in secondary metabolism for this ciliated morphospecies. Nonetheless, these metabolites - which are the first non-aldehydic terpenoids so far isolated from ciliates - represent a unique whole that constitutes a reliable taxonomic tool at the morphospecies level. Epirarisetenolide (5) and rarisetenolide (3), in this order, showed higher toxicity towards nonproductive ciliates than the chemically more reactive natural epoxide 4 and the semisynthetic aldehyde/protected-aldehyde froms 8/7a/7b. This inverse trend of biological vs. chemical effectiveness suggests that these cytotoxic agents interact noncovalently with membrane receptors.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790813
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  • 108
    Electronic Resource
    Electronic Resource
    A Tricyclic Template Derived from (2S,4R)-4-Hydroxyproline for the Synthesis of Protein Loop Mimetics (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2235-2248 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A tricyclic diketopiperazine, formally derived by coupling (2S,4S)-4-aminoproline (Pro(NH2)) and (2S,4R)-4-(carboxymethyl)proline (Pro(CH2COOH)), is synthesized starting from readily available (2S,4R)-4-hydroxyproline. The resulting tricyclic template has carboxy and amino groups to which a peptide chain may be attached. The Fmoc-protected template 5 is incorporated into the cyclic molecule cyclo(-Ala1-Asn2-Pro3-Asn4-Ala5-) (6) where Pro(NH2)7 = Pro(CH2COOH)8 represents the template, using solid-phase peptide synthesis with cyclization in solution. The molecule is shown by NMR and dynamic simulated annealing methods to adopt a preferred conformation in aqueous solution, which includes an extended backbone at the residues Asn2-Pro3-Asn4, and a type-Iβ-turn at . These studies show that this novel template may be used in the synthesis of cyclic peptide and protein mimetics having defined secondary structure in aqueous environments.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790817
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  • 109
    Electronic Resource
    Electronic Resource
    Masthead (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790701
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  • 110
    Electronic Resource
    Electronic Resource
    Synthesis and Enzymatic Evaluation of Substrates and Inhibitors of β-Glucuronidases (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1757-1784 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The phosphono and the tetrazolyl analogues 4 and 5 of 4-methylumbelliferyl β-D-glucuronide (=(4-methyl-2-oxo-2H-1-benzopyran-7-yl β-D-glucopyranosid)uronic acid; 6) were synthesized and evaluated as substrates of β-glucuronidases. Similarly, the phenylcarbamate 7 and its phosphono analogue 8 were prepared and evaluated as inhibitors. To examine the diastereoselectivity of the phosphorylation, we also synthesized the protected L-ido-D-gluco-, and D-galacto-configurated phospha-glycopyranuronates 12, 13, 21, 22, 34 and 35. Two strategies were followed. In the first one, the glucuronic acid 19 was decarboxylated to 11 and further transformed, via 20, into the trichloroacetimidate 10 (Scheme 2). Phosphorylation of 10 with (MeO)3P yielded the diastereoisomers 12 and 13, the diastereoselectivity depending on the solvent. In MeCN, 12 and 13 were obtained in a ratio of 1:1, while in non-participating solvents the L-ido 12 was by far the major diastereoisomer. The acetate 11 was inert to (MeO)3P, but reacted with (PhO)3P to the anomeric mixture 21/22, in keeping with a stabilizing 1,3-interaction in the intermediate phosphonium salt. Similarly, the phospha-galacturonates 34 and 35 were prepared from the galactoside 23 via the enol ether 26, the lactone 27, and the acetates 28/29 that were also transformed into the trichloroacetimidate 33 (Scheme 3). In the second, higher-yielding strategy, phosphorylation of the pentodialdehyde 39 to 40/41 was followed by hydrolysis and acetylation to the phospha-glucuronates 43/44 (Scheme 4). Transesterification to 45/46, selective deacetylation to 48/49, and formation of the trichloroacetimidates 50/51 were followed by glycosidation and deprotection to 4. The tetrazole 5 was prepared from the lactones 54/55 via the N-benzylamides 57/58 that were treated with TfN3 to give the N-benzyltetrazoles 59/60 (Scheme 4). These were transformed into the trichloroacetimidates 63/64, glycosylated to 65, and deprotected. The O-carbamoylhydroximo-lactone 7 derived from the glucuronate 67/68, and the phosphonate analogue 8 were prepared by established methods. The phosphonate 4 is slowly hydrolyzed by the E. coli β-glucuronidase, but neither 4 nor the tetrazole 5 are affected by the bovine liver β-glucuronidase (Table 4). The phenylcarbamate 7 of D-glucarhydroximo-1,5-lactone, but not its phosphonate analogue 8, is an inhibitor (KI = 8 m̈M) of the E. coli β-glucuronidase. The bovine liver β-glucuronidase is inhibited strongly by 7 (IC50 = 0.2 m̈M) and weakly by 8 (IC50 = 2mM).
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790703
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  • 111
    Electronic Resource
    Electronic Resource
    Protein-Loop Mimetics: A Diketopiperazine-Based Template to Stabilize Loop Conformations in Cyclic Peptides Containing the NPNA and RGD Motifs (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1825-1842 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We explore here an approach to mimic the structures and biological functions of protein loops in small synthetic molecules, by grafting the loop of interest onto an organic template comprising a bicyclic diketopiperazine, prepared by the formal coupling of (2S,4S)-4-aminoproline (Pro(NH2)) and aspartic acid (Asp). The Fmoc-protected template 4 is used to prepare cyclo(-Ala1-Asn2-Pro3-Asn4-Ala5- Ala6-Temp-) (5) and cyclo(-Ala1-Arg2-Gly3-Asp4-Temp-) (6) (where Temp = template derived from 4), containing the Asn-Pro-Asn-Ala (NPNA) and Arg-Gly-Asp (RGD) motifs. The conformational properties of these molecules are studied in aqueous solution by NMR and simulated-annealing methods. The NPNA motif, an immunodominant epitope on the circumsporozoite surface protein of the malaria parasite Plasmodium falciparum, is shown to adopt a stable type-I β-turn in 5. The template in 5 adopts a preferred conformation with Pro(NH2)χ1 ≍ -35° and the Asp moiety χ1 ≍ 70°. A different template conformation is inferred for 6, with Pro(NH2)χ1 ≍ 0°, but the ARGD loop appears by NMR to undergo rapid conformational averaging. Solid-phase binding assays reveal that 6 displays modest antagonist activity towards both the integrin αIIbβ3 and αvβ3 receptors.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790708
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  • 112
    Electronic Resource
    Electronic Resource
    Synthesis, Molecular Recognition, and Enzymology of Oligonucleotides Containing the Non-standard Base Pair between 5-Aza-7-deazaisoguanine and 6-Amino-3-methylpyrazin-2(1H)-one, a Donor-Acceptor-Acceptor Purine Analog and an Acceptor-Donor-Donor Pyrimidine Analog (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1881-1898 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A 6-aminopyrazin-2(1H)-one (pyADD), when incorporated as a pyrimidine-base analog into an oligonucleotide chain, presents a H-bond acceptor-donor-donor pattern to 5-aza-7-deazaisoguanine (puDAA), the complementrary donor-acceptor-acceptor purine analog. Reported here are the syntheses of the phosphoramidite of the 2′-deoxyribonucleoside bearing the puDAA base, oligonucleotides containing this nucleoside unit, the enzyme-assisted synthesis of oligoribonucleotides containing the pyADD ribonucleoside, and the molecular-recognition properties of this non-standard base pair in an oligonucleotide context. A series of melting experiments suggests that the pyADD · puDAA base pair contributes to the relative stability of a duplex structure approximately the same as an A · T base pair, and significantly more than mismatches between these non-standard bases and certain standard nucleobases. The pyADD nucleoside bisphosphate is accepted by T4 RNA ligase, but the triphosphate of the pyADD nucleoside was not incorported by T7 RNA polymerase opposite the puDAA nucleobase in a template. Oligonucleotides containing the pyADD base slowly undergo a reversible first-order reaction, presumably an epimerization process to give the α-D-anomer. These experiments provide the tools for laboratory-based use of the pyADD · puDAA base pair as a component of an oligonucleotide-like molecular-recognition system based on an expanded genetic alphabet.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790711
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  • 113
    Electronic Resource
    Electronic Resource
    Stereoselektive Synthese von 2′-O-(2-Methoxyethyl)ribonucleosiden: Nachbargruppenbeteiligung der Methoxyethoxy-Gruppe bei der Ribosylierung von Heterocyclen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1930-1938 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective Synthesis of 2′-O-(2-Methoxythyl)ribonucleosides: Neighboring-Group Participation of the Methoxythoxy Group in the Ribosylation StepA new access to 2′-O-(2-methoxyethyl)ribonucleosides, building blocks for second-generation antisense oligonucleotides, is presented. The influence of various reaction parameters on the coupling reaction of 2-O-(2-methoxyethyl)-D-ribose derivatives with heteroaromatic bases as the key step was investigated, and reaction conditions were optimized with regard to formation of the desired β-D-anomers. With Sn2+ salts as promotors in polar solvents, these β-D-anomers were formed with a high degree of steroselectivity.
    Additional Material: 4 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790716
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  • 114
    Electronic Resource
    Electronic Resource
    Synthesis of 4′-C-Acylated Thymidines (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1980-1994 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two synthetic pathways towards 4′-C-acylthymidines are presented. These modified mononucleosides are precursors of the 2′-deoxyribonucleotide 4′-C-radical. They were converted into their corresponding 3′-O-[(2-cyanoethyl) N,N-diisopropylphosphoramidites] 3a-c and incorporated in oligonucleotides by solid-phase synthesis. The structure of some modified nucleosides was revealed by X-ray crystal-structure analysis.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790720
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  • 115
    Electronic Resource
    Electronic Resource
    Oligosaccharide Analogues of Polysaccharides. Part 9. Synthesis and Thermolysis of Acetylenosaccharide-Derived 1,2-Dialkynylbenzenes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2004-2022 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermolysis of the 1,2-bis(glucosylalkynyl)benzenes 6 and 16 was studied to evaluate the effects of intramolecular H-bonding on the activation energy of the Bergman-Masamune-Sondheimer cycloaromatization, and to evaluate the use of the cycloaromatization for the synthesis of di-glycosylated naphthalenes. The dialkynes were prepared by cross-coupling of the O-benzylated or O-silylated glucosylalkynes 1 and 4 (Scheme 1). Thiolysis of the known 1, or acetolysis of 1, followed by deacetylation (→2→3) and silylation gave 4. Cross-coupling of 1 or 4 with iodo- or 1,2-diiodobezene depended upon the nature of the added amine and on the protecting group, and led to the mono- and dialkynylbenzenes 5 and 6, or 12, 13, and 15, respectively. The benzyl ethers 5 and 6 gave poor yields upon acetolysis catalyzed by BF3 · OEt2, while Ac2O/CoCl2 · 6 H2O transformed 6 in good yields into the regioselectively debenzylated 10. Desilylation of 7 and 13 gave the alcohols 8 and 14, respectively. Thermolysis of 6 in PhCl gave 22 and 23, independently of the presence or absence of 1,4-cyclohexadiene; 23 was formed from 22 (Scheme 2). Acetolysis of 22 gave the hexaacetate 24 that was completely debenzylated by thiolysis, yielding the diol 26 and trans-stilbene, evidencing the nature and position of the bridge between the glucosyl moieties (Scheme 3). Thiolysis of 22 yielded the unprotected 2,3-diglucosylnaphthalene 28, a new type of C-glycosides. Depending upon conditions, hydrogenation of 22 led to 29 (after acetylation), 30, or 32. NMR and particularly NOE data evidence the threo-configuration of the bridge. The structure of 23 was confirmed by hydrolysis to the diol 34 and diphenylacetaldehyde, and by correlation of 23 with 22 via the common product 31. Formation of 22 is rationalized by a Bergman cyclization to a diradical, followed by regioselective abstraction of a H-atom from the BnO—C(2) group, and diastereoselective combination of the doubly benzylic diradical (Scheme 4). While thermolysis of 3 in EtOH sets in around 140°, 16 did not react at 160° and decomposed at 180-220°. No evidence for intramolecular H-bonds of 16, as compared to 14, were found.
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    Synthesis and Crystal Structure of Tricaesium Heptaphosphide-Ammonia(1/3) Cs3P7·3 NH3 (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2083-2087 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Recrystallization of Cs3P7 from liquid NH3 yields the triammoniate Cs3P7·3 NH3, which loses the weakly bound NH3 of crystallization below 253 K. A low-temperature crystal-structure analysis shows that Cs3P7· NH3 consists of a framework of heptaphosphanortricyclane anions P73- and Cs+ cations with NH3 molecules completing the coordination of the cations. The framework is built from Cs3P7 layers connected by only few Cs…P interactions, the interlayer gap being filled by a two-dimensional network of NH3. The Cs7P7 part of the structure completes a family of alkali-metal-polyphosphide substructures which range from ∞1[RbP7]2- or ∞1[CsP11]2- chains over ∞2[Cs2Pn]- layers (n = 7, 11) to now ∞3[Cs3P7] frameworks.
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  • 117
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    Electrospray-Ionization Mass Spectrometry. Part 2.Part 1: [1]. Neighboring-Group Participation in the Mass-Spectral Decomposition of 4-Hydroxycinnamoyl-spermidines (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2137-2151 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The three mono substituted N-[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines 1-3 have been studied by positive-ion electrospray-ionization tandem mass spectrometry (ESI-MS/MS). Because of the neighboring-group participation, the MS/MS of [1 + H]+ and [2 + H]+ are essentially similar, while compound 3 can be easily distinguished from 1 and 2 because of the characteristic ions at m/z 218. However, with the source collision-induced dissociation (source-CID) MS/MS technique, the compounds 1 and 2 can be unambiguously distinguished by the signal of the pyrrolidinium ion (m/z 72) from their daughter ion (m/z 275). The source-CID MS/MS of the labeled compound N-(4-aminobutyl)-N-(3-aminopropyl)-N-[3-(4- hydroxyphenyl)prop-2-en[15N]amide] ([15N(4)]-2) provide more information on the decomposition mechanisms and proved the occurrence of a partial transamidation reaction 2→1 during the measurement.
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  • 118
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    Synthesis of Lipid Derivatives of Colchicine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2346-2352 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of glycero-lipids linked to colchicine derivatives is reported. The lipid structures are designed to perform two-dimensional crystallization experiments with tubulin, the structural subunit protein of microtubules.
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  • 119
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    Synthesis and Characterization of the (5′ → 5″)-Ester of Adenosine 5′-diphosphate with α-D-ribofuranose cyclic 1″,2″-phosphate: A NAD derivative produced during tRNA splicing (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1005-1010 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and spectroscopic properties of the (5′ → 5″)-ester of adenosine 5′-diphosphate with ribofuranose cyclic 1″, 2″-phosphate 3, a recently discovered metabolite produced during tRNA splicing, are reported.
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  • 120
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    Efficient Preparation and X-Ray Structure Analyses of (2R)-N-pyruvoyl- and (2R)-N-(phenylglyoxyloyl)bornane-10,2-sultamDedicated to Prof. Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1059-1066 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An efficient synthesis of the two title compounds is reported, as well as their X-ray crystal-structure analyses. A discussion based on stereoelectronic considerations rationalizes the first example of a crystalline SO2/C(O) syn-periplanar conformer of a N-acylbornane-10,2-sultam.
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    Unparalleled, Enormous Metal-Carbon Bond Strength in PdCH2I+Dedicated to Albert Eschenmoser on the occasion of his 70th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1-5 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermalized Pd+ cations activate methyl iodide by selective cleavage of a C—H bond under formation of PdCH2I+ and an H-atom. This finding implies that the interaction energy between the metal cation and the CH2I fragment and thus the metal-carbon bond strength exceeds 103 kcal/mol. Theory predicts that the energetically most favorable isomer of this ion exhibits the Pd+—CH2—I structure, which is stabilized by an unprecedented bridging interaction between the two heavy atoms Pd and I.
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  • 122
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    Regioselektive 1,3-Dipolare Cycloadditionen eines ‘Thiocarbonyl-methanids’ ((Alkylidensulfonio)methanids) mit aromatischen SulfinenAls ‘Sulfine’ werden Thiocarbonyl-S-oxide bezeichnet (s.z.B. [1]). (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 31-40 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regioselective 1,3-Dipolar Cycloadditions of a ‘Thiocarbonyl-methanide’ ((Alkylidenesulfonio)methanide) with Aromatic SulfinesReaction of the spirocyclic 2,5-dihydro-1,3,4-thiadiazole 7 and thiobenzophenone S-oxide (6a) in THF at 45° yielded the spirocyclic 1,3-dithiolane 1-oxide 8, thiirane 9, and the diazane derivative 10 in a ratio of 61:15:23 (Scheme 2). The formation of 8 is rationalized by a 1,3-dipolar cycloaddition of ‘thiocarbonyl-methanide’ 1, generated from 7 by thermal elimination of N2, and the C=S bond of sulfine 6a. Cyclization of intermediate 1 leads to thiirane 9. Under the same conditions, 7 and adamantane-2-thione S-oxide (6b) or 2,2,4,4-tetramethyl-3-thioxocyclobutanone S-oxide (4) reacted to give only 9 and 10 but no cycloadduct of type 8 (Scheme 4). With the aim to favor the formation of 8, a mixture of 6a and 1.1 equiv. of 7 was heated to 45° without any solvent in a sealed tube. The ratio of products was only slightly different from that of the thermolysis in THF. An analogous experiment with 7 and 9H-fluorene-9-thione S-oxide (6c) yielded cycloadduct 13 and 9 (Scheme 5). It is most interesting that the 1,3-dipolar cycloadditions of 1 and the sulfines 6a and 6c proceeded with different regioselectivity. A reaction mechanism for the unexpected formation of 10 is proposed in Scheme 7. The key step is the base-catalyzed ring opening of 7 and the nucleophilic addition of the thereby formed thiolate 21 onto the sulfonium ion 19.
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    The Chemistry of Stable Carbenes. Part 2Part 1: [15].. Benzoin-type condensations of formaldehyde catalyzed by stable carbenes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 61-83 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stable carbenes derived from thiazole, 1H-imidazole, and 4H-1,2,4-triazole are efficient catalysts for benzointype condensations of formaldehyde. Catalysts derived from N-substituted thiazolium salts trimerize formaldehyde to dihydroxyacetone (II). Catalysts based on 1,4-disubstituted 4H-1,2,4-triazol-1-ium salts give glycolaldehyde (I) as the main product and no II, whereas N,N′-disubstituted 1H-imidazol-3-ium salts yield mixtures of both products. The isolation of several intermediates in the catalytic cycle provide a better insight into the reaction mechanism.
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  • 124
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    Die Reduktion von Azulen mit Natrium zum Kontaktionen-Tripel [Dinatrium-(1,1′,6,6′-Tetrahydro-6,6′-bi(azulen)-1,1′-diid)-bis (diglyme)]∞91. Mitteilung über Wechselwirkungen in Molekülkristallen; 90. Mitteilung [1].Teil der Dissertation von C. Arad, Universität Frankfurt, 1995.Edgar Heilbronner zum 75. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 92-100 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Sodium Reduction of Azulene to the Contact-Ion Triple [Disodium-(1,1′,6,6′-Tetrahydro-6,6′-bi(azulene)-1,1′-diide)-bis(diglyme)]∞Colorless air-sensitive single crystals can be grown from a diglyme solution after the reduction of azulene by a Na-metal mirror. Structure determination at 150 K reveals a dimer dianion, in which the seven-membered rings are connected in 6,6′-positions and doubly diglyme-solvated Na+ counter cations η5-coordinate to the five-membered rings. Based on preceding cyclovoltammetric measurements in aprotic azulene solutions as well as on extensive MNDO enthalpy of formation calculations, a proposal is forwarded how possibly the contact ion triple is formed along a microscopic pathway.
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  • 125
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    Synthesis and Tritium Labelling of 6β-Amino-4,5α-epoxymorphinans and their 14-hydroxy derivatives as potential affinity labelling probes with μ opioid agonist activity (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 133-136 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of 6β-(methylfumaramido) and 6β-chloroacetamido derivatives 1b and 1c of 6β-amino-7,8- didehydromorphinan and the corresponding 14-hydroxy derivatives 1e and 1f are described. The 7,8-dihydro derivatives of these compounds were synthesized in inactive (2b,c,e,f) as well as in tritiated form (3b,c,e,f).
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    URL: http://dx.doi.org/10.1002/hlca.19960790114
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  • 126
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    Synthesis of 6-Substituted 1-Deazapurine 2′-Deoxyribonucleosides (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 169-178 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of 6-substituted 1-deazapurine 2′-deoxyribonucleosides is described. Glycosylation of the 1-deazapurine (imidazo[4,5-b]pyridine) anions with the α-D-halogenose 5 gives stereoselectively N7- and N9- regioisomers. 1H-NMR NOE and 13C-NMR spectroscopy are used for unambiguous assignment of isomers, and 15N-NMR chemical shifts are correlated with σpara Hammett constants and point charges.
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  • 127
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    Synthese von ‘Push-Pull’- Eninen29. Mitt. über Aminoacrylderivate. 28. Mitt.: [1], Kurzmitteilung: [2]. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 179-191 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of ‘Push-Pull’ Enynes‘Push-pull’ enynes 1a-1f are easily available by Pd catalyzed coupling of stannyl-ynamines 15 and silylynamines 16 with β-iodo-enones 8 (Schemes 7 and 8).
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  • 128
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    10-Alkyl-10-demethylcolchicines (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 208-212 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that colchicine (4) can regiospecifically be transetherified at C(10) by heating in ROH in the presence of (RO)4M (M = Ti, Zr; cf. Scheme 2). (PrO)4Zr in PrOH gives better yields than (PrO)4Ti in PrOH, and also in the catalytic variant of the conversion is (PrO)4Zr more effective than (PrO)4Ti.
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    URL: http://dx.doi.org/10.1002/hlca.19960790121
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  • 129
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    Synthesis of (15N2)[17O]Urea, (15N2)[O2, O4-17O2]Uridine, and (15N3)[O2-17O]Cytidine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 244-254 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general synthetic approach for the synthesis of 15N- and 17O-doubly labelled pyrimidine nucleosides is described. The 15N isotopes in uridine and the 17O isotope in the urea-derived carbonyl group of uridine and cytidine originate from (15N2)[17O]urea (5) which was synthesized from 15NH4Cl, thiophosgene (1), and H2[17O]. The third 15N isotope of cytidine in 4-position stems from the substitution of the 1,2,4-triazole moiety of (15N2)[O2-17O]uridine derivative 8a/b with 15NH4OH. Hydrolysis of the same key intermediate 8a/b with Na[17O]H/H2[17O] introduced the second 17O isotope into the 4-position of uridine. The 15N- and 17O-NMR spectra of the target compounds 12 and 14 in phosphate-buffered H2O serve as references for heteronuclear NMR spectra of labelled RNA fragments.
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    Total Synthesis of (+)-Elacomine and (-)-Isoelacomine, Two Hitherto Unnamed Oxindole Alkaloids from Elaeagnus commutata (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 151-168 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic elacomine ((±)-2), a hemiterpene spiro oxindole alkaloid from Elaeagnus commutata, was synthesized in five steps from 6-methoxytryptamine (19) in 16% overall yield (Scheme 3). The final oxidative rearrangement of the corresponding β-carboline precursor (±)-21 furnished isoelacomine ((±)-22) as a by-product (6% overall yield). A more elaborate route that started from L-tryptophan furnished (+)-2 and (-)-22 with optical purities of 76% (Scheme 4) and established the absolute configuration of these compounds. A reinvestigation of the alkaloidal content of the roots of E. commutata showed that both elacomine and isoelacomine occur naturally in racemic form.
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    Synthesis of 4-acetylcolchicine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 203-207 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of 4-acetylcolchicine (1) by Swern dehydrogenation of the corresponding mixture of 4-[(R)-1-hydroxyethyl]- and 4-[(S)-1-hydroxyethyl]colchicine (3a and 3b, respectively) is described (cf. Scheme). The X-ray analysis of 1 (cf. Fig.), crystallized from MeOH, showed the presence of MeOH in the crystals.
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    A Novel Amination Reaction with Diphenyl Phosphorazidate: Synthesis of α-amino-acid derivatives (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 213-219 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of enolates of α-unsubstituted carboxamides 3 with diphenyl phosphorazidate (DPPA) and di(tert-butyl) dicarbonate (‘Boc anhydride’) in THF at -78° yielded 2-{[(tert-butoxy)carbonyl]amino}carboxamides 5 (Scheme 2) which are derivatives of α-amino acids. In this reaction, DPPA acts as an electrophilic amination reagent. A reaction mechanism is proposed in Scheme 3.
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    URL: http://dx.doi.org/10.1002/hlca.19960790122
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    Metal Complexes with Macrocyclic Ligands. Part XLIPart XL: [1].. Nickel(II) and copper(II) complexes with mono-N-functionalized dithiadiazamacrocycles (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 235-243 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three N2S2 macrocycles (3, 10, 12) carrying an amino group as a pendant arm have been synthesized and their complexation properties towards Ni2+ and Cu2+ studied. The crystal structures of the Cu2+ complexes with 10-methyl-1,4-dithia-7,10-diazacyclododecane-7-ethanamine (3) and 11-methyl-1,4-dithia-8,11-diazacyclotetradecane-8-ethanamine (10) show that, in both cases, the Cu2+ is pentacoordinated by the four donor atoms of the macrocycle and the amino group of the side chain. In aqueous solution, however, two forms of the complexes with stoichiometries [MLH] and [ML] (M = Cu2+ or Ni2+) have been observed. In [MLH], the amino group is protonated and does not bind to the metal ion, whereas in [ML] the amino group is bound, and a pentacoordinated geometry results. The pKa values for the equilibrium [ML] + H+⇌[MLH]+ decrease in the order 12 〉 10 〉 3, indicating that the 2-aminoethyl side chain binds better to the Cu2+ than the 3-aminopropyl side chain. Cyclic voltammetry for the Cu2+/Cu+ pair shows that the 2-aminoethyl pendant arm stabilizes the Cu2+ oxidation state, when the metal ion is in the 14-membered ring (10), whereas it stabilizes Cu+ for the 12-membered macrocycle (3).
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    URL: http://dx.doi.org/10.1002/hlca.19960790124
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    Synthesis of a Building Block for a Nucleic-Acid Analog with a Chiral, Flexible Peptide Backbone (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 288-294 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present here a nine-step synthesis of the thymine-containing amino ester 1, starting from commercially available methyl N-[(tert-butoxy)carbonyl]-L-serinate. Amino ester 1 is considered as a building block for the preparation of a new nucleic-acid analog with a chiral, flexible polyamide backbone. Key steps in the synthesis are the vitamin-B12-catalyzed addition of 3-bromo-N-[(tert-butoxy)carbonyl]-L-alaninate 2 to ethyl acrylate and the homologation of the corresponding N-protected α-amino acid 4 into the β-amino ester 6 by Arndt-Eistert chemistry. The latter was found to proceed with 10% inversion of configuration at the asymmetric center in 6. Resolution to enantiomerically pure material, however, was easily achieved by simple crystallization of 1.
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    URL: http://dx.doi.org/10.1002/hlca.19960790128
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    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 20-20 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790132
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  • 136
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    Model Studies Towards a Novel Fragment Coupling for the Synthesis of Mycalamides and Related Natural Products (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 346-352 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The mycalamides (3) and related natural products contain an α-hydroxy acid 1 bound via an amide bond to an α-alkoxy amine. The high density of functional groups in the ‘coupling region’ renders the coupling reactions delicate. Model studies showed that an alternate assembly of the molecular halves is possible using an ‘Umpolung’ of an α-alkoxy isocyanate, e.g. 13. Thus, combination of the ester 12 with 13 led to the α-keto amide 16. The further elaboration of 16 to the α-hydroxy-amide structure found in the mycalamides is reported.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790203
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  • 137
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    Preparation, Structure, and Reactivity of Thioxo and Imino Derivatives of the Triolide (and Pentolide) from (R)-3-Hydroxybutanoic Acid (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 319-345 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of the triolide 1 from (R)-3-hydroxybutanoic acid with Lawesson's reagent 5 leads to the mono-, di-, and trithio derivatives 6-8 which can be isolated in pure form (20-40% yields), and which have crystal structures very similar to the parent triolide 1 (Fig. 1). Similarly, pentolide 3 is converted to mixtures of various thio derivatives, three of which are separated (10-12) by HPLC and fully characterized. The X-ray structures of the mono- and of one of the dithiopentolides (10, 12) differ remarkably from each other (Fig. 3). Reduction of the thiotriolides 6-8 (NaBH4, R3SnH, Cl3SiH, Raney-Ni) gives 12-membered rings containing up to three ether groups (chiral crown ethers, 15, 17-19) in poor yields. The thiotriolides react spontaneously and in yields of up to 96% with ammonia, certain primary amines, and hydroxylamine to give imine and oxime derivatives with intact 12-membered-ring backbones (20, 22-24, 30, see crystal structures in Figs. 4-7). The rigid structure of all the derivatives of triolide 1 puts the C=O, C=S, and C=NR O-, S-, and N-atoms in juxtaposition (a feature reminiscent of the side chains in the iron-binder enterobactin, Fig. 6). Imines containing PPh2 groups are prepared (30, 33, 35) from the thiotriolides and tested as chiral ligands for PdII-catalyzed 1,3-diphenyallylations (→ 37, enantiomer ratio up to 77:23). The reactions described demonstrate that multiple reactions of the triolide 1 from (R)-3-hydroxybutanoic acid which proceed through tetrahedral intermediates are possible without ring opening - the skeleton is remarkably stable, and this might be exploited as a template for bringing up to three pendent substituents into close proximity to allow a study of their interactions and cooperative properties. Also, the di- and trithio derivatives 7 and 8 could be used for cross-linking in molecules containing primary NH2 groups.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790202
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  • 138
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    Chiral Acylsilanes in Organic Synthesis. Part 3Part 2: [1].. Silicon-directed stereoselective preparation and Ireland ester-enolate rearrangement of O-acyl-substituted α-silylated allyl alcohols (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 391-404 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereocontrolled addition of alk-1-enylmetal reagents to the chiral (alkoxymethyl)-substituted acylsilanes (±)-6 gave rise to α-silylated allyl alcohols, which were converted to the corresponding acetates or propionates 11-16 (Scheme 2). Deprotonation and silylation with Me3SiCl afforded - in an Ireland ester-enolate-accelerated Claisen rearrangement - stereoselectively αδ-silylated γδ-unsaturated carboxylic acids 18-24 (Scheme 4). The Me3Si groups in α-position to the COOH group of these compounds were removed chemoselectively in presence of the chiral silyl group in δ-position by treatment with Bu4NF · 3 H2O or Et3N · 3 HF (→ 27-32; Scheme 5). The reaction sequence allows a novel stereocontrolled access to chiral C-frameworks possessing a vinylsilane moiety with its full reaction potential.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790208
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  • 139
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    Epoxyfocardin and Its Putative Biogenetic Precursor, Focardin, Bioactive, New-Skeleton Diterpenoids of the Marine Ciliate Euplotes focardii from Antractica (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 439-448 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: From the ciliate Euplotes focardii, collected from Ross Sea coastal waters, Antarctica, a new-skeleton diterpenoid, epoxyfocardin, was isolated as a 85:15 mixture of hemiacetals 8a and 8b. The absolute configuration of 8a/8b was determined from Mosher's esters 11a/11b and 12a/12b. Focardin 9a/9b, most likely a biogenetic precursor of 8a/8b, was also isolated as a minor component. Focardin, and particularly epoxyfocardin, proved to be toxic towards representatives of ciliate communities from Antarctic, temperate, tropical, and equatorial environments, constituting the first example of ecologically relevant metabolites from ciliate species that inhabit polar ecosystems.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790211
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  • 140
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    The Tandem Pauson-Khand ReactionDedicated to Hans-Dieter Scharf on the occasion of his 65th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 461-476 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conditions for the novel tandem Pauson-Khand reaction have been explored. The highly functionalized tetracyclic compounds 11c, 11d, and 16 were prepared from the ene-diynes 4c, 4d, and 10 by treatment with 2 equiv. of [Co2(CO)8] and 4-methylmorpholine N-oxide (NMO) or Me3NO in yields of 24, 22, and 53%, respectively (Table). In the presence of 1-3 equiv. of H2O added to the NMO used for induction of the Pauson-Khand reaction of 6d, a mixture of cyclopentanones 17/18 and cyclopentenones 12/13 was obtained (Scheme 5). The first example of a [Co2(CO)6]-induced highly stereoselective ene reaction is described. To account for these results, the formation of intermediates are proposed (Schemes 6 and 7) which hitherto have not been considered in the mechanistic description of the Pauson-Khand reaction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790214
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  • 141
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    Synthesis of 4-Substituted 1H-Benzimidazole 2′-Deoxyribonucleosides and Utility of the 4-Nitro Compound as Universal Base (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 488-498 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The stereoselective synthesis of 4-substituted 1H-benzimidazole 2′-deoxyribonucleosides is described. Regioisomeric (N1 and N3) β-D-deoxyribonucleosides 2a-c and 3a-c were formed. 13C-NMR Chemical shifts of the 1H-benzimidazole 2′-deoxy-β-D-ribofuranosides were correlated with point charges of C-atoms as well as with Hammett constants of the exocyclic substituents. Phosphonate and phosphoramidite building blocks of 4-nitro-1H-benzimidazole 2′-deoxyribofuranoside (2a) were prepared (see 4a, b). Oligonucleotides of the d(A20) type were synthesized in which the two central dA bases were replaced by 4-nitro-1H-benzimidazole residues. They were hybridized with oligomeric dT and related oligomers having the other conventional bases opposite to the 4-nitro-1H-benzimidazole moieties. Within these duplexes (12·13, 12·14, 12·15, and 12·16), the destabilization was almost independent of the mismatch which is required for a universal base. The thermodynamic data indicate that the 4-nitro-1H-benzimidazole residues do not form H-bonds with opposite bases but are stabilizing the duplex by stacking interactions and favorable entropic changes.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790216
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  • 142
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    Ein neues 3-Amino-2H-azirin als Aib-Pro-Baustein: Synthese des C-terminalen Nonapeptids von Trichovirin I 1BHerrn Professor Edgar Heilbronner zum 75. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 527-540 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A New 3-Amino-2H-azirine as an Aib-Pro Synthon: Synthesis of the C-Terminal Nonapeptide of Trichovirin I 1BThe synthesis of methyl N-(2,2-dimethyl-2H-azirin-3-yl)-L-prolinate (3), a novel 3-amino-2H-azirine, is described (Scheme 2). It is shown that the reaction of COCl2 with thioamide 5 is remarkably faster than with the corresponding amide 4, and the yield of 3 is much better in the synthesis starting with 5. The 3-amino-2H-azirine 3 has been used as a building block of the dipeptide moieties Aib-Pro in the synthesis of nonapeptide 17 (Schemes 4 and 5), the C-terminal 6-14 segment of the peptaibole trichovirin I 1B. The structure of 17 was established by single-crystal X-ray crystallography (Figs.1 and 2).
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790220
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  • 143
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    Ein alternativer Zugang zum PQQ-TriesterHerrn Professor Siegfried Hauptmann zum 65. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 658-662 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An Alternative Approach to the PQQ-TriesterA novel approach to triethyl 5-methoxy-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylate (6), a close precursor of the redox cofactor PQQ of bacterial dehydrogenases, is described. Diethyl 2-oxopent-3-ynedioate (5) was used as building block for the annelation of the quinoline unit. The annelation proceeds in two steps via the diastereoselective formation of diethyl (Z)-4-{[2-(ethoxycarbonyl)-5-methoxyindol-6-yl]amiono}-2-oxopent-3-enedioate ((Z)-7) followed by acid-catalyzed cyclocondensation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790309
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  • 144
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    Hydrolytic Breakdown of the Euplotins, Highly Strained, Adaptive, Hemiacetal Esters of the Marine Ciliate Euplotes crassus: A Mimic of Degradative Pathways in Nature and a Trick for the Assignment of the Absolute ConfigurationPresented in part by G.G. at ‘XXI Convegno Nazionale della Divisione di Chimica Organica’, Terrasini (Palermo), 28 September-2 October 1993. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 710-717 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Euplotin C((+)-3), the most abundant of the niche-exploitation terpenoids of the marine ciliate morphospecies Euplotes crassus, was found to undergo degradation in mildly basic H2O/MeOH by initial hydrolysis of the acetate group, followed by, in turn, hydrolytic ring-A and ring-C opening and ring-A reclosure with indiscriminate C(1)/C(15) methanol trapping to give four diastereoisomeric aldehydic hemiacetals 5-8 in similar proportions; 7, as a model for its congeners, proved biologically inactive. From these, the absolute configuration was assigned via Mosher's ester methodology. These processes may be assumed to mimic inactivation of the euplotins in sea water. Degradation of (+)-3 in either stronger base or acidic medium was also examined.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790313
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  • 145
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    Palladium-Catalyzed Syntheses of Polyethynyl-Substituted 2,2′-BithiophenesDedicated to Prof. Dr. Miha Tišler, Ljubljana/Slovenia, on the occasion of his 70th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 755-766 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of polyethynyl-substituted 2,2′-bithiophenes 2 and related 5,5′-dicarbaldehyde derivatives 1 are described. The treatment of easily available polybrominated 2,2′-bithiophenes 3 and 2,2′-bithiophene-5,5′-dicarbaldehydes 4 with phenyl or (trimethylsilyl)acetylene in the presence of PdII and CuI in (i-Pr)2NH yields substituted polyethynyl-2,2′-bithiophene compounds. The Me3Si protecting groups can be removed by protodesilylation under basic conditions to give the corresponding terminal ethynyl groups. These polyethynyl-bithiophenes could be interesting precursors for the synthesis of macrocycles with interesting properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790318
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  • 146
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    Dendrophanes: Novel Steroid-Recognizing Dendritic Receptors. Preliminary Communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 779-788 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The H2O-soluble dendritic cyclophanes (dendrophanes) 3-5 of first to third generation with molecular weights up to nearly 20 kD were synthesized, purified, and characterized. Cyclophane 2, which served as the initiator core (generation zero), was prepared from tetrabromocyclophane 10 in a four-step sequence which involved as the first transformation a high-yielding, four-fold Pd(0)-catalyzed Suzuki cross-coupling reaction with 4-(benzyloxy)-phenyl-boronic acid to give 18. The X-ray crystal-structure analysis of tetrabromocyclophane 10 displayed an open, nearly rectangular box with opposite aromatic walls being 8.3 and 11.4 Å apart and of suitable size for the incorporation of steroidal substrates. 1H-NMR Binding titrations in borate-buffered D2O/CD3OD 1:1 showed that cyclophane-tetracarboxylate 2 forms 1:1 inclusion complexes with steroids (Table 2). Complexation was found to be enthalpically driven with higher binding affinities measured for the more apolar substrates. 1H-NMR Titrations in the same solvent also provided clear evidence for core-selective complexation of testosterone (21) by the dendrophanes 3 (1st), 4 (2nd), and 5 (3rd generation) carrying up to 108 carboxylate surface groups. The stability of the corresponding 1:1 complexes was hardly affected by the size of the dendritic shell, although some generation-dependent conformational changes in the receptor cavity seemed to take place. Remarkably, host-guest exchange kinetics in all recognition processes were fast on the 1H-NMR time scale.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790320
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  • 147
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    Synthesis and X-Ray Structure of Bis{S-(1-lithio-3,3-diphenylprop-2-enyl)-N-methyl-S-phenylsulfoximine- Tetrahydrofuran (1/2)} Complemented by Model ab initio Calculations of α-Lithiosulfoximines (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 820-826 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The gas-phase structure of N,S,S-trimethylsulfoximine 1 and of its monolithiated isomers 2-4 was calculated by ab initio methods. It was found that a Li—C—S—N four-membered chelate 2 is the most stable isomer. The second minimum 4 shows N—Li—O complexation and is only slightly higher in energy. Li-Contacts with the C(α) atom and the sulfoximine O-atom in 3 are energetically disfavored by 6.1 kcal/mol. The two transition states 5 and 6 suggest an interconversion mechanism of 2 to 3 with 4 as an intermediate. A comparison of 4 with the crystal structure of the THF solvate 7, which was prepared by the addition of BuLi to (±)-S-(3,3-diphenylprop-2-enyl)-N-methyl-S-phenylsulfoximine (8) at low temperature in THF, demonstrates that the coordination geometry in the solid state is in good agreement with the calculated structure. The (1-lithioallyl)sulfoximine 7 crystallized as a centrosymmetric dimeric aggregate featuring an eight-membered ring with the atomic sequence (Li—N—S—O)2. The O-atoms of two THF molecules and the sulfoximine O- and N-atoms are coordinated to the Li-atom in a tetrahedral orientation. After metallation, a significant shortening of the S—C(α) bond is observed. Remarkably, only one of the two possible diastereoisomeric enantiomer pairs is found in the solid state.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790324
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790401
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  • 149
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    Enantioselective Reduction of Electrophilic C=C Bonds with sodium tetrahydroborate and ‘semicorrin’ cobalt catalysts (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 961-972 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ‘Semicorrin’ cobalt complexes, prepared in situ from cobalt(II) chloride and the corresponding ligands, are efficient catalysts for the enantioselective reduction of electrophilic C=C bonds with NaBH4. The best selectivities (〉 90% ee) are achieved with α,β-unsaturated carboxamides and carboxylates. Analogous α,β-unsaturated nitriles, sulfones, and phosphonates afford enantiomeric excesses of 50-70%. Interestingly, in the reduction of α,β-unsaturated sulfones, the highest enantioselectivities were obtained with unsymmetrical ‘semicorrins’, whereas in all other cases C2-symmetric ‘semicorrins’ proved to be superior.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790404
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  • 150
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    The Asymmetric Synthesis of Fused Cyclopentenone Ring Systems: A formal asymmetric total synthesis of (-)-isocomene (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1026-1046 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optically active tricyclic oxazolidine lactams 10 have been prepared using two different routes (Scheme 1). They can be obtained by acid-mediated intramolecular cyclization of bicyclic lactams 13 via their acyliminium intermediates producing appended five-, six-, and seven-membered tricyclic systems. Alternatively, 10 can be prepared by cyclocondensation of chiral amino alcohols with cyclopentane-1,2-dicarboxylic acids 12 to give the imide which is reduced or alkylated to the amino alcohols and cyclized to a diastereoisomer mixture of 10. Alkylation of 10 (R″ = H) via its enolate gives stereospecifically α-quaternary products 10 (R″ = alkyl). Degradation of the latter with MeLi or Red-Al® followed by mild acid hydrolysis and aldol cyclization produces the bicyclic ketones 14 and 15 as 1:1 mixtures, readily separated and isolated in 〉 99% ee. This sequence produced a known non-racemic intermediate 69 for the synthesis of (-)-isocomene.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790412
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  • 151
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    The X-Ray Crystal Structure and Packing of a Hexakis-adduct of C60: Temperature dependence of weak C—H…O interactions (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1047-1058 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structure and packing of the C60 hexakis-adduct 1 has been determined from accurate X-ray diffraction measurements. The largest deformations on the C60 surface occur within the bridged pyracylene subunits; the bridgehead 6-6 bond lengths of the four cyclopropane and the two cyclohexene rings, attached in pseudo-octahedral positions, are close to 1.6 Å. Significant deformations also occur in the remaining part of the C60 skeleton, not bridged to any of the functional groups. The crystal packing, analyzed at 270, 230, and 180 K is characterized by a three-dimensional network of 14 weak C—H…O interactions. For most of these, H…O and C…O distances decrease on cooling the crystal specimen, indicating that they are attractive in nature. On the other hand, the shortest, nearly linear C(sp3)—H…O contact seems to be repulsive, since cooling of the crystal specimen leads to significantly longer H…O and C…O distances and to a reduction of the C—H…O linearity. Probably as a result of this intermolecular expansion, strain is produced in the crystal. Depending on the crystal-mounting procedure, this can lead either to a disordered structure (below ca. 200 K), or to destruction of the crystal specimen.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790413
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  • 152
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    A Ring Enlargement from Seven- to Ten-Membered-Ring sulfonamide derivatives (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1121-1128 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of 3-(dimethylamino)-2,2-dimethyl-2H-azirine (1a) with 4,5-dihydro-7,8-dimethoxy-1,2-benzothiazepin-3-one 1,1-dioxide (4) in dioxane at room temperature gave the correspondingly substituted 4H-1,2,5-benzothiadiazecin-6-one 1,1-dioxide 5a in 64% yield (Scheme 2). The structure of this novel ten-membered ring-enlargement product was established by X-ray crystallography (Fig.). Under more vigorous conditions (refluxing dichloroethane), 5a was formed together with the isomeric 6a, both in low yield. The 3-(dimethylamino)-2H-azirines 1b and 1c reacted sluggishly to give the two isomeric ring-enlargement products of type 5 and 6 in yields of 24-29% and 2-4%, respectively (Table 1). Even less reactive is 2,2-dimethyl-3-(N-methyl-N-phenylamino)-2H-azirine (1d), which reacted with 4 in MeCN only at 65°. Under these conditions, besides numerous decomposition products, only traces of 5d and 6d were formed. No ring enlargement was observed with the sterically crowded 1e, which bears an isopropyl group at C(2).
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790420
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    Oligosaccharide Analogues of Polysaccharides. Part 7. Synthesis of a monosaccharide-derived monomer for amylose and cyclodextrin analogues (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1159-1168 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of monomers of type C (Scheme 1) is described. In a first approach, chloro-acetyl-addition to the dioxolane 2 (Scheme 2), followed by treatment of the resulting chlorides 3 (α-D/β-D 1:3) with excess AgOTf and Bu3SnC≡CSiMe3 gave the axial C-alkynyl-glycoside 4 (31%) and the C-arylglycoside 5 (29%). The structure of the dialkyne 6, obtained by deacetylation of 4, was established by X-ray analysis. The yield of the C-alkynyl-glycoside was slightly improved by protecting the C(4)-ethynyl group as the triethysilyl derivative, but not by substituting the benzyl by allyl or 2,6-difluorobenzyl groups. Silylation of the diol 1 with (chloro)diethyl[2-(trimethylsilyl)ethynyl]silane (19) resulted in 90% of the monosilyl ether 20. HO—C(3) of 20 should favor coordination of a Lewis acid to O—C(6), and intramolecular, inverting acetal opening should lead to the product of axial alkynylation. Indeed, treatment of 20 with in situ generated BuAlCl2, followed by treatment of the crude product with 0.1M HCl in MeOH, gave the dialkynylated triol 22 in yields of 85 to 90%. Under similar conditions, the disilyl ether 21 reacted more slowly to 22 (75%). The slower reaction correlates with the assumed intramolecular interaction of the precoordinated Lewis acid with O—C(6) in 20.
    Additional Material: 1 Ill.
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790501
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  • 155
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    Chemistry and the Origin of LifeThis article is a slightly extended version in English of a chapter written in German by A.E. ‘Zur Frage nach der Entstehung des Lebens’ to appear in ‘Mensch und Natur’, Festschrift zur 250-Jahr-Feier der Naturforschenden Gesellschaft, Zürich (Ed. G. Benz), October 1996. M.V.K. has been responsible for the translation from German into English.Dedicated to Vlado Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1249-1259 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The natural genesis of life on Earth is a hypothesis of evolutionary science; it is the task of synthetic organic chemistry to test this hypothesis experimentally. The aim of an experimental aetiological chemistry is not primarily to delineate the pathways along which our (‘natural’) life on Earth could have originated, but to provide decisive experimental evidence, through the realization of model systems (‘artificial chemical life’), that life can arise as a result of the organization of organic matter.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790503
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  • 156
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    Synthesis, Conformational Properties, and Synthetic Applications of Novel Optically Pure α,α-Disubstituted (R)- and (S)-Glycines (‘α-Chimeras’) Combining Side Chains of Asp, Glu, Leu, Phe, Ser, and ValDedicated to Prof. Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1315-1337 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of novel open-chain and cyclic conformationally constrained α,α-disubstituted (R)- and (S)-glycine derivatives (‘α-chimeras’) combining side chains of Asp, Glu, Leu, Phe, Ser, and Val have been efficiently synthesized by using α-alkylation of racemic 4-monosubstituted 2-phenyl-1,3-oxazol-5(4H)-ones of type 5, resolution after reaction with (S)-phenylalanine cyclohexylamide (8) as chiral auxiliary, a novel azlactone/dihydrooxazole interconversion reaction to synthesize optically pure α-substituted (R)- and (S)-serine derivatives coupled with succinimide-ring formation of aspartic-acid derivatives. Based on X-ray structures of (R,S)-9b, (R,S)-11c, (R,S)-18, and (S,S)-30, the absolute configuration of these novel amino-acid building blocks could be unambiguously determined and their preferred conformations in the crystalline state be assessed. The high preference of the open-chain derivatives (R,S)-1, (S,S)-3, and (R,S)-11c for β-turn type-I conformations, as well as of the succinimide derivatives (R,S)-2, (S,S)-19, (S,S)-24, (S,S,S)-26, and (R,S)-29 for β-turn type-II conformations and of (S,S)-4, (R,S)-18, (R,S)-23, and (S,S)-30 for β-turn type-II′ conformations could be confirmed in solution by using CD and NMR spectroscopy. Finally, the spiro derivatives (R,S)-29 and (S,S)-30 incorporating the ‘α-chimera’ of Asp/Glu constitute doubly constrained peptide building blocks combining the properties of α-substituted prolines and aspartimides.
    Additional Material: 12 Ill.
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  • 157
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    Isotactic and Atactic Copolymerization of Styrene and Carbon Monoxide Using Cationic Palladium-Phosphino(dihydrooxazole) Complexes. Preliminary Communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1387-1392 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cationic palladium complexes 2 containing coordinated phosphino(dihydrooxazole) ligands 1 give catalytic systems which allow the production of alternating styrene-carbon monoxide copolymers. Depending on the symmetry of the ligand, the copolymers are produced either with a highly isotactic or with an essentially completely atactic microstructure. Termination of the polymeric chain is mainly due to a β-H-elimination reaction. Both linear ((E)-PhCH=CHCO—) and branched (CH2=C(Ph)CO—) unsaturated end groups were identified.
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  • 158
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    Reisolation of Carotenoid 3,6-Epoxides from Red Paprika (Capsicum annuum) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1435-1443 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cucurbitaxanthin A (= (3S,5R,6R,3′R)-3,6-epoxy-5,6-dihydro-β,β- carotene-5,3′-diol; 5), cucurbitaxanthin B (= (3S,5R,6R,3′S,5′R,6′S)-3,6,5′, 6′-diepoxy-5,6,5′,6′-tetrahydro-β,β-carotene-5,3′-diol; 6), the epimeric cucurbitachromes 1 and 2 (= (3S,5R,6R,3′S,5′R,8′S)- and (3S,5R,6R,3′S,5′R,8′R)-3,6,5′, 8′-diepoxy-5,6,5′,6′-tetrahydro-β,β-carotene-5,3′-diol, resp.; 9/10), cycloviolaxanthin (= (3S,5R,6R,3′S,5′R,6′R)-3,6,3′, 6′-diepoxy-5,6,5′,6′-tetrahydro-β,κs-carotene-5,5′-diol; 8), and capsanthin 3,6-epoxide (= (3S,5R,6R,3′S,5′R)-3,6-epoxy-5,6-dihydro -5,3′-dihydroxy-β,κ-caroten-6′-one; 7) were isolated from red spice paprika (Capsicum annuum var. longum) and characterized by their 1H- and 13C-NMR, mass, and CD spectra.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790516
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  • 159
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    The Deoxygenation and Isomerization of Artemisinin and Artemether and Their Relevance to Antimalarial ActionDedicated to the memory of our late colleague Professor Wolfgang von Oppolzer (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1475-1487 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The treatment of artemisinin (1) and β-artemether (6) with Zn dissolving in AcOH for a few hours results in mono-deoxygenation giving deoxyartemisinin (5) and deoxy-β-artemether (7), respectively, as the sole product. In contrast, submission of 1 to FeCl2 · 4 H2O in MeCN at room temperature for 15 min causes only isomerization, (3aS,4R,6aS,7R,10S,10aR)-octahydro-4,7-dimethyl-8-oxo-2H-10H-furo[3,2-i] benzopyran-10-yl acetate (8) and (3R)-3-hydroxydeoxyartemisinin (9) being produced in 78 and 17% yield, respectively. The action of FeCl2 · 4 H2O in MeCN on 6 is similar. Under the same conditions, 6 gives products analogous to 8 and 9 accompanied by an epimeric mixture of 2-[4-methyl-2-oxo-3-(3-oxobutyl)cyclohexyl]propanaldehyde in yields of 32, 23, and 16%, respectively. No epoxide is formed on repeating the last two experiments in the presence of cyclohexene. The deoxygenation of 1 and 6 by Zn is rationalized in terms of its oxophilic nature. The catalyzed isomerization of 1 and 6 by Fe2+ is attributed to the redox properties of the Fe2+/Fe3+ system.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790520
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  • 160
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    Extrem aufgeweitete Tetrathiafulvalene mit Polyen-Spacern. Carotinoide Tetrathiafulvalene. Polymethin-tetracyanotetrathiafulvalen-Radikalkationen, eine neue Klasse von ViolenenProf. Dr. Siegfried Hünig zum 75. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1497-1517 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Extremely Expanded Tetrathiafulvalenes with Polyene Spacers. Carotinoid Tetrathiafulvalenes. Polymethin-Tetracyanotetrathiafulvalene Radical Cations, a New Class of ViolenesThe synthesis of extended tetrathiafulvalenes 11 with di-, tetra-, hexa-, octa-, deca- and dodecamethine spacers is described by a PPh3-induced Wittig-reaction-like condensation of the corresponding polyenedials 10 with 2-thio-1,3-dithiole-4,5-dicarbonitrile (9). By the same procedure, the dimethyloctamethine- and the tetramethyl-hexadecamethine-tetrathiafulvalenes 14 and 15, respectively, were obtained. The extended tetrathiafulvalenes represent multistep vinylogous redox systems of the ‘violene type’. They can be oxidized to give the cyanine-like radical cations, e.g. 11sem, 14sem, and 15sem, and the dications, e.g. 11ox, 14ox, and 15ox; their UV/VIS/NIR spectra are reported. The crystal and molecular structure of (all-E)-2,2′-(octa-2,4,6-trien-1,8-diylidene)bis[1,3-dithiole-4,5-dicarbonitrile] (11e) was determined: it is a rod-like, planar molecule; in the crystal, it forms staples along the longest molecule axis. The CV measurements confirm that the redox potentials of 11, 14, and 15 decrease asymptotically with the increasing length of the spacer. Because of the close relationship of the extended tetrathiafulvalenes (ETTF's) to the carotinoids, they are named ‘caroviologenes’; they formally belong to the class of molecular wires.
    Additional Material: 11 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790522
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  • 161
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    An Unexpectedly Stable Chiral Hydrido-Solvent Complex of RuII: A mechanistic link in the enantioselective hydrogenation of pyrones. Preliminary communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1587-1591 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ligand (6,6′-dimethoxybiphenyl-2,2′-diyl)bis[3,5-di(tert-butyl)phenylphosphine] (1) forms an unexpectedly stable hydrido-bis-solvento complex of composition [RuH(isopropanol)2(1)]BF4, (2) under the conditions used in the enantioselective hydrogenation of pyrones. The structure of 2, determined by X-ray diffraction, represents the first well-characterized chiral five-coordinate bis-phosphine ruthenium-hydride complex stable as a solvento complex, and provides a structural link in the enantioselective pyrone hydrogenation cycle catalyzed by [Ru(OAc)2(1)]. Using the arene complex [RuH(p-cymene)(1)]BF4 (3), the chiral pocket of coordinated 1 is shown to be relatively rigid, via NMR spectroscopy. This is reflected in restricted rotation about one of the four P-[3,5-di(tert-butyl)phenyl] P—Cipso bonds at room temperature.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790608
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  • 162
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    Carbenoid Reactions of Dimethyl Diazomalonate with Aromatic Thioketones and 1,3-Thiazole-5(4H)-thiones (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1785-1792 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dimethyl diazomalonate (4) and thiobenzophenone (2a) do not react in toluene even after warming to 50°. After addition of catalytic amounts of Rh2(OAc)4, a smooth reaction under N2 evolution afforded a mixture of thiiranedicarboxylate 5 and (diphenylmethylidene)malonate 6 (Scheme 2). A reaction mechanism via an intermediate ‘thiocarbonyl ylide’ 7, formed by the addition of the carbenoid species 8 to the S-atom of 2a, is plausible. Similar reactions were carried out with 9H-xanthene-9-thione (2b), 9H-thioxanthene-9-thione (2c, Scheme 4), and 1,3-thiazole-5(4H)-thione 18 (Scheme 6). In the cases of 2b and 2c, spirocyclic 1,3-dithiolanetetracarboxylates 14a and 14b, respectively, were obtained as the third product. Reaction mechanisms for their formation are proposed in Scheme 5: S-transfer from intermediate thiirane 12 to the carbenoid species yielded thioxomalonate 15 which underwent a 1,3-dipolar cycloaddition with ‘thiocarbonyl ylide’ 16. An alternative is the formation of ‘thiocarbonyl ylide’ 17 via carbene addition to 15, followed by 1,3-dipolar cycloaddition with 2b and 2c, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790704
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  • 163
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    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1741-1741 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790622
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  • 164
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    Pyranosyl-RNA (‘p-RNA’): NMR and Molecular-Dynamics Study of the Duplex Formed by Self-pairing of Ribopyranosyl-(C-G-A-A-T-T-C-G)4th Communication in the series ‘Pyranosyl-RNA (‘p-RNA’)’. For the 3rd communication, see [1]. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2316-2345 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solution structure of the duplex formed by self-pairing of the p-RNA octamer β-D-ribopyranosyl-(2′→4′)-(CGAATTCG) was studied by NMR techniques and, independently, by molecular-dynamics calculations. The resonances of all non-exchanging protons, H-bearing C-atoms, P-atoms, and of most NH protons were assigned. Dihedral angle and distance constraints derived from coupling constants and NOESY spectra are consistent with a single dominant conformer and corroborate the main structural features predicted by qualitative conformational analysis. The duplex displays Watson-Crick pairing with antiparallel strand orientation. The dihedral angles β and ∊ in the phosphodiester linkages differ considerably from the idealized values. Model considerations indicate that these deviations from the idealized model allow better interstrand stacking and lessen unfavorable interactions in the backbone. The average base-pair axis forms an angle of ca. 40° with the backbone. The resulting interstrand π-π stacking between either two purines, or a purine and a pyrimidine, but not between two pyrimidines, constitutes a characteristic structural feature of the p-RNA duplex. A 1000-ps molecular-dynamics (MD) calculation with the AMBER force field resulted in an average structure of the same conformation type as derived by NMR. For the backbone torsion angle ∊, dynamically averaged coupling constants from the MD calculation agree well with the experimental values, but for the angle β, a systematic difference of ca. 25° remains. The two base pairs at the ends of the duplex are calculated to be highly labile, which is consistent with the high exchange rate of the corresponding imino protons found by NMR.
    Additional Material: 23 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790820
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790301
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  • 166
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    Nucleotides. Part IL.Part XLVIII: [1]. Synthesis and Characterization of Cordycepin-Trimer-Vitamin and -Lipid Conjugates Potential Inhibitors of HIV-1 ReplicationDedicated to Prof. Dr. Richard Neidlein on the occasion of his 65th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 619-633 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of biodegradable 2′- and 5′ -ester and 2′- and 5′ -carbonate conjugates of the antivirally active 3′-deoxyadenylyl-(2′-5′)-3′-deoxyadenylyl-(2′-5′)-3′-deoxyadenosine (cordycepin-trimer core) with the vitamins, E, D2, and A and the lipids 1,2-di-O-palmitoylglycerol and 1,2-di-O-hexadecylglycerol were achieved first by preparation of the trimeric educts 19-21 (Scheme 1). Secondly, these substances were condensed with the lipophilic residues via a succinate or carbonate linker and then deprotected by β-elimination of the npeoc and npe protecting groups and acid treatment for detritylation without harming the ester and carbonate functions, respectively (Scheme 2). Metabolically stable cordycepin-trimer-vitamin and -lipid conjugates are a new class of bioconjugates that inhibit HIV-1-induced syncytia formation with IC50 values of 7, 18, and 24 m̈M for 39, 29, and 42, respectively, and inhibit HIV-1 reverse transcriptase (RT) activity from 14 to 96% (see Table). Of the nine conjugates tested, inhibition of HIV-1 replication by 28, 29, 32, 40, and 42 may be attributed in part to the activation of the RNase L/PKR antiviral pathways. Trimer conjugate 42 showed the greatest inhibition of HIV-1 replication, i.e., a 120-fold decrease in HIV-1-induced syncytia formation and an 88% inhibition of HIV-1 reverse transcriptase (RT). This inhibition of replication of HIV-1 by 42 can be attributed in part to the activation of recombinant, human RNase L. The inhibition of HIV-1 replication by the cordycepin-trimer-vitamin and -lipid conjugates is significantly greater than that observed for the (2′-5′) A-trimer core or cordycepin-trimer core.
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    URL: http://dx.doi.org/10.1002/hlca.19960790305
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  • 167
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    Synthesis of Coumarin Derivatives as Inhibitors of Platelet Aggregation (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 651-657 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In a search for the inhibitors of platelet aggregation, certain coumarin derivatives were synthesized and evaluated for antiplatelet activity against thrombin(Thr)-, arachidonic acid(AA)-, collagen(Col)-, and platelet-activating-factor(PAF)-induced aggregation in washed rabbit platelets. These compounds were synthesized from 4-hydroxycoumarin (1) or naphthalen-1-ol via alkylation and Reformatsky-type condensation (Schemes 1-3). Among them, 4-[(2,3,4,5-tetrahydro-4-methylidene-5-oxo-2-phenylfuran-2-yl)methoxy]-2H-1-benzopyran-2-one (6b) showed potent antiplatelet effects on AA- and PAF-induced aggregation with IC50 values of 8.21 and 103.67 m̈M, respectively (see Tables 1 and 2). The antiplatelet potency of 6b against PAF-induced aggregation could be further improved by introducing a proper substituent at the 2-phenyl group of the lactone ring.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790308
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  • 168
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    Synthese monodisperser linearer und cyclischer Oligomere der (R)-3-Hydroxybuttersäure mit bis zu 128 Einheiten (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 670-701 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Monodisperse Linear and Cyclic Oligo[(R)-3-hydroxybutanoates] Containing up to 128 Monomeric UnitsUsing benzyl ester/(tert-butyl)diphenylsilyl ether protection, (COCl)2/pyridine esterification conditions, and a fragment-coupling strategy (with H2/Pd-C debenzylation and HF · pyridine desilylation), linear oligomers of (R)-3-hydroxybutanoic acid (3-HB) containing up to 128 3-HB building blocks (mol. weight 〉 11 000 Da) are assembled (Schemes 1,2,5, and 6). In contrast to the previously employed protecting-group combination, and due to the low-temperature esterifying conditions, this procedure leads to monodisperse oligomers: all steps occur without loss of single 3-HB units. The product oligomers with two, one, and no terminal protecting groups (mostly prepared in multi-gram amounts) are characterized by all standard spectroscopic methods, especially by mass spectroscopy (Figs. 2 and 3), by their optical activity, and by elemental analyses. Cyclization of the oligo[(R)-3-hydroxybutanoic acids] with up to 32 3-HB units, using thiopyridine activation and CuBr2 for the ring closure, produces oligolides consisting of up to 128 ring atoms (Scheme 7). Mixed oligolides containing 3-HB and (R)-3-hydroxypentanoic units are prepared from the corresponding linear trimers, using Yamaguchi's method for the ring closure (Scheme 8 and Fig.4 (X-ray crystal structures of two folded conformers)). Comparisons of melting points (Table 1), of [α]36520 values (Tables 2 and 3), of 1H-NMR coupling constants (Table 3), and of molecular volume/hydroxyalkanoate unit (Table 4) of linear and cyclic oligomer derivatives and of the high-molecular-weigh polymer show that the monodisperse oligomers appear to be surprisingly good models for the polymer. Besides this insight, our synthesis is supplying the samples to further test the role of P(3-HB) (ca. 140 units) as a component of complexes forming channels through cell-wall phospholipid bilayers.
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    URL: http://dx.doi.org/10.1002/hlca.19960790311
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    The Active Centres of Agelastatin A, a Strongly Cytotoxic Alkaloid of the Coral Sea Axinellid Sponge Agelas dendromorpha, as Determined by Comparative Bioassays with Semisynthetic DerivativesPresented in part by M.D. at the ‘Giornate di Chimica delle Sostanze Naturali’, Amalfi, Italy, 29 May to 1 June 1994.Dedicated to Professor Mario Piattelli on the occasion of his 70th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 727-735 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Agelastatin A (1), an unusual alkaloid of the axinellid sponge Agelas dendromorpha from the Coral Sea, can be selectively acetylated (→7) or methylated at OH—C(8a) (→4), peracetylated (→8) or permethylated at OH—C(8a), NH(5), and NH(6) (→5), or, finally, subjected to C(9)—C(8a) (→14) or C(5b)—C(8a) β-elimination (→11-13), in a regiospecific manner or not, depending on the reaction conditions. Under acidic conditions, compound 12 adds H2O or MeOH, regioselectively though not endo/exo stereoselectively, giving transoid/cisoid mixtures 1/18 or 4/19, respectively. Similarly 11 or 13 add MeOH to give mixtures (-)-2/20 or 15/16, respectively. Compound 13 also adds AcOH giving mixture 8/17. The intermediate cisoid form obtained on treatment of 21 with H3O+ undergoes N(5)—N(6) bridging affording pentacyclic 22 which constitutes a proof for the cisoid configuration. From conformational studies, rules are devised that allow assigning the configuration of these compounds from NMR data. In vitro comparative cytotoxicity assays of these compounds show that for high cytotoxic activity, such as of 1 in vivo, unsubstituted OH—C(8a), H—N(5), H—N(6) moieties are needed in the natural B/D transoid configuration.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790315
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  • 170
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    Synthesis and Luminescence of Novel EuIII Complexing Agents and Labels with 4-(Phenylethynyl)pyridine Subunits (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 789-802 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of novel 4-(phenylethynyl)pyridine subunits containing H2O-soluble complexing agents and their luminescence with EuIII ions are reported. Ligands with high luminescence intensities as well as quantum yields were obtained. Also the prepared labeling reagents as antibody conjugates gave the highest quantum and luminescence yields reported for H2O-soluble EuIII labels.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790321
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  • 171
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    Struktur des Zwischenproduktes einer (i-PrO)2 TiMe2-Methylierung: NMR-Untersuchungen und Konformationsanalyse (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 827-836 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Structure of the Intermediate of a Methylation with (i-PrO)2TiMe2: NMR Investigations and Conformational Analysis
    Notes: The intermediate of the reaction of 3-(1-nitro-2-oxocyclohexyl)propanal ((±)-1) with (i-PrO)2TiMe2 was shown to be a titanium salt of 6-(hydroxynitroryl)decano-9-lactone. 1H-, 13C-, HSQC-, 1H,1H-TOCSY- and long-range HMBC-NMR spectra of this intermediate indicate a complexation of the carbonyl O-atom and an O-atom of the hydroxynitroryl group to the same Ti-atom. The product of the reaction (t-3-methyl-c-6-nitro-2-oxabicyclo[4.4.0]decan-r-1-ol, (±)-2) was reacted with several titanium reagents to give also titanium salts of the 6-(hydroxynitroryl)decano-9-lactone. Monte Carlo studies and MM3* force-field calculations of the anion of 6-(hydroxynitroryl)decano-9-lactone, 3′, resulted in a conformation of the ten-membered ring (only 9.57 kJ/mol higher in energy than the global minimum), which allows the Ti-atom to coordinate to the O-atom of the N(O)OH and C=O group at the same time. With the suggested structure of 3′ · TiRn, we are able to explain the selectivity of the reaction.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790325
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  • 172
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    Determination of the Configuration of Wine Lactone (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1559-1571 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The intense sweet and coconut-like smelling odorant 1, named ‘wine lactone’, was isolated from different wine varieties. The chemical structure of this compound, which has not yet been detected in wine or a food, was identified by high-resolution mass spectrometry (HR-MS) as 3a,4,5,7a-tetrahydro-3,6-dimethylbenzofuran-2(3H)-one. For the evaluation of the configuration of wine lactone, stereochemically controlled syntheses were developed. All eight isomers were characterized by NMR, MS, IR, and CD measurements. The configuration of ‘wine lactone’ was in agreement with synthesized (3S,3aS,7aR)-enantiomer (1a) on the basis of enantioselective GC. For this isomer, the lowest odor threshold (0.02 pg/1 air) was detected.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790606
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  • 173
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    Säurekatalysierte Umlagerung von 9-Hydroxyfuranoeremophilanen zu Eremophilanlactonen14. Mitt. über Petasites-Inhaltsstoffe; 13. Mitt.: [1].Die systematischen Namen für Furanoeremophilane 1-3 und Eremophilanlactone (‘Eremophilenolide’) 4-6 lauten: 7-R-3,4a,5-Trimethyl-4,4a,5,6,7,8,8a,9-octahydronaphtho[2,3-b]furan-9-ol (1-3) und 7-R-3,4a,5-Trimethyl-4,4a,5,6,7,8,8a,9-octahydronaphtho[2,3,-b] furan-2(4H)-on (4-6, vgl. Exper. Teil). Im Allg. Teil werden die in der Literatur verwendeten Trivialnamen Furanoeremophilane und Eremophilanlactone (‘Eremophilenolide’) mit der in Schema 1 und 2 angegebenen Numerierung verwendet. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1592-1606 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acid-Catalyzed Rearrangement of 9-Hydroxyfuranoeremophilanes to Eremophilanlactones9-Hydroxyfuranoeremophilanes 1-3 are the main components of freshly harvested rhizomes of P. hybridus (furanopetasin chemovar, Table, Fig. 1). They easily and quantitatively rearrange in the presence of traces of acid to give an epimeric mixture of 8-H-eremophilanlactones 4-6 (eremophilenolides, Table, Figs. 4 and 5). Besides subsequent oxidation (see 4 → 7), this is the most important reaction occuring during drying and storage of rhizomes of P. hybridus (Figs. 1 and 3). A reasonable mechanism of the rearrangement is presented, and spectroscopic structure elucidation of 8-H-eremophilanlactones is discussed.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790609
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  • 174
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    The Addition of tert-Butyl (Me3Ċ) and (tert-Butoxy)carbonylmethyl (Me3CO2CĊH2) radicals to alkynes in solution studied by ESR spectroscopy (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1670-1682 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Absolute rate constants and their temperature dependence are determined by time-resolved electron spin resonance for the addition of Me3Ċ to 20 and of Me3CO2CĊH2 to six mono- and disubstituted alkynes in solution. For Me3Ċ the rate constants show polar alkyne-substituent effects which are, however, weaker than for the corresponding alkenes. For Me3CO2ĊH2, the rate constants do not vary strongly with alkyne substitution and probably increase with increasing reaction exothermicity. Both radicals react generally slower with alkynes than with alkenes which is discussed in terms of the state correlations. Several vinyl-type radical adducts of Me3Ċ to alkenes are characterized by electron spin resonance, and their spectral data indicate linear or bent configurations of the radical carbon depending on the substitution.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790617
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  • 175
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    Leucascandrolide A, a New Type of Macrolide: The first powerfully bioactive metabolite of calcareous sponges (Leucascandra caveolata, a new genus from the coral sea) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 51-60 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Leucascandrolide A ((+)-1), a doubly O-bridged 18-membered macrolide of a new type, i.e., showing little C1-branching vs. extensive 1,3-dioxygenation and a peculiar side chain, was isolated from a calcareous sponge of a new genus, Leucascandra caveolata BOROJEVIC and KLAUTAU from the Coral Sea. Transesterification of (+)-1 gave the methyl ester 3, derived from the side chain, and the 5-hydroxy derivative (+)-2, derived from the macrolide portion and with the natural configuration at C(5) (axial). Mosher's MTPA esters 4 and 5 obtained from (+)-2 showed scattered Δδ = (δ(S) - δ(R)) data. However, inversion of the configuration at C(5) led, via ketone (+)-6, to the less encumbered 5-equatorial hydroxy derivative (+)-7, whose MTPA esters 8 and 9 gave consistent Δδ data, allowing the assignment of the absolute configuration of (+)-7, and hence of (+)-1. The structural novelty of (+)-1 and its powerful antifungal and cytotoxic activities are likely to renew interest in calcareous sponges, previously limited to scarcely biologically active 2-aminoimidazoles.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790107
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  • 176
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    Enantioselektive Synthese von α-Phosphinoketonen. Vorläufige Mitteilung (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 118-122 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantioselective Synthesis of α-PhosphinoketonesThe first asymmetric (C—P)-connective synthesis of α-phosphinoketones in high enantiomeric purity (e.e.91-97%) is described. Key step is the highly diastereoselective phosphinylation of SAMP-hydrazones (S)-2 to produce α-phosphinohydrazones (S,R)-3, isolated as the more stable borane adducts. Subsequent ozonolysis afforded the title compounds (R)-4, potential ligands for enantioselective homogeneous catalysis.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790112
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  • 177
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    Gas-Phase Reactions of Aliphatic Alcohols with ‘Bare’ FeO+ (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 123-132 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ion/molecule reactions of ‘bare’ FeO+ with linear and branched aliphatic alcohols have been examined by Fourier-transform ion-cyclotron resonance mass spectrometry. Depending on the chain length of the alcohol, three different types of reactions can be distinguished: (i) Oxidation of the alcohols in the α-positions, to yield the corresponding carbonyl-Fe+ complexes, involves an initial O—H bond activation of the alcohol resulting in the formation of RO—Fe+—OH as the central intermediate. (ii) The formation of Fe(OH)2+, concomitant by loss of the corresponding neutral alkenes, competes with the generation of neutral OFeOH and a carbocation R+. These couples point to the existence of an intracomplex acid-base equilibrium and are connected with each other by a proton transfer from either acid to the other, e.g. i-C3H7+ + OFeOH⇄C3H6 + Fe(OH)2+. The process is driven by the Lewis acidity of FeO+ and starts with the abstraction of a hydroxide anion from the alcohol. (iii) For longer alcohols, e.g. pentanol, functionalization of non-activated C—H bonds which are remote from the O functionality is observed. Here, the OH group of the alcohol serves as an anchor, which directs the reactive metal-oxide cation toward a particular site of the hydrocarbon chain.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790113
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  • 178
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    A 1H-NMR Spectroscopic Investigation of the Conformation of the Acetamido Group in Some Derivatives of N-Acetyl-D-allosamine and -D-glucosamine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 269-287 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The population of the conformations obtained by rotation around the C(2)—N and the N—C(O) bonds of AllNAc, GlcNAc, and GlcNMeAc derivatives was investigated by 1H-NMR spectroscopy. The AllNAc-derived α-D-and β-D-pyranosides 4-7, the AllNAc diazirine 16, and the GlcNAc-derived axial anomers α-D-8-10 prefer the (Z)-anti-conformation. A significant population of the (Z)-syn-conformer in the (Z)-syn/(Z)-anti-equilibrium for the equatorial anomers β-D-8-10 and the GlcNAc diazirine 17 was evidenced by an upfield shift of H—C(2), downfield shifts of H—C(1) and H—C(3), and by NOE measurements. The population of the (Z)-syn-conformation depends on the substituent at C(1) and is highest for the hexafluoroisopropyl glycoside. The population of the (Z)-syn-conformation of β-D-14 decreases with increasing polarity of the solvent, but a substantial population is still observed for solutions in D2O. Whereas the α-D-anomers of the hemiacetal 22 and the methyl glycoside 21 prefer the (Z)-anti-conformation in D2O solution, the corresponding β-D-anomers are mixtures of the (Z)-anti-and (Z)-syn-conformers. The diazirine 17 self-associates in CD2Cl2 solution at concentrations above 0.005M at low temperatures. The axial anomers of the GlcNMeAc derivatives α-D-26-28 are 2:1 to 3:1 mixtures of (Z)-anti-and (E)-anti-conformers, whereas the corresponding β-D-glycosides are ca. 1:3:6 mixtures of (Z)-syn-, (Z)-anti-, and (E)-anti-conformers.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790127
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  • 179
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    Compounding Selectivity in Reactions of Diastereoisomeric Radical Intermediates. An experimental demonstration that the yield of a product from a diastereotopic-group-selective reaction can significantly exceed the level of group selectivity (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 21-30 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reduction of a bis-radical precursor, 6-phenyl-1,1-bis[3-(phenylselanyl)propyl]-3a,4-dihydro-1H,3H-cyclopenta[c]furan-5-one (6), with 3 equiv. of Ph3SnH provides mixtures of cis,cis- or cis,trans-angular triquinane products (3aα,5aβ,8β,8aR*)- and (3aα,5aα,8β,8aR*)-hexahydro-3a-propyl-8-phenyl-5H-dicyclopenta[b,c]furan-7(8H)-one (cis,cis-12/cis,trans-12), in yields that vary from 50%/50% to 91%/6% depending on the reaction concentration. A mechanistic model for this process is proposed that involves a non-selective phenylselenium-group abstraction step followed by successive kinetic resolutions of diastereoisomeric radical intermediates. This reaction shows how yields in group-selective reactions can be compounded to levels above that ostensibly permitted by the level of the group-selective step.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790104
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  • 180
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    Formation of Cyclic Ketals from Hydroxyalkyl Enol Ethers, a stereoelectronically controlled endo-trig-cyclization processDedicated to Albert Eschenmoser on the occasion of his 70th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 41-50 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acid-catalyzed cyclic ketal formation vs. hydrolysis of a series of hydroxyalkyl cyclic enol ethers in the presence of 1 equiv. of H2O, and acid-catalyzed cyclic-ketal formation (same ketals as above) vs. methanolysis of a series of mixed pent-4-enyl hydroxyalkyl ketals with N-bromosuccinimide in the presence of 4 equiv. of MeOH led to the same result: the intramolecular cyclization processes occur at similar rates as the intermolecular H2O or MeOH attacks independently of the size of the rings formed (five-, six-, or seven-membered), by cyclizations. These results can be explained by the facts that, due to stereoelectronic effects which impose a torsional strain to the sp2 hybridized O-atom, the cyclization activation enthalpy decreases, as the length of the hydroxyalkyl chain increase (ease of cyclization: 7 〉 6 〉 5), whereas the entropy factor favors the cyclization in the reverse fashion (ease of cyclization: 5 〉 6 〉 7). The various reaction pathways have been examined using the semi-empirical Hamiltonian AM1, and the results obtained confirm that large-ring formation is enthalpically much favored over the cyclization processes leading to small rings (ease of cyclization: 7 〉 6 〉 5).
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790106
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  • 181
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    Solvent-shared radical ion pairs [pyrene·⊖Na⊕(C2H5)2]∞: ESR evidence for two different aggregates in solution, room temperature crystallization, and structural proof of another polymorphic modificationPart 85 of Interactions in Molecular Crystals; Part 84: [1].Dedicated to Kurt Dehnicke on the occasion of his 65th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 84-91 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reduction of pyrene with sodium in aprotic diethyl ether allows to crystallize the extremely air-sensitive radical ion pair pyrene-sodium-diethylether. The single-crystal structure determination at 130 K shows that each sodium counter cation, solvated by one diethyl-ether molecule, is η3- and η6-coordinated to one of the short-axis six-membered rings of two pyrene radical anions. The resulting dibenzene-sodium sandwiches form a string, in which the hydrocarbon planes are canted to each other by 62°. In the pyrene radical-anion skeleton, no distortion due to its negative charge can be detected relative to that of the neutral molecule. From the temperature-dependent signal multiplets of preceding ESR investigations, the solvent-separated pyrene radical anion as well as two different contact radical-ion pairs had been identified and their structures in solution approximated by potential-energy estimates. Referring to the recently discovered long-axis Na⊕ contact ion pair polymorph, crystallized at lower temperatures, the structure reported here represents the second and probably thermodynamically more stable one. Both the ESR and the structural results provide some insight into the multidimensional networks of equilibria in aprotic solution, which are activated by alkali-metal reduction of unsaturated organic compounds.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790109
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  • 182
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    Mechanism of Transfer of a Basic Drug across the Water/1,2-Dichloroethane Interface: The case of quinidine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 101-117 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electrochemical transfer of quinidine across the H2O/1,2-dichloroethane interface was investigated by cyclic voltammetry, so as to determine its lipophilicity. The formal transfer potential was measured as a function of the pH of the aqueous phase. Both singly and doubly protonated quinidine cations can transfer across the interface, and their formal Gibbs free energies of transfer were observed to be 7.7 and 31.2 kJ mol-1, respectively. Between pH 0 and 3, only the doubly charged quinidine was present in the aqueous phase and was observed to transfer. Between pH 3 and 6, the transfer of both cations occurred. The proportion of doubly charged quinidine decreased progressively in this pH range and disappeared completely above pH 6. The overall process was analyzed using a thermodynamic model. The relationship between the various forms of quinidine in both phases and pH was established and found to be in good agreement with the experimental results. With this model, the acid-base equilibrium constants in the organic phase could be calculated as pKa1o = 9.66 ± 0.21 and pKa2o = 14.20 ± 0.16 (the subscripts a1o and a2o refer to the first and second dissociation constants). This study illustrates how the partition of ionic species can be taken into account in the determination of lipophilicity and in the description of the passive transfer of organic drugs.
    Additional Material: 11 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790111
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  • 183
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    Kinetics and Mechanisms of Racemization: 5-Substituted Hydantoins (= Imidazolidine-2,4-diones) as Models of Chiral Drugs (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 767-778 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A chiral center in a drug molecule increases the complexity of synthetic, metabolic, pharmacological, and clinical studies, an additional problem being a possible lack of configurational stability. Here, we report detailed kinetic and mechanistic studies on the deuteration and racemization of seven 5-monosubstituted hydantions (= imidazolidine-2,4-diones) used as model compounds. Using 1H-NMR and chiral RP-HPLC, rates of reaction and thermodynamic parameters of activation were determined for the reactions of deuteration and racemization. Energies of deprotonation were obtained by molecular-orbital calculations performed at the AM1 level. It is demonstrated that the deuteration and racemization of 5-monosubstituted hydantoins follow general-base catalysis. The identical (within experimental errors) activation energies of deuteration and racemization indicate that the two reactions share a common reaction mechanism. The fact that the pseudo-first-order rate constants of deuteration are about half of those of racemization suggests that deuteration occurs with inversion of configuration. Very large differences in reaction rates were observed between the seven compounds, indicating the marked influence of substituents on chiral stability. These results, together with the small isotope effects observed, and the comparison between experimental activation energies and calculated energies of deprotonation, suggest a SE2 push-pull mechanism for the racemization of 5-monosubstituted hydantoins.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790319
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  • 184
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    Synthesis, and Solution and Solid-State Structures of (η3-Allyl){(4S)-4-benzyl-2-[2′-(diphenylphosphino)phenyl]- 4,5-dihydrooxazole-P,N}palladium(II) Hexafluorophosphates. Comparison with Dichloro{(4S)-2-[2′-(diphenylphosphino)phenyl]-4,5-dihydro-4-phenyloxazole-P,N}zinc(II) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 803-812 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Crystal and solution structures of the [PdII(η3-allyl)] and of the [PdII(η3-1,3-diphenylallyl)] complexes, 4 and 5, respectively, with (4S)-4-benzyl-2-[2′-(diphenylphosphino)phenyl]-4,5-dihydrooxazole (3) were determined by X-ray crystallography and 2D-NMR spectroscopy. Complex 4 proved to be disordered with both diastereoisomeric complexes in the crystal. The results of X-ray and NMR experiments demonstrate a good agreement between solution and solid-state equilibria of the two isomers. A comparison with dichloro{(4S)-2-[2′-(diphenylphosphino)phenyl]-4,5-dihydro-4-phenyloxazole-P,N}zinc(II) (2b) shows a surprising conformational stability of the coordinated phosphinooxazole ligand 3.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790322
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  • 185
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    1,5-Dipolare Elektrocyclisierung von Acyl-substituierten ‘Thiocarbonyl-yliden’ zu 1,3-Oxathiolen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 855-874 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1,5-Dipolar Electrocyclization of Acyl-Substituted ‘Thiocarbonyl-ylides’ to 1,3-OxathiolesThe reaction of α-diazoketones 15a, b with 4,4-disubstituted 1,3-thiazole-5(4H)-thiones 6 (Scheme 3), adamantanethione (17), 2,2,4,4-tetramethyl-3-thioxocyclobutanone (19; Scheme 4), and thiobenzophenone (22; Scheme 5), respectively, at 50-90° gave the corresponding 1,3-oxathiole derivatives as the sole products in high yields. This reaction opens a convenient access to this type of five-membered heterocycles. The structures of three of the products, namely 16c, 16f, and 20b, were established by X-ray crystallography. The key-step of the proposed reaction mechanism is a 1,5-dipolar electrocyclization of an acyl-substituted ‘thiocarbonyl-ylide’ (cf. Scheme 6). The analogous reaction of 15a, b with 9H-xanthen-9-thione (24a) and 9H-thioxanthen-9-thione (24b) yielded α,β-unsaturated ketones of type 25 (Scheme 5). The structures of 25a and 25c were also established by X-ray crystallography. The formation of 25 proceeds via a 1,3-dipolar electrocyclization to a thiirane intermediate (Scheme 6) and desulfurization. From the reaction of 15a with 24b in THF at 50°, the intermediate 26 (Scheme 5) was isolated. In the crude mixtures of the reactions of 15a with 17 and 19, a minor product containing a CHO group was observed by IR and NMR spectroscopy. In the case of 19, this side product could be isolated and was characterized by X-ray crystallography to be 21 (Scheme 4). It was shown that 21 is formed - in relatively low yield - from 20a. Formally, the transformation is an oxidative cleavage of the C=C bond, but the reaction mechanism is still not known.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790327
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  • 186
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    Nonreductive Enantioselective Ring Opening of N-(Methylsulfonyl)dicarboximides with Diisopropoxytitanium α,α,α′,α′-Tetraaryl-1,3-dioxolane-4,5-dimethanolate (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 875-894 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bicyclic and tricyclic meso-N-(methylsulfonyl)dicarboximides 1a-f are converted enantioselectively to isopropyl [(sulfonamido)carbonyl]-carboxylates 2a-f by diisopropoxytitanium TADDOLate (75-92% yield; see Scheme 3). The enantiomer ratios of the products are between 86:14 and 97:3, and recrystallization from CH2Cl2/hexane leads to enantiomerically pure sulfonamido esters 2 (Scheme 3). The enantioselectivity shows a linear relationship with the enantiomer excess of the TADDOL employed (Fig.3). Reduction of the ester and carboxamide groups (LiAlH4) and additional reductive cleavage of the sulfonamido group (Red-Al) in the products 2 of imide-ring opening gives hydroxy-sulfonamides 3 and amino alcohols 4, respectively (Scheme 4). The absolute configuration of the sulfonamido esters 2 is determined by chemical correlation (with 2a,b; Scheme 6), by the X-ray analysis of the camphanate of 3e (Fig. 1), and by comparative 19F-NMR analysis of the Mosher esters of the hydroxy-sulfonamides 3 (Table 1). A general proposal for the assignment of the absolute configuration of primary alcohols and amines of Formula HXCH2CHR1R2, X = O, NH, is suggested (see 11 in Table 1). It follows from the assignment of configuration of 2 that the Re carbonyl group of the original imide 1 is converted to an isopropyl ester group. This result is compatible with a rule previously put forward for the stereochemical course of reactions involving titanium TADDOLate activated chelating electrophiles (12 in Scheme 7). A tentative mechanistic model is proposed (13 and 14 in Scheme 7).
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790328
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  • 187
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    Mimicking the Vancomycin Carboxylate Binding Site: Synthetic receptors for sulfonates, carboxylates, and N-protected α-amino acids in water (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 942-960 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The novel H2O-soluble cyclophanes 1 and 2 incorporating different anion-recognition sites were prepared in short synthetic routes (Schemes 1 and 2) as first-generation mimics of the natural, D-Ala-D-Ala binding antibiotic vancomycin. The X-ray crystal structure of 1, a tris(hydrochloride)salt, revealed an open, preorganized cavity of sufficient size for the incorporation of small aliphatic residues (Fig. 3). In the crystal, molecules of 1 are arranged in parallel stacks, generating two types of channels, an ‘intra-stack’ channel passing through the cyclophane cavities and an ‘inter-stack’ channel located between cyclophane stacks (Fig. 4). The strongest intermolecular interactions between macrocycles in the crystal are C=O…H—N H-bonds between the carboxamide residues of adjacent cyclophanes in neighboring stacks (Fig. 5). The ‘intra-’ and ‘inter-stack’ channels incorporate the three ordered Cl- counterions and several, partially ordered solvent molecules (4 MeOH, 1 H2O) (Fig. 6). Counterion Cl(2) is located within the ‘intra-stack’ channel and interacts with a protonated piperazinium N-atom and both ‘intra-stack’ MeOH molecules. The two other counterions, Cl(1) and Cl(3), are located within the ‘inter-stack’ channel. They are connected to two MeOH and one H2O molecules and also interact both with the NH2+ group of the protonated spiropiperidinium ring in 1, forming an infinite, chain-like H-bonding network …Cl(1)…HOH…MeOH…Cl(3)…HNH…Cl(1′)…. Both ‘intra-’ and ‘inter-stack’ MeOH molecules undergo weak CH…π interactions with neighboring aromatic rings. Cyclophane 1 complexed aromatic sulfonates in 0.5M KCl/DCl buffer in D2O, whereas the tetrakis(quaternary ammonium) receptor 2 bound the sodium salts of aliphatic and aromatic carboxylates and sulfonates, of N-acylated α-amino acids as well as of N-acetyl-D-alanyl-D-alanine (Ac-D-Ala-D-Ala), a substrate of vancomycin, in pure H2O. In all of these complexes, ion pairing between the cationic recognition site in the periphery of the cyclophane receptor and the anionic substrates represents the major driving force for host-guest association. The 1H-NMR analysis of complexation-induced changes in chemical shift clearly demonstrated that, in solution, this ion pairing exclusively takes place outside the cavity. Nevertheless, the macrocyclic bridges are essential for the efficiency of the anion-recognition sites in the two cyclophane receptors 1 and 2. Control compounds 3 and 4 possess nearly the same anion-recognition sites than 1 and 2, but lack their macrocyclic preorganization; as a consequence, they do not form stable ion-pairing complexes with mono-anionic substrates in the considered concentration ranges ( 〈 50 mM) in D2O.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790403
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  • 188
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    Synthesis of 2′-Deoxy-5-(isothiazol-5-yl)uridine and Its Interaction with the HSV-1 Thymidine Kinase (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1462-1474 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2′-Deoxy-5-(isothiazol-5-yl)uridine (12) was synthesized starting from 2′-deoxy-5-iodouridine using a Pd-catalysed cross-coupling reaction with propiolaldehyde diethyl acetal followed by deprotection and ring closure using thiosulfate. 2′-Deoxyuridine 12 has a particular place among the 5-heteroaryl-substituted 2′-deoxyuridines in that it has a high affinity for herpes simplex virus type 1 (HSV-1)-encoded thymidine kinase (TK) without antiviral activity. Biochemical studies revealed that 12 is a substrate for viral TK. We further investigated the interaction of 12 with the HSV-1 thymidine kinase. The conformation of 12 in solution was established by NMR spectroscopy. The most stable conformer 12A has the S-atom of the isothiazole ring placed in the neighbourhood of the C(4)=O group of the pyrimidine moiety. The compound was docked in its most stable conformation in the active site of HSV-1 TK and subjected to energy minimization. This demonstrated that the isothiazole moiety binds in a cavity lined by the side chains of Tyr-132, Arg-163, Ala-167, and Ala-168 and that the C(3) atom of the isothiazole moiety is located in close proximity of the phenolic O-atom of Tyr-132 and the aliphatic part of the Arg-163 side chain.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790519
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  • 189
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    C-Glycoside Analogues of N4-(2-Acetamido-2-deoxy-β-D-glucopyranosyl)-L-asparagine: Synthesis and conformational analysis of a cyclic C-glycopeptideDedicated to Michael Hanack on the occassion of his 65th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1519-1532 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of C-glycosidic analogues 15-22 of N4-(2-acetamido-2-deoxy-β-D-glucopyranosyl)-L-asparagine (Asn(N4GlcNAc)) possessing a reversed amide bond as an isosteric replacement of the N-glycosidic linkage is presented. The peptide cyclo(-D-Pro-Phe-Ala-CGaa-Phe-Phe-) (CGaa = C-glycosylated amino acid; 24) was prepared to demonstrate that 3-[(3-acetamido-2,6-anhydro-4,5,7-tri-O-benzyl-3-deoxy-β-D-glycero-D-guloheptonoyl)amino]-2-[(9H-fluoren-9-yloxycarbonyl)amino]propanoic acid (22) can be used in solid-phase peptide synthesis. The conformation of 24 was determined by NMR and molecular-dynamics (MD) techniques. Evidence is provided that the CGaa side chain interacts with the peptide backbone. The different C-glycosylated amino acids 15-21 were prepared by coupling 3-acetamido-2,6-anhydro-4,5,7-tri-O-benzyl-3-deoxy-β-D-glycero-D-gulo-heptonic acid (4) with diamino-acid derivatives 8-14 in 83-96% yield. The synthesis of 4 was performed from 2-(acetamido-3,4,6-tri-O-benzyl-2-deoxy-β-D-glucopyranosyl) tributylstannane (2) by treatment with BuLi and CO2 in 83% yield. Similarly, propyl isocyanat yielded the glycoheptonamide 7 in 52% from 2. Compound 2 was obtained from 2-acetamido-3,4,6-tri-O-benzyl-2-deoxy-D-glucopyranose (1) by chlorination and addition of tributyltinlithium in 74% yield. A procedure for a multigram-scale synthesis of 1 is given.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790602
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  • 190
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    Stereoselektive Synthese des Nitrit-reduzierenden Cofaktors Häm d1 ausgehend von HämatoporphyrinKurzmitteilungen: [1].Prof. Burchard Franck zum 70. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1572-1586 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Steroselective Synthesis of the Nitrite-Reducing Cofactor Heme d1 from HematoporphyrinThe nitrite-reducing cofactor heme d1 has been synthesized in a few synthetic steps, starting from readily accessible hematoporphrin dimethyl ester (rac-4a). A single-crystal structure analysis of the synthesized target molecule rac-10a unequivocally established the constitution and the relative configuration of heme d1 (3).
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790607
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  • 191
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    Synthesis of Glycosylrifamycins, a new type of semisynthetic rifamycins (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1611-1619 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Glycosylrifamycins, a new type of semisynthetic rifamycin derivatives, can be easily obtained by reaction of 3-(2-aminoethylthio)rifamycin SV (2) with a glycosyl compound carrying a coupling group, such as isothicyanate or carboxy. We prepared O-acetylated and free glucopyranosyl and arabinopyranosyl derivatives of rifamycin S and SV (see 3-10). Additionally, derivatives with D-saccharo-1,4-lactone and with shikimic acid were obtained (see 11-15). Glycosylrifamycins show an interesting inhibitory power on Gram-positive bacteria (Table).
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790611
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  • 192
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    Ionization and Partitioning Profiles of Zwitterions: The case of the anti-inflammatory drug azapropazone (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1683-1695 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Azapropazone (1) is a non-steroidal anti-inflammatory drug (NSAID) whose chemical structure is markedly different from that of other agents in this class and challenges our understanding of structure-activity and structure-permeation relationships. Using a variety of experimental and computational techniques, we studied 1 for its molecular structure in the gas phase and non-protic polar solvents, protonation/deprotonation equilibra, tautomerism, and pH-lipophilicity profiles (octan-1-ol/H2O and dodecane/H2O). Other NSAIDs and model compounds were also examined for comparison. Due to its very low acidic pKa1, 1 exists in the physiological pH range as a zwitterion and as an anion. Some pharmacological implications of these findings are discussed.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790618
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  • 193
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    Antiplatelet α-Methylidene-γ-butyrolactones: Synthesis and evaluation of quinoline, flavone, and xanthone derivatives (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1620-1626 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: As a continuation of our previous studies on the synthesis and antiplatelet activity of coumarin derivatives of α-methylidene-γ-butyrolactones, certain quinoline, flavone, and xanthone derivatives were synthesized and evaluated for antiplatelet activity against thrombin (Thr)-, arachidonic acid (AA)-, collagen (Col)-, and platelet-activating factor (PAF)-induced aggregation in washed rabbit platelets. These compounds were synthesized from quinolin-8-ol, flavon-7-ol, and xanthon-3-ol, respectively, via alkylation and Reformatsky-type condensation (Schemes 1-3). By the comparison with comparison with coumarin α-methylidene-γ-butyrolactone 3a, flavone and xanthone derivatives, 3b and 3c, respectively, are more selective in which only AA- and collagen-induced aggregation are strongly inhibited. Most of the quinoline derivatives (9a-e) exhibited broad-spectrum antiplatelet activities.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790612
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  • 194
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    Transfer Mechanism of Ionic Drugs: Piroxicam as an agent facilitating proton transfer (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1651-1669 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Facilitated proton transfer may be of potential significance in pharmacokinetic and pharmacodynamic processes. Here, we show that the NSAID piroxicam and its N-and O-methylated derivatives act as ionophores for proton transfer across the H2O/1,2-dichloroethane interface. Investigations by cyclic voltammetry showed that the proton transfer occurs by interfacial protonation of the ionophore. The dissociation constants of the three compounds in the organic phase were calculated by Matsuda's theory. With this particular transfer mechanism, the present study exemplifies how electrochemistry at a liquid/liquid interface can be applied to calculate the fundamental thermodynamic parameters related to the pharmacokinetic behavior of ionic drugs.
    Additional Material: 12 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790616
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  • 195
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    Asymmetric Synthesis of 3-Hydroxyprolines by Photocyclization of C(1′)-Substituted N-(2-Benzoylethyl)glycine Esters (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1843-1862 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chiral N-(2-benzoylethyl)-N-tosylglycine esters 5a-h and the α-amino-γ-keto ester 6 were prepared from γ-(tosylamino) alcohols 7a-h. Irradiation of compounds 5a-c, e gave cis-3-hydroxyproline esters 20-23 (Scheme 6), partly with complete asymmetric induction by the C(1′)-substituent, whereas 6 gave enantiomerically pure 4-hydroxy-4-phenyl-L-proline esters 24 in good yield but low de (Scheme 6). The de of the photocyclization depended on the nature and/or size of the C(1′)-substituents. Irradiation of ketones 5d and 5f, bearing H-atoms at C(γ) with respect to the keto function, gave cyclobutanols (Scheme 9) in low yields besides the preferred Norrish-type-II cleavage product. Cyclopentanol 25 was a by-product of the photocyclization of 5c as a result of H—C(δ) abstraction from the t-Bu group. The structure of products 20, 22, and 24a, b was established by NMR or X-ray analyses.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790709
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  • 196
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    Optisch aktive 3-Amino-2H-azirine als Bausteine für enantiomerenreine αα-disubstituierte α-Aminosäuren: Synthese von Isovalin-Synthonen und Einbau in ein Trichotoxin-A-50-SegmentHerrn Professor Conrad Hans Eugster zum 75. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1903-1915 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optically Active 3-Amino-2H-azirines as Synthons for Enantiomerically Pure αα-Disubstiuted α-Amino Acids: Syntheses of Isovaline Synthons and a Segment of Trichotoxin A-50The synthesis of a novel 3-amino-2-methyl-2-[2-(phenylsulfonyl)ethyl]-2H-azirine derivative 12 with a chiral substituent at the amino group is described. Chromatographic separation of the diastereoisomer mixture gave pure diastereoisomers which, after an electrochemical cleavage of the phenylsulfonyl group, yielded the (S)- and (R)-isovalin (Iva) synthons 13a and 13b, respectively. The absolute configuration of the precursor molecule 12b was established by X-ray crystallography. The Iva synthons were successfully used in the synthesis of the C-terminal pentapeptide Z-Leu-Aib-(R)-Iva-Gln-Valol of the peptaibole Trichotoxin A-50 and its epimer.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790713
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  • 197
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    Chiral Diselenides from Benzylamines: Catalysts in the Diethylzinc Addition to Aldehydes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1957-1966 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of new chiral diselenides with a N-atom in the side chain was prepared by a short synthetic sequence (Scheme 1). Only 1 mol-% of these diselenides catalyzed very effectively the diethylzinc addition to various aromatic and α,β-unsaturated aldehydes yielding the secondary alcohols in up to 98% ee (Scheme 2 and Tables 1 and 2). An asymmetric amplification was observed with these catalysts. Detailed NMR studies were performed to characterize the catalytically active species.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790718
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  • 198
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    Total Synthesis of (-)-(2R)-Dihydromyricoidine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1995-2003 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An asymmetric synthesis of the spermidine alkaloid (-)-(2R)-dihydromyricoidine (5) was performed by employing two ring-enlargement reactions. The chiral center was introduced by a diastereoselective Michael addition of perhydropyridazine (7) to the α,β-unsaturated ester 6. The (Z)-C=C bond was obtained by a selective Wittig reaction. The synthetic compound 5 was found to have a negative value for the specific rotation. This is in contrast to that of the natural product reported in the literature. Therefore, as an outcome of this synthesis, the absolute configuration of the natural alkaloid should be inverted to be as shown in structure V.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790721
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  • 199
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    A Formal Total Synthesis of (+)-Methyl Trachyloban-18-oate and (+)-Methyl 16-Oxo-17-norkauran-18-oate: Regio- and Diastereoselective Preparation of Methyl (13S)-13-Hydroxyisoatisiren-18-oate from (-)-Abietic AcidDedicated to Prof. Alessandro Ballio on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2035-2041 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel preparation of methyl (13S)-13-hydroxyisoatisiren-18-oate (4), a key-intermediate in a synthesis of (+)-methyl trachyloban-18-oate ((+)-1), from (-)-abietic acid, is described. Since (-)-1 has been previously converted into (-)-methyl 16-oxo-17-norkauran-18-oate ((-)-16), our preparation of 4 constitutes also a formal total synthesis, from (-)-abietic acid, of (+)-16. Key steps in this approach were the allene photoaddition to podocarp-8(14)-en-13-one (5) and the conversion of the endo-toluene-4-sulfonate 11 into the exo-benzoate 12b.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790724
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  • 200
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    Synthesis and Characterization of Non-standard Nucleosides and Nucleotides Bearing the Acceptor-Donor-Donor Pyrimidine Analog 6-Amino-3-methylpyrazin-2(1H)-one (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1863-1880 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 6-Aminopyrazin-2(1H)-one, when incorporated as a pyrimidine-base analog into an oligonucleotide chain, presents a H-bond acceptor-donor-donor pattern to a complementary purine analog. When paired with the corresponding donor-acceptor-acceptor purine in oligonucleotides, the heterocycle selectively contributes to the stability of the duplex, presumably by forming a base pair of Watson-Crick geometry joined by a non-standard H-bonding pattern. Aspects of the nucleoside chemistry, including syntheses of the β-furanosyl ribonucleoside 1, the ribonucleoside triphosphate 2 and the ribonucleoside bisphosphate 3 of 6-aminopyrazin-2(1H)-one are reported here. In aqueous solution, the ribonucleoside 1 was found to undergo acid- and base-catalyzed rearrangement with an apparent half-life of ca. 63 h at neutral pH and 30°. The rearrangement appears to be specific acid- and base-catalyzed. The thermodynamically most stable compound formed during this rearrangement reaction was isolated by HPLC and shown to be the β-pyranosyl form 4 of the 6-aminopyrazin-2(1H)-one nucleoside in its 4C1 chair conformation. This reactivity of 1 under physiological conditions may explain why Nature does not use this particular heterocyclic system to implement an acceptor-donor-donor H-bonding pattern in the genetic alphabet.
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    URL: http://dx.doi.org/10.1002/hlca.19960790710
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