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  • 1995-1999  (1,168)
  • 1990-1994
  • 1996  (1,168)
  • Life and Medical Sciences  (629)
  • Organic Chemistry  (539)
  • Nuclear reactions
  • 1
    Electronic Resource
    Electronic Resource
    Fullerene-Acetylene Molecular Scaffolding: Chemistry of 2-functionalized 1-ethynylated C60, oxidative homocoupling, hexakis-adduct formation, and attempted synthesis of C1242- (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 6-20 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the way to the fullerene-acetylene hybrid carbon allotropes 2 and 6, the oxidative homocoupling of the 2-functionalized 1-ethynylated C60 derivatives 11, 12, 14, and 15 was investigated. Under Glaser-Hay conditions, the two soluble dumbbell-shaped bisfullerenes 17 and 18, with two C60 moieties linked by a buta-1,3-diynediyl bridge, were formed in 52 and 82% yield, respectively (Scheme 2). Cyclic-voltammetric measurements revealed that there is no significant electronic communication between the two fullerene spheres via the buta-1,3-diynediyl linker. Removal of the 3,4,5,6-tetrahydro-2H-pyran-2-yl (Thp) protecting groups in 18 gave in 80% yield the highly insoluble dumbbell 19 with methanol groups in the 2,2′-positions of the buta-1,3-diynediyl-bridged carbon spheres. Attempted conversion of 19 to the all-carbon dianion 6 (C1242-) via base-induced elimination of formaldehyde was not successful presumably due to exo-dig cyclization of the formed alkoxides. The occurrence of this cyclization under furan formation was proven for 2-[4-(trimethylsilyl)buta-1,3-diyn-1-yl][60]fullerene-1-methanol (21), a soluble model compound for 19 (Scheme 3). To compare the properties of ethynylated fullerene mono-adducts to those of corresponding higher adducts, hexakis-adducts 26 and 28 with an octahedral functionalization pattern resulting from all-e (equatorial) additions were prepared by the reversible-template method of Hirsch (Scheme 4). Reaction of the ethynylated mono-adducts 25 or 13 with diethyl 2-bromomalonate/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in the presence of 1,9-dimethylanthracene (DMA) as reversible template led to 26 and 28 in 28 and 22% yield, respectively. Preliminary experiments indicated a significant change in reactivity and NMR spectral properties of the fullerene addends with increasing degree of functionalization.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790103
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  • 2
    Electronic Resource
    Electronic Resource
    Oligonucleotide Synthesis on Polystyrene-Grafted Poly(tetrafluoroethylene) Support (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 137-150 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Supports consisting of a thin layer (2-10%) of polystyrene (PS) grafted onto a poly(tetrafluoroethylene) (PTFE) core are an interesting alternative to controlled pore glass (CPG) carriers in oligonucleotide synthesis. The beads are mechanically stable, do not show significant swelling, and allow effective removal of substrates by short washing steps with organic solvents. PTFE-PS as an organic polymer has generally more hydrophobic properties than inorganic polymer supports and, therefore, is well compatible with organic solvents such as anhydrous MeCN. We found PTFE with a content of 2-3% PS graft to be a very good support for the synthesis of oligonucleotides of extended length. In comparison, PTFE with 5-10% grafted PS is especially useful for large-scale syntheses. Functionalization procedures minimized in the extent of side reactions are described as well as examples for the use of the supports in syntheses of oligonucleotides both on large scale and of extended chain length.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790115
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  • 3
    Electronic Resource
    Electronic Resource
    Säure-Additionen an ‘Push-Pull’-Enine. Erste Beobachtung einer Umlagerung von 5-Chloro-5-(dialkylamino)pentadienalen30. Mitteilung über Aminoacryl-Derivate. 29. Mitteilung: [1]. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 192-202 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of ‘Push-Pull’ Enynes with Acids. First Observation of a Rearrangement of 5-Chloro-5-(dialkylamino)pentadienals‘Push-pull’-enynes 7 react easily with HCl as well as with AcOH to give 5-amino-5-chloropentadienals 8 and 5-(acetoxy)-5-aminopentadienals 13 as well as the corresponding ketones. In view of a postulated rearrangement of compounds 8 and 13 (Scheme 2), both types of compounds have been treated with traces of acid. While no definite reaction is observed in case of 13, HCl-addition products 8 easily and quantitatively rearrange to give 2H-pyran-2-iminium chlorides 10 which are the postulated intermediates of the rearrangement 8→12 (Scheme 2).
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790119
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  • 4
    Electronic Resource
    Electronic Resource
    The Self-Recognition and Self-Assembly of Folic Acid Salts in Isotropic Water Solution (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 220-234 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-assembly of alkaline folates in isotropic water solutions, with or without added salts, has been investigated by small-angle neutron-scattering, circular-dichroism, and NMR techniques. The assembled species are chiral, cylindrical aggregates of finite length, composed of stacked tetramers; each tetramer is formed by Hoogsteen-bonded folate residues. The assembly process is more efficient in the presence of an excess of NaI ions, leading to longer aggregates with stronger tetramer-tetramer-tetramer interaction. In pure water, the rods are shorter and the tetramer-tetramer interaction weaker. Association between folates can be detected by circular-dichroism spectroscopy starting from a concentration of 6· 10-4 mol l-1, well below the critical concentration for the formation of the cholesteric mesophase (ca. 0.5 mol l-1).
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790123
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  • 5
    Electronic Resource
    Electronic Resource
    Oligosaccharide Analogues of Polysaccharides. Part 6. Orthogonal protecting/activating groups in an improved binomial synthesis of ‘acetyleno-oligosaccharides’ (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 255-268 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bromination of the monosilylated dialkynylated monomer 1a (Scheme 1), dimer 3 and tetramer 5 by N-bromosuccinimide (NBS) in the presence of CF3COOAg gave 2, 4, and 6, respectively, in over 93%. Similar conditions led to bromodesilylation. Either silyl group of the diprotected monomer 1c was selectively removed by bromolysis. On the one hand, bromodesilylation of 1c gave 2 in yields varying between 80 and 99%. On the other hand, bromodesilylation of 7, obtained from 1c by hydrolytic removal of the tetrahydro-2H-pyran-2-yl (Thp) group, yielded 91% of 8. Mechanistic considerations suggested that the deprotective bromination should be improved by replacing the Me3Si by a Me3Ge group. Indeed, bromodegermylation of 1b was quantitative and ca. 60 times faster than bromodesilylation of 1c. The Me3Si and Me3Ge groups can be used for an orthogonal protection/activation of dialkynes. This was shown by desilylating 12 to 11 (Scheme 2), while bromination yielded 13. Both reactions proceeded in high yields; 9 was isolated as a minor by-product of 13. The reactivity towards bromolysis decreases in the series H-DOPS 〉 Me3Ge ≈ H 〉 Me3Si 〉 Thp-DOPS (DOPS = [dimethyl(oxy)-p ropyl]dimethylsilyl). Orthogonal bromolysis of DOPS- and Me3Ge-substituted dialkynes is slightly more selective than the one of Me3Si- and Me3Ge-substituted analogues. Coupling of 7 with the bromoalkyne 2 gave the dimer 15 (76%), 14 (2%), and 16 (4%) (Scheme 3). The binomial synthesis was optimized so that each cycle, doubling the size of the precursor, requires the minimal number of transformations (Scheme 4). The orthogonally protected monomer 1b, dimer 19, and tetramer 22 were, on the one hand, hydrolyzed to the alcohols 18 (95%), 21 (91%), and 24(91%), respectively, and, on the other hand, bromodegermylated to 2 (99%), 4 (97%), and 6(93%). Cross-coupling of 18 with 2, 21 with 4, and 24 with 6 gave the orthogonally protected dimer 19 (73%), tetramer 22 (87%), and octamer 25 (83%), respectively.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790126
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  • 6
    Electronic Resource
    Electronic Resource
    Novel Thiazole-Containing Complexing Agents and Luminescence of Their Europum(III) and Terbium(III) Chelates (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 295-306 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of novel thiazole-containing complexing agents and their luminescence properties with EuIII and TbIII ions are reported. One of these terpyridine analogues was also tested as an EuIII labelling reagent, and its luminescence properties as an antibody conjugate were studied.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790129
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  • 7
    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.19960790201
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  • 8
    Electronic Resource
    Electronic Resource
    Corniculoside, a New Biosidic Ester Secoiridoid from Halenia corniculata (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 363-370 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chemical screening of the secondary metabolites from Halenia corniculata L. (CORNAZ), by LC/UV,LC/TSP-MS (thermospray), and LC/ES-MS (electrospray) was used for the targeted isolation of corniculoside (1), a new biosidic ester secoiridoid. The structure was established as 7-β-[(E)-4′-O-(β-D-glucopyranosyl)caffeoyloxy]-sweroside by 1D- and 2D-NMR, LC/UV, LC/MS, and FAB-MS data, in combination with chemical reactions.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790205
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  • 9
    Electronic Resource
    Electronic Resource
    New Addition Reactions of Organometal Compounds with 4,4-Dimethyl-1,3-thiazole-5(4H)-thionesDedicated to Professor H. Suschitzky on the occasion of his 80th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 371-384 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Addition reactions of organometallic reagents with 4,4-disubstituted 1,3-thiazole-5(4H)-thiones were studied. Whereas the reactions with alkyllithium and alkyl Grignard reagents occurred in the thiophilic manner, the carbophilic addition was observed with allyllithium and allyl Grignard reagents. A radical reaction mechanism is proposed for rationalizing these observations (Scheme 5). A radical cyclization of the prepared 5-allyl-4,5-dihydro-1,3-thiazole-5-thiol derivatives yielded 1,6-dithia-3-azaspiro[4.4]non-2-enes (Table 4).
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790206
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  • 10
    Electronic Resource
    Electronic Resource
    Synthesis of Highly Fluorinated Di-O-alk(en)yl-glycerophospholipids and Evaluation of Their Biological Tolerance (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 405-425 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of various fluorocarbon/fluorocarbon and fluorocarbon/hydrocarbon rac-1,2- and 1,3-di-O-alk(en)ylglycerophosphocholines and rac-1,2-di-O-alkylglycerophosphoethanolamines (see Fig.2), which may be used as components for drug-carrier and delivery systems, are described together with some results concerning their biological tolerance. They were obtained by phosphorylation of perfluoroalkylated rac-di-O-alk(en)ylgly-cerols using POCl3, then condensation with choline tosylate or N-Boc-ethanolamine (2-[(tert-butoxy)carbonyl-amino]ethanol) followed by Boc-deportection (Schemes 6-8). The fluorcarbon/fluorocarbon 1,2-di-O-alkylgly-cerols were prepared by O-alkylation of rac-1-O-benzylglycerol using perfluoroalkylated mesylates, then hydrogenolysis for benzyl deprotection (Scheme 1). The two different hydrophobic chains in the mixed fluorocarbon/fluorocarbon and fluorocarbon/hydrocarbon 1,2-di-O-alk(en)ylglycerols were introduced starting from 1,2-O-iso-propylidene- then O-trityl-protected glycerols or from 1,3-O-benzylidene-glycerol (Schemes 3 and 4). The perfluoroalkylated O-alkenylglycerols were obtained by O-alkylation of a glycerol derivative using an ω-unsaturated alkenyl reagent, the perfluoroalkyl segment being connected onto the double bond in a subsequent step (Schemes 1) and 3. The perfluoroalkylated symmetrical and mixed 1,3-di-O-alkylglycerols were synthesized by displacement of the Cl-atom in epichlorohydrin by perfluoroalkylated alcohols, then catalytic (SnCl4) opening of the oxirane ring of the resulting alkyl glycidyl ethers in neat alcohols (Scheme 5). When injected intravenously into mice, acute maximum tolerated doses higher than 1500 and 2000 mg/kg body weight were observed for the fluorinated glycerophosphocholines, indicating a very promising in vivo tolerance.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790209
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  • 11
    Electronic Resource
    Electronic Resource
    Diazoaldehyde Chemistry. Part 3Part 1: [1a]; Part 2: [1b].. Synthesis of 4-acyl-1H-1,2,3-triazole derivatives (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 449-453 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ten new α-diazo-β-oxoaldehydes were condensed with aniline, ammonia, hydroxylamine, and semicarbazide to yield new 4-acyl-(1-substituted)-1H-1,2,3-triazoles in moderate-to-good yields. The method is simple and regiospecific. The latter feature makes this method superior to the widely used acylacetylene + azide approach.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790212
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  • 12
    Electronic Resource
    Electronic Resource
    N7-DNA: Synthesis and Base Pairing of Oligonucleotides Containing N7-(2-Deoxy-β-D-erythro-pentofuranosyl)guanine (N7Gd) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 477-487 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of oligonucleotides containing N7-(2-deoxy-β-D-erythro-pentofuranosyl)guanine (N7Gd; 1) is described. Compound 1 was prepared by nucleobase-anion glycosylation of 2-amino-6-methoxypurine (5) with 2-deoxy-3,5-di-O-(4-toluoyl)-α-D-erythro-pentofuranosyl chloride (6) followed by detoluoylation and displacement of the MeO group (8→10→1). Upon base protection with the (dimethylamino)methylidene residue (→11) the 4,4-dimethoxytrityl group was introduced at OH—C(5′) (→12). The phosphonate 3 and the phosphoramidite 4 were prepared and used in solid-phase oligonucleotide synthesis. The self-complementary dodecamer d(N7G—C)6 shows sigmoidal melting. The Tm of the duplex is 40°. This demonstrates that guanine residues linked via N(7) of purine to the phosphodiester backbone are able to undergo base pairing with cytosine.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790215
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  • 13
    Electronic Resource
    Electronic Resource
    Studied Directed toward the Synthesis of Phomenoic Acid. Part 2Part 1: [1].. Stereocontrolled Synthesis of the C(7)-to-C(14) Segment (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 518-526 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Starting from the esters (2E,4S)-6 and (2E,4R)-6, bromo aldehydes (S)-9 and (R)-9 as well as bromo alcohols (S)-10 and (R)-10, respectively, were prepared. Bromo alcohol (R)-8 was converted to the diol (2E,4R)-16. Ozonolysis of the latter led to aldehyde (R)-17, which was transformed, by a Wittig reaction, to (2R,4E,6R)-18, corresponding to the C(7)-to-C(14) segment of phomenoic acid (1). Attempts to improve the yields by applying a Julia coupling of (R)-23, which was prepared from (2E,4R)-7, with (R)-24 were unsuccessful. Finally, the coupling of the iodo derivative (2E,4S)-28 with the lithiated derivative of 1,3-dithiane 30 by the Corey-Seebach ‘Umpolung’ led to (3S,4E)-32 which is a derivative of the C(7)-to-C(14) segment of 1, suitable for further transformations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790219
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  • 14
    Electronic Resource
    Electronic Resource
    Channel-Forming Activity of 3-Hydroxybutanoic-Acid Oligomers in Planar Lipid BilayersDedicated to Prof. Dr. Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 507-517 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Monodisperse and polydisperse oligomers and polymers of 3-hydroxybutanoic acid (3-HB) containing 8, 16, ca. 28, 32, ca. 60, 64, 96, and ca. 3000 monomer units were incorporated into palmitoyl-oleoyl-phosphatidyl choline (POPC) planar bilayers. At concentrations of 0.1-5% of oligo(3-HB), the resulting phospholipid bilayers showed typical single-channel behavior for Rb+ and Ba2+ ions, using the patch clamp technique. Thus, channel-forming activity of a pure polyester has been demonstrated for the first time (Figs. 1, 3, and 6). Single-channel activity depends upon the following structural parameters of the 3-HB derivatives: unprotected OH and COOH groups on the chain ends; chain length ≥ 16 monomer units; no high-molecular-weight as in P(3-HB). The results are discussed in view of the Ca2+-specific channel formed with the P(3-HP)/Ca · PPi complex from genetically competent Escherichia coli and in view of the ubiquitous occurrence of low-molecular-weight P(3-HB) in prokaryotic and eukaryotic organisms. A simple model for the channel-causing structure is proposed, based on the proven tendency of oligo- and poly(3-HB) to form ca. 50-Å thick lamellar crystallites.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790218
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  • 15
    Electronic Resource
    Electronic Resource
    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 568-570 
    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.19960790224
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  • 16
    Electronic Resource
    Electronic Resource
    Synthese der p-CumaroylspermidineProf. Edgar Heilbronner zum 75. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 548-559 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Synthesis of p-CumaroylspermidinesThe synthesis of three mono[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines, 10, 20, and 28, three bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines, 6, 16, and 25, and one tris[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]-spermidine is described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790222
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  • 17
    Electronic Resource
    Electronic Resource
    Further C-Alkylations of Cyclotetrapeptides via Lithium and Phosphazenium (P4) Enolates: Discovery of a New Conformation (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 588-608 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Four cyclotetrapeptides containing one (1, 2) or two (3, 4) chiral amino acids have been C-alkylated or C-hydroxyalkylated through Li+ or phosphazenium (P4 · H+) enolates. The reactions are completely diastereoselective (by NMR or HPLC analysis) with respect to the newly formed backbone stereogenic centres (Tables 2 and 3). The reactivity of the polylithiated species responsible for these alkylations is such that only highly reactive electrophiles (MeI, BnBr, primary allylic halides, aldehydes, CO2) can be employed. It is shown that the position, and thus the chirality sense, of the newly formed stereogenic centre in a given cyclotetrapeptide backbone is controlled by the positioning of N-methyl groups in the starting material (cf. cyclo(-MeLeu-Gly-D-Ala-Sar-) (3) and cyclo(-Leu-Sar-MeDAla-Gly-) (4) in Scheme 1). With Schwesinger's phosphazene P4-base, all NH groups are first benzylated and C-benzylation then takes place at a sarcosine, rather than an N-benzylglycine residue (Table 3). In contrast to open-chain N-benzyl peptides, the N-benzylated cyclotetrapeptides could not be debenzylated under dissolving-metal conditions (Na/NH3). Conformational analysis (NMR spectroscopy and X-ray diffraction) shows that the prevailing species have cis/trans/cis/trans(ctct) peptide bonds (zigzag conformation of Ci backbone symmetry, Figs. 2-4). However, a hitherto unknown conformation of cyclotetrapeptides has been found in CDCl3 solutions of the hydroxyalkylated products 18-21 (obtained with EtCHO and PhCHO as electrophiles; Fig.4). The new conformation has four trans peptide bonds and is believed to result mainly from intramolecular H-bond formation, involving the newly generated alkyl- or arylserine residue. This assumption has also been supported by modelling (TRIPOS force field, SYBYL, see Fig.5 and Table 6). The structure may be considered as a β-turn mimic.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790303
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  • 18
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    Electronic Resource
    1,1,2,2-Tetraethynylethanes: Synthons for Tetraethynylethenes and Modules for Acetylenic Molecular ScaffoldingDedicated to Nikolai S. Zefirov on the occasion of his 60th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 634-645 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of functionalized 1,1,2,2-tetraethynylethanes (= 3,4-diethynylhexa-1,5-diynes) as synthons for tetraethynylethenes (3,4-diethynylhex-3-ene-1,5-diynes) and as building blocks for three-dimensional acetylenic molecular scaffolding targeting the synthesis of the molecular carbon belts 3 and 4 is reported (Scheme 1). Reaction of diethyl oxalate and (trialkylsilyl)ethynyl Grignard reagents afforded the silyl-protected 3,4-diethynylhexa-1,5-diyne-3,4- diols 7 and 8 which were transformed in high yields into the cyclic carbonate 9 and the cyclic orthoesters 10-13, respectively (Scheme 2). The solid-state structures of 9 and 10 were elucidated by X-ray crystallography. The alkyne protecting groups in 9, 10, and 12 were smoothly removed to give the free tetraynes 14-16 as relatively stable oils in nearly quantitative yields (Scheme 3). Orthoesters 15 and 16 underwent Pd-catalyzed cross-coupling with iodobenzene to give the tetraphenyl derivatives 17 and 18 (Scheme 4). Thermal acid-catalyzed elimination of the orthoester moieties in 12 and 13 produced the silyl-protected tetraethynylethenes 19 and 20 and concluded a novel, simple three-step synthesis of these fully two-dimensionally conjugated π-chromophores (Scheme 5).
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790306
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    Synthesis of Carbocyclic C-Nucleosides Containing Nonnatural Pyrimidine Bases (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 702-709 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of new carbocyclic C-nucleosides with a cis-4′-(hydroxymethyl)cyclopent-2′-enyl sugar moiety and unnatural pyrimidine bases (2-6) were synthesized in racemic form in two steps starting from the easily accessible cyclic carbonate 1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790312
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  • 20
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    First Fullerene[60]-Containing Thermotropic Liquid Crystal. Preliminary Communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 736-741 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and liquid-crystalline and thermal properties of a fullerene[60] functionalized by a framework containing two cholesterol derivatives through a methanofullerene structure are reported. The targeted fullerene derivative showed high thermal stability.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790316
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  • 21
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    Nucleic-Acid Analogs with Restricted Conformational Flexibility in the Sugar-Phosphate Backbone (‘Bicyclo-DNA’). Part 6Part 5: [1].. Probing the influence of torsion angle γ on DNA-duplex stability: Syntheis and properties of oligodeoxynucleotides containing [(3′S,5′S)-2′-deoxy-3′,5′-ethano-β-D-ribofuranosyl]adenine and -thymine (‘5′-epi-bicyclodeoxynucleosides’)Prelimimanry communication: [2]. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1129-1146 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the thymine- and adenine-containing 5′-epi-bicyclodeoxynucleosides 7 and 8 as well as of the corresponding building blocks 13 and 14 for oligonucleotide syntheis according to the phosphoramidite methodology is described. A conformational analysis of 7 and 8 by 1H-NMR spectroscopy, refined by molecular modeling, shows the preferred conformation of the furanose unit in these nucleosides to be of the 1′-exo/2′-endo type. The 5′-OH group on the carbocyclic ring prefers to be axially oriented, thus placing torsion angle γ in the unusual -syn-clinal (-sc) range. These epi-bicyclodeoxynucleosides were successfully incorporated into DNA decamers. From UV/melting curves of such decamers with DNA and RNA complements, a duplex destabilization of -2 to -9° per residue was observed. An oligonucleotide built completely from 5′-epi-bicyclothymidine shows no detectable affinity to its DNA or RNA complement anymore. CD Spectra of duplexes containing 5′-epi-bicyclodeoxynucleotide units are very similar to the natural reference systems, indicating no major structural changes. A molecular-dynamics simulation of a heptamer duplex containing one 5′-epibicyclothymidine residue in the center reveals a conformational change of its carbocyclic unit placing torsion angle γ in the (for the free mononucleoside unfavorable) -anti-clinal (-ac) conformation in the duplex. The role of torsion angle γ on DNA duplex stability is discussed.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790421
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  • 22
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    The Enantioselective Synthesis of β-Amino Acids, their α-hydroxy derivatives, and the N-terminal components of bestatin and microginin (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1203-1216 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: L-Aspartic acid by tosylation, anhydride formation, and reduction with NaBH4 was converted into (3S)-3-(tosylamino)butan-4-olide (8; Scheme 1). Tretment of 8 with ethanolic trimethylsilyl iodide gave the N-protected deoxy-iodo-β-homoserine ethyl ester 9. The latter, on successive nucleophilic displacement with lithium dialkyl-cuprates ( → 10a-e), alkaline hydrolysis ( → 11a-e), and reductive removal of the tosyl group, produced the corresponding 4-substituted (3R)-3-aminobutanoic acids 12a-e (ee 〉 99%). Electrophilic hydroxylation of 8 ( → 19; Scheme 3), subsequent iodo-esterification ( → 21; Scheme 4), and nucleophilic alkylation and phenylation afforded, after saponification and deprotection, a series of 4-substituted (2S, 3R)-3-amino-2-hydroxybutanoic acids 24 including the N-terminal acids 24e ( =3) and 24f ( =4) of bestatin and microginin (de 〉 95%), respectively.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790426
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  • 23
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    A New Textual Analysis of the Prelog Erlkönig Legend: An Interdisciplinary Approach to Scientific History and Literary Criticism (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1241-1248 
    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.19960790502
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  • 24
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    Total Synthesis of (+)-(8S,13R)-CyclocelabenzineDedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1295-1304 
    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 (+)-cyclocelabenzine (1a) and its (-)-(13S)-epimer 1b is described using optically active (+)-(3S)-3-amino-3-phenylpropionic acid as the chiral building block. The isoquinolin-1-one fragment 15 was synthesized by a modified Bischler-Napieralski reaction. The relative configuration of the (-)-isomer was determined by an X-ray crystal-structure analysis, which enabled us to determine the absolute configuration of natural (+)-1a as (8S,13R).
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790506
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  • 25
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    A Reinvestigation of the Oxidative Rearrangement of Yohimbane-Type Alkaloids. Part BPart A: [1].. Formation of Oxindol (= 1,3-Dihydro-2H-indol-2-one) Derivatives (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1361-1378 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The methanolysis of the epimeric 7-chloro-7H-yohimbine derivatives 2 and 3 was reinvestigated. In case of the 7α-epimer 2, the reaction was uneventful and conformed with earlier observations, i.e., under sufficiently mild conditions, only the imino ether 4 (= imino ether A) was produced. Under the same conditions, the less reactive β-isomer 3 furnished a mixture of both imino ethers 4 and 5, accompanied by the elimination product 11, and by equal amounts of yohimbine (1) and 3,4,5,6-tetradehydroyohimbine (12), which are believed to arise through a disproportionation process of the putative intermediate 5,6-didehydroyhimbine (23). The nature of this divergent reactivity and of the ready equilibration of 4 and 5 was investigated by means of extensive force-field and semi-empirical calculations (AM1 and PM3) of various conformers of the compounds 2-5 and of some possible reaction intermediates.
    Additional Material: 7 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790510
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  • 26
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    2-Alkynylcyclopent-2-enols from 2-Alkynylcyclohex-2-enones via 1-Alkynyl-7-oxabicyclo[4.1.0]heptan-2-ones (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1428-1434 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2-Alkynylcyclohex-2-enones 1a-c and 2a-c react with H2O2/NaOH in MeOH to afford 1-alkynyl-7-oxabicyclo[4.1.0]heptan-2-ones 3a-c and 4a-c, respectively. The 3-unsubstituted bicyclic epoxy ketones 3a, 3b, and 4a, 4b react further with H2O2/NaOH, undergoing ring contraction and (formal) decarbonylation to give 2-alkynylcy-clopent-2-enols 5a, 5b, and 6a, 6b, respectively. Epoxy ketones 3 are also obtained under neutral conditions on irradiation (λ = 350 nm) of cyclohexenones 1 in air-saturated benzene solution. Similarly, under neutral conditions oxo-cycloalkenecarbonitriles 8 react (thermally) with H2O2 in MeCN to give the oxabicyclic carbonitriles 9.
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    URL: http://dx.doi.org/10.1002/hlca.19960790515
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  • 27
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    Xylose-DNA Containing the Four Natural Bases (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1451-1461 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligonucleotides containing (2′-deoxy-β-D-xylofuranosyl)guanine have been prepared. For this purpose 2-aminoadenosine (5) was synthesized and converted to 2′-deoxy-β-D-xyloguanosine (1). The related 2′-deoxy-β-D-xyloisoguanosine (3) and 2′-deoxy-β-D-xyloxanthosine (4) were also synthesized. Compound 1 was converted to the phosphonate and phosphoramidite building blocks 10 and 11, respectively. The oligodeoxynucleotide (5′-3′)d(xG-xT-xA-xG-xA-xA-xT-xT-xC-xT-xA-xC-T) (18) formed a duplex with the same Tm as the parent (5′-3′)-(G-T-A-G-A-A-T-T-C-T-A-C) (19), but with an inverted CD spectrum.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790518
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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.19960790601
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  • 29
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    Biosynthesis of Dodecano-4-lactone in Ripening Fruits: Crucial Role of an Epoxide-Hydrolase in Enantioselective Generation of Aroma Components of the Nectarine (Prunus persica var. nucipersica) and the Strawberry (Fragaria ananassa) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1488-1496 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Early steps in the biosynthesis of dedecano-4-lactone in ripening nectarines (Prunus persica var. nucipersica) and strawberries (Fragaria ananassa) were studied using cis-9,10-([18O]epoxy)heptadecanoic acid (5), instead of the natural C18-precursor epoxystearic acid 1. In nectarines, an epoxide hydrolase catalyzed, in a highly regio- and enantioselective reaction, the attack of H2O at C(9) of epoxy acid 5 yielding preferentially (9R,10R)-dihydroxyheptadecanoic acid 6a as an early intermediate and, after further metabolization, undecano-4-lactone 7a. In strawberries, the same transformation exhibited only a moderate regio- and enantioselectivity. Besides the previously established function of phytogenic epoxide hydrolases involved in generating oxygenated fatty-acid derivatives, which are used in cutin monomer production and plant defence, a new rǒle for these enzymes now emerges in the enantioselective production of aroma components of ripening fruits. The cis-9,10-([18O]epoxy)heptadecanoic acid (≥ 98% 18O; 5) was obtained in good overall yield (68%) from isopropyl (9Z)-heptadec-9-enoate using 18O2 gas in a free-radical chain reaction in the presence of 2-methylbutanal.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790521
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    Synthese von Trifluoromethyl-substituierten Schwefel-Heterocyclen unter Verwendung von 3,3,3-Trifluorobrenztraubensäure-DerivatenVorgetragen von H. H. am ‘Symposium on Fluoroorganic Chemistry’, Warschau, 17. Mai 1996. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1537-1548 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Trifluoromethyl-Substituted Sulfur Heterocycles Using 3,3,3-Trifluoropyruvic-Acid DerivativesThe reaction of methyl 3,3,3-trifluoropyruvate (1) with 2,5-dihydro-1,3,4-thiadiazoles 4a, b in benzene at 45° yielded the corresponding methyl 5-(trifluoromethyl)-1,3-oxathiolane-5-carboxylates 5a, b (Scheme 1) via a regioselective 1,3-dipolar cycloaddition of an intermediate ‘thiocarbonyl ylide’ of type 3. With methyl pyruvate, 4a reacted similarly to give 6 in good yield. Methyl 2-diazo-3,3,3-trifluoropropanoate (2) and thiobenzophenone (7a) in toluene underwent a reaction at 50°; the only product detected in the reaction mixture was thiirane 8a (Scheme 2). With the less reactive thiocarbonyl compounds 9H-xanthene-9-thione (7b) and 9H-thioxanthene-9-thione (7c) as well as with 1,3-thiazole-5(4H)-thione 12, diazo compound 2 reacted only in the presence of catalytic amounts of Rh2(OAc)4. In the cases of 7a and 7b, thiiranes 8b and 8c, respectively, were the sole products (Scheme 3). The crystal struture of 8c has been established by X-ray crystallography (Fig.). In the reaction with 12, desulfurization of the primarily formed thiirane 14 gave the methyl 3,3,3-trifluoro-2-(4,5-dihydro-1,3-thiazol-5-ylidene)propanoates (E)-and (Z)-15 (Scheme 4). A mechanism of the Rh-catalyzed reaction via a carbene addition to the thiocarbonyl S-atom is proposed in Scheme 5.
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    URL: http://dx.doi.org/10.1002/hlca.19960790604
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    Duplex Stability of 7-Deazapurine DNA: Oligonucleotides containing 7-bromo- or 7-iodo-7-deazaguanine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1549-1558 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The oligonucleotide building blocks, the phosphonates 1a, b and the phosphoramidites 2a, b derived from 7-iodo- and 7-bromo-7-deaza-2′-deoxyguanosines 3a, b were prepared. They were employed in solid-phase oligonucleotide synthesis of the alternating octamers d(Br7c7G-C)4 (8) and d(I7c7G-C)4 (9) as well as the homo-oligonucleotides d[(Br7c7G)5-G] (11) and d[(I7c7G)5-G] (12). The melting profiles and CD spectra of oligonucleotide duplexes were measured. The Tm values as well as the thermodynamic data were determined and correlated to the major-groove modification of this DNA. The self-complementary octamers 8 and 9 form more stable duplexes compared to the parent oligomer d(G-C)4. The heteroduplex of d[(I7c7G)5-G] (12) with d(C6) is slightly destabilized (ΔTm = -12°) over that of d[(c7G)5-G] with d(C6). However, the complex of 12 with poly(C) is more stable than that of d[(c7G5-G)] with poly(C).
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790605
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    The Crystal and Molecular Structure of [Co(tmen)3](C7H7SO3)3·2 H2O and [Co(tmen)3](CH3SO3)2·EtOH·1/2 H2O (tmen = 2,3-dimethylbutane-2,3-diamine) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1607-1610 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The molecular structure of the two complex ions [Co(tmen)3]3+ and [Co(tmen)3]2+ (tmen = 2,3-dimethylbutane-2,3-diamine) have been determined at 100 K. They show a strong trigonal distortion of the octahedral CoN6 core. The twist angle is 26.2(2.8)° for CoII and 43.9(9)° for CoIII. Average Co—N distances are 2.193(13) Å (CoII) and 1.997(2) Å (CoIII), 0.02 Å longer than for the main body of CoIII amines.
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    URL: http://dx.doi.org/10.1002/hlca.19960790610
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    Hydrolytical Cleavage of TAR-RNA, the trans-Activation Responsive Region of HIV-1, by a Bis(guanidinium) Catalyst Attached to Arginine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1967-1979 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Guanidinium compounds imitating the bis(arginine) structural motif of staphylococcal nuclease (e.g. 3) are known to be powerful catalysts for phosphoryl transfer reactions in dipolar aprotic solvents. Compound 3 also accelerates the hydrolysis of RNA (H2O, pH 7). However, due to diminished substrate affinity in H2O, the rate effects are less pronounced in aqueous than in aprotic solution. To test if a synthetic ribonuclease may be derived from the bis(guanidinium) moiety of 3 by the addition of RNA-binding substructures, the TAR sequence of HIV-1 was chosen as a target. The arginine residue of compound 4 serves as an extremely simplified mimic of tat, a protein responsible for boosting the viral transcription by complex formation with TAR. Here, we present the synthesis of 4 and its ability to bind and to cleave efficiently the truncated TAR sequence 1. In addition, the synthesis of an acridine arginine conjugate, 19, is reported in preliminary form. Compound 19 associates with 1 and completely blocks the cleavage induced by 4.
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    URL: http://dx.doi.org/10.1002/hlca.19960790719
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    Lipid Masking and Reactivation of Angiotensin Analogues (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2023-2034 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Studies on post-angioplastic restenosis have shown the implication of angiotensin II (Ang) as a myoproliferative mediator. The antiproliferative efficacy of non-peptide Ang antagonists on the rat carotid model is of 50%, whereas a continuously infused peptide antagonist at low doses totally blocks neointimal growth. To explore the feasibility of depot forms of Ang that may be introduced during angioplasty and thus prevent restenois, lipid-masked Ang analogues of the following general structure were prepared: [Xxx°, Yyy1]Ang with Xxx = decanoyl or palmitoyl and Yyy = Ser, Cys, Asp, β-lactoyl, 3-mercaptopropanoyl or succinyl. All fatty acylated peptides [Xxx°, Yyy1]Ang were practically inactive, and O- or S-esterified Ser and Cys peptides underwent intramolecular transcylation giving inactive Nx-acylated peptides. O-Acylated [β-mercaptopropanoyl1]Ang were easily hydrolyzed into their biologically active[Yyy1]Ang forms, either by mild saponification or by lipase activity.
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    URL: http://dx.doi.org/10.1002/hlca.19960790723
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    From Inactive Nortopsentin D, a Novel Bis(indole) Alkaloid Isolated from the Axinellid Sponge Dragmacidon sp. from Deep Waters South of New Caledonia, to a Strongly Cytotoxic Derivative (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2075-2082 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nortopsentin D (5), a bis(indole) alkaloid unique for bearing a 2-amino-methylimidazole appendage at the central 1H-imidazol-5(4H)-one nucleus, was isolated in abundance, besides the putative biogenetic precursor 6 of its appendage, from the deep-water axinellid sponge Dragmacidon sp. Structural elucidation of 5 by NMR and MS methods heavily relied on its N-methyl derivatives 8-11. Unusually for topsentin-type structures, natural 5 and semisynthetic methyl derivatives 8 and 10 proved inactive on KB tumoural cells, while introduction of the last three methyl groups, amazingly led to highly cytotoxic 11.
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    URL: http://dx.doi.org/10.1002/hlca.19960790804
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    Model Studies for the Coenzyme-B12-Catalyzed Methylmalonyl→Succinyl Rearrangement. The Importance of Hydrophobic Peripheral Associations (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2100-2113 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interaction between a vitamin B12 derivative containing a peripheral C18 alkyl chain (see 1a) and a (methyl)thiomalonate substrate bearing alkyl chains of various length at the thioester group (see 5) was investigated. A catalytic cycle was established for the methylmalonyl→succinyl rearrangement by using electrochemistry and photolysis (see Scheme 3). Increased yields of the succinate relative to the reduction product were obtained (2:3 ratio), when the reaction was run in MeOH/H2O, and when both the substrate and the catalyst had an octadecyl substituent capable of hydrophobic interactions.
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    URL: http://dx.doi.org/10.1002/hlca.19960790807
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    Electrospray-Ionization Mass Spectrometry. Part III.Part II:[1] Acid-Catalyzed Isomerization of N,N′-Bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines by the Zip ReactionPresented by L.B. at the ‘29. Diskussionstagung der Arbeitsgemeinschaft Massenspektrometrie der Deutschen Physikalischen Gesellschaft’, on May 28-31, 1996, University of Bremen, Germany. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2152-2163 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electrospray tandem mass spectra (ESI-MS/MS) of the three N,N′-bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines 1-3 displayed the same fragment-ion signals. These isomers could not be differentiated by ESI-MS/MS, since their fragmentation patterns are similar. (E,E)-N-(3-[15N]Aminopropyl)-3,3′-bis(4- hydroxyphenyl)-N,N′-(butane-1,4-diyl)bis[prop-2-enamide] ([15N(1)])-(1) was synthesized in order to get further information about the fragmentation mechanisms. The comparison of the ESI-MS/MS of 1 and [15N(1)]-1 revealed a transamidation, the Zip reaction, under mass-spectral conditions of the [1 + H]+ ions. Because of this reaction, the three isomers 1-3 could not be distinguished.
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    Total Synthesis of Cyclothialidine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2219-2234 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A total synthesis of cyclothialidine (1), a new DNA gyrase inhibitor isolated from Streptomyces filipinensis, is described. The synthetic concept was tested by preparing the lactone 13 (Scheme 2) which contains the characteristic bicyclic core entity of 1. Key features of the synthesis of 1 are: preparation of 3,5-dihydroxy-2,6-dimethylbenzoic acid (23) from 3,5-dihydroxybenzoic acid (19) by two consecutive Mannich aminomethylation/hydrogenation sequences; benzylic N-bromosuccinimide bromination of an ester derivative 25 thereof and its subsequent coupling with Boc-Ser-Cys-OMe (11); cyclization of the ω-hydroxy acid 29 29 to the 12-membered lactone 30 using preferably Mitsunobu conditions; completion of the peptidic side chains of 1 using Boc strategy (Scheme 4). Optically pure cis-N-(tert-butoxycarbonyl)-3-hydroxy-L-proline ((-)-14) was obtained by resolution of the racemate via an efficient reaction sequence containing a lipase-catalyzed enantiospecific acetate hydrolysis (Scheme 3). The structure of natural 1 was confirmed by comparison with the synthetic material. The synthetic route described provides also easy access to analogues of 1, e.g., via the intermediate 30.
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  • 39
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    Donor/Acceptor-Substituted Tetraethynylethenes: Systematic Assembly of Molecules for Use as Advanced Materials (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2249-2281 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A comprehensive series of tetraethynylethenes (= 3,4-diethynylhex-3-ene-1,5-diynes, TEEs) bearing electrondonating (p-methoxyphenyl or p-aminophenyl) and/or electron-accepting (p-nitrophenyl) groups was prepared via [Pd]-catalyzed Sonogashira cross-coupling reactions. The electronic and photonic properties of these molecules were investigated with a special emphasis on the effects caused by degree and pattern of donor/acceptor substitution around the central TEE core. This analysis showed that intramolecular donor-acceptor interactions, as evidenced by a long-wavelength charge-transfer band, are considerably more effective in TEEs 44 and 46, with trans and cis, linearly-conjugated electronic pathways between donor and acceptor, than in 11, with a geminal, cross-conjugated electronic pathway. UV/VIS Spectroscopy revealed a steady bathochromic shift of the longest-wavelength absorption band (λmax) as the number of donor-acceptor conjugation paths increased upon changing from bis-arylated (11, 44, and 46) to tetrakis-arylated (14, 31, and 35) TEEs. The position of the longest-wavelength absorption was also found to be strongly dependent on the nature of the N-substituents in the R2NC6H4-donor groups. Electronic emission spectroscopic investigations demonstrated a considerable solvent dependency of the fluorescence of donor-acceptor-substituted TEEs such as 11 or 44, in agreement with the presence of highly polarized excited states in these molecules. Correspondingly, fluorescence spectra of TEEs bearing only donor or acceptor substituents showed little solvent dependency. The large majority of the donor/acceptor-substituted TEEs are thermally and environmentally stable molecules. They can be stored for months as solids in the air at room temperature, and many decompose only upon heating to temperatures above 200°. X-Ray analysis showed the conjugated C-atom scaffolds of 44, 46, and 67 to be essentially planar, whereas the aryl substituents in 28 and 30 are rotated out of the plane of the TEE core by varying degrees.
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    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 913-913 
    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.19960790330
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  • 41
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    β-Peptides: Synthesis by Arndt-Eistert homologation with concomitant peptide coupling. Structure determination by NMR and CD spectroscopy and by X-ray crystallography. Helical secondary structure of a β-hexapeptide in solution and its stability towards pepsin (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 913-941 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The β-hexapeptide (H-β-HVal-β-HAla-β-HLeu)2-OH (2) was prepared from the component L-β-amino acids by conventional peptide synthesis, including fragment coupling. A cyclo-β-tri- and a cyclo-β-hexapeptide were also prepared. The β-amino acids were obtained from α-amino acids by Arndt-Eistert homologation. All reactions leading to the β-peptides occur smoothly and in high yields. The β-peptides were characterized by their CD and NMR spectra (COSY, ROESY, TOCSY, and NOE-restricted modelling), and by an X-ray crystal-structure analysis. β-Sheet-type structures (in the solid state) and a compact, left-handed or (M) 31 helix of 5-Å pitch (in solution) were discovered. Comparison with the analogous secondary structures of α-peptides shows fundamental differences, the most surprising one at this point being the greater stability of β-peptide helices. There are structural relationships of β-peptides with oligomers of β-hydroxyalkanoic acids, and dissimilarities between the two classes of compounds are a demonstration of the power of H-bonding. The β-hexapeptide 2 is stable to cleavage by pepsin at pH 2 in H2O for at least 60 h at 37°, while the corresponding α-peptide H-(Val-Ala-Leu)2-OH is cleaved instantaneously under these conditions. The implication of the described results are discussed.
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  • 42
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    Novel Cholic-Acid-Type Sterones of Deltocyathus magnificus, a deep-water scleractinian coral from the Loyalty Islands, SW Pacific (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 982-988 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cholic-acid-type 3-keto steroids 1-8, all isolated from Deltocyathus magnificus (2-8 after CH2N2 treatment), are the first secondary metabolites obtained from a deep-water scleractinian. Steroids 1-3, 5, and 7 are new, their main structural oddities being loss of C(24) (see 5) or hydroxylation in the side chain (see 3 and 7). Steroids 4, 6, and 8, from the same coral, were previously known from other marine organisms.
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  • 43
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    Acid-Catalysed Backbone Rearrangement of cholesta-6,8(14)-dienes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 989-998 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rearrangement of 5α- and 5β-cholesta-6,8(14)-dienes (13a and 13b, resp.) in the presence of anhydrous toluene-4-sulfonic acid in acetic acid leads to 5α- and 5β-12(13 → 14)-abeo-cholesta-8,13(17)-dienes (15a and 15b, resp.) via 5α- and 5β-cholesta-8,14-dienes (14a and 14b, resp.), respectively. Epimerization at C(20) of the spirosteradienes 15a and 15b occurs with increasing reaction time. Molecular-mechanics calculation of the relative stabilities of these compounds and of congeners thereof is in agreement with the observed reaction pathway.
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  • 44
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    Asymmetric Synthesis of the Alkaloids mayfoline and N(1)-acetyl-N(1)-deoxymayfoline (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1085-1094 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The total syntheses of the spermidine alkaloids (-)-mayfoline (11) and (+)-N(1)-acetyl-N(1)-deoxymayfoline (12) are described. These macrocyclic lactams belong to the most interesting conjugates of the polyamine derivatives very commonly found in nature. The enantioselective syntheses were achieved through resolution of the methyl 3-amino-3-phenylpropanoate (2) by recrystallization of its (+)-L-tartrate salt. Construction of the 13-membered ring ensued through condensation, reductive ring expansion (internal bond cleavage), and finally a transamidation reaction involving a second ring expansion.
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  • 45
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    Electrochemical Behaviour and Antioxidant Activity of Some Natural Polyphenols (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1147-1158 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of natural polyphenols (chlorogenic acid (9), cordigol (11), cordigone (12), danthrone (1), 1,5-dihydroxy-3-methoxyxanthone (2), eriosematin (7), flemichin D (8), frutinone A (6), mangiferin (4), quercetin (5), 1,3,6,7-tetrahydroxyxanthone (3) and verbascoside (10)) were investigated for their redox properties using cyclic voltammetry. The antioxidant properties of these compounds were also examined in two models, namely lipid peroxidation in rat synaptosomes and AAPH-mediated oxidation of serum albumin. Compounds with a catechol group (9, 4, 5, 3 and 10) were oxidized below 0.4 V and inhibited lipid peroxidation with IC50 values between 2 and 8 μM. Compounds having one or more isolated phenolic groups and showing an oxidation potential between 0.45 and 0.8 V (11, 12 and 8) inhibited lipid peroxidation with IC50 between 7 and 9 μM, except 2 (0.45 V), danthrone (0.96 V) and eriosematin which showed no or modest antioxidant activity. Some of the investigated compounds also protected albumin from oxidation, but no structure-activity relationship was apparent, suggesting that other factors beside redox potential influence this activity.
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  • 46
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    Glycosylidene Carbenes. Part 23. Regioselective glycosidation of deoxy- and fluorodeoxy-myo-inositol derivativesDedicated to Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1169-1191 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To demonstrate the relevance of the kinetic acidity of individual OH groups for the regioselectivity of glycosylation by glycosylidene carbenes, we compared the glycosylation by 1 of the known triol 2 with the glycosylation of the diol D-3 and the fluorodiol L-4. Deoxygenation with Bu3SnH of the phenoxythiocarbonyl derivative of 5 (Scheme 1) or the carbonothioate 6 gave the racemic alcohol (±)-7. The enantiomers were separated via the allophanates 9a and 9b, and desilylated to the deoxydiols D- and L-3, respectively. The assignment of their absolute configuration is based upon the CD spectra of the bis(4-bromobenzoates) D- and L-10. The (+)-(R)-1-phenylethylcarbamates 13a and 13b (Scheme 2) were prepared from the fluoroinositol (±)-11 via (±)-4 and the silyl ether (±)-12 and separated by chromatography. The absolute configuration of 13a was established by X-ray analysis. Decarbamoylation of 13a ( → L-12) and desilylation afforded the fluorodiol L-4. The H-bonds of D-3 and L-4 in chlorinated solvents and in dioxane were studied by IR and 1H-NMR spectroscopy (Fig. 2). In both diols, HO—C(2) forms an intramolecular, bifurcated H-bond. There is an intramolecular H-bond between HO—C(6) and F in solutions of L-4 in CH2Cl2, but not in 1,4-dioxane; the solubility of L-4 in CH2Cl2 is too low to permit a meaningful glycosidation in this solvent. Glycosidation of D-3 in dioxane by the carbene derived from 1 (Scheme 3) followed by acetylation gave predominantly the pseudodisaccharides 18/19 (38%), derived from glycosidation of the axial OH group besides the pseudodisaccharides 16/17 (13%) and the epoxides 20/21 (7%), derived from protonation of the carbene by the equatorial OH group. Similarly, the reaction of L-4 with 1 (Scheme 4) led to the pseudodisaccharides 28/29 (46%) and 26/27 (14%), derived from deprotonation of the axial and equatorial OH groups, respectively. Formation of the epoxides involved deprotonation of the intramolecularly H-bonded tautomer, followed by intramolecular alkylation, elimination, and substitution (Scheme 4). The regio- and diastereoselectivities of the glycosidation correlate with the H-bonds in the starting diols.
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    A Novel Asymmetric Benzoin Reaction Catalyzed by a Chiral Triazolium Salt. Preliminary communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1217-1221 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using the chiral triazolium salt 1 as catalyst, a novel asymmetric variant of the benzoin reaction is reported. For the first time, the scope of the method is extended to a broader range of aromatic aldehydes 2, affording the acyloins 3a-h in yields of 22-72% and enantiomeric excesses up to 86%.
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    Erratum (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1239-1239 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790429
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    Variation and SelectionDedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1260-1278 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ninety years ago, chemical methods were used in solving biological problems. In our days, methods of evolutionary biology are going to be used in solving problems of synthetic chemistry.
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    URL: http://dx.doi.org/10.1002/hlca.19960790504
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  • 50
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    Oligosaccharide Analogues of Polysaccharides. Part 8. Orthogonally Protected Cellobiose-Derived Dialkynes. A Convenient Method for the Regioselective Bromo- and Protodegermylation of Trimethylgermyl- and Trimethylsilyl-protected DialkynesDedicated to Vlado Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1279-1294 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cellobiose-derived dialkynes 14 and 15 were prepared by glycosidation of the acceptor 9 with the thioglycosides 12(82%) and 13(85%), respectively. The acceptor 9 was prepared from the known alcohol 2 via the lactone 7 in five steps (48% overall), and the donors 12 and 13 were prepared from the alkynylated anhydroglucose derivative 10 (60% overall). Acetolytic debenzylation of 14 and 15 (→16 and 17, resp.) followed by deacylation of 16 yielded 60% of the cellobiose-derived dialkyne 18. Deacylation of 14 (→19), methoxymethylation (→20) and trimethylgermylation led to the orthogonally protected dialkyne 21 (69% overall). Protodesilylation of 21 with K2CO3/MeOH gave 22 (90%), while the Me3Ge group was selectively removed with CuBr (19 mol-%) in THF/MeOH to give 20 (95%). Treatment of 21 with aqueous HCl solution led to 19 (80%). Bromodegermylation of 21 (NBS/AgOOCCF3) led to a mixture of 23 (85%) and 24 (11%). Similar conditions using CuBr instead of AgOOCCF3 gave exclusively the bromoalkyne 23 (93%). The temperature dependence of the δ values of the OH resonances of 18 in (D6)DMSO evidence a strong intramolecular H-bond between C(5′)—O…HO—C(5).
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    Analogs of Cinchona Alkaloids Incorporating a 9,9′-Spirobifluorene MoietyDedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1338-1360 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Cinchona alkaloid analogs (+)- and (-)-5 with a quinuclidine-2-methanol residue attached to C(2) of a 9,9′-spirobifluorene moiety were prepared as a racemic mixture by reacting lithiated 2-bromo-9,9′-spirobifluorene 7 with (2-ethoxycarbonyl)quinuclidine (±)-6 to give ketone (±)-8, followed by diastereoselective reduction with diisobutylaluminum hydride (DIBAL-H). The absolute configuration at C(9) and C(8), i.e., at the methanol bridge and the adjacent quinuclidine C-atom, in the two enantiomers of 5 is identical to the configuration at the corresponding centers in (-)-quinine (1) and (+)-quinidine (2), respectively. For the optical resolution of (±)-5, a chiral stationary phase for HPLC was prepared by covalently bonding quinine via a thiol spacer to a silica-gel surface. The enantiomer separation was accomplished at an α value of 1.61 with (±)-5 being eluted last, in agreement with 1H-NMR studies in CDCl3 which showed that (+)-5 underwent a more stable host-guest association with quinine than (-)-5. 1H{1H} Nuclear Overhauser effect (NOE) difference spectroscopical analysis of the host-guest associations with quinine in CDCl3, combined with computer-model examinations, allowed the assignment of the absolute configurations as (+)-(8R,9S)-5 and (-)-(8S,9R)-5. A detailed conformational analysis displayed excellent agreement between the results of computational methods (Monte Carlo multiple minimum simulations, analyses of the total energy as a function of the flexible dihedral angles in the molecule) and 1H{1H}-NOE difference spectroscopical data. It was found that (-)-5 and (+)-5 differ significantly in their conformational preference from their natural counterparts quinine (1) and quinidine (2). Whereas the natural alkaloids prefer the ‘open’ conformation, with the quinuclidine N-atom pointing away from the quinoline ring, analog (±)-5 adopts preferentially (by ca. 4 kcal mol-1) a ‘closed’ conformation, in which the quinuclidine N-atom points into the cleft of the 9,9′-spirobifluorene moiety. Since the basic quinuclidine N-atom in the ‘closed’ conformation is sterically shielded from forming strong H-bonds, the new Cinchona alkaloid analogs form less stable host-guest associations via H-bonding than quinine or quinidine.
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    A New Approach to the Synthesis of Long-Chain Polypropionates Based on the Diels-Alder Monoadditions of 2,2′-Ethylidenebis[3,5-dimethylfuran]Part of the Ph.D. thesis of J.A., University of Lausanne, 1995. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1393-1414 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acidic condensation of 2,4-dimethylfuran with acetaldehyde provided 2,2′-ethylidenebis[3,5-dimethylfuran] (7) which added 1 equiv. of methyl bromopropynoate to give a major adduct 8. Regio- and stereoselective hydroboration of the latter 7-oxanorbornadiene derivative followed by alcohol protection and methanolysis of its β-bromoacrylate moiety gave (1RS,2RS,4RS,5SR,6SR,1′RS)-methyl 4-[1′-(3″,5″-dimethylfuran-2″-yl)ethyl]-3,3-dimethoxy-6-exo-[(2-methoxy)ethoxy]-1,5-endo-dimethyl-7-oxabicyclo[2.2.1]heptane-2-endo-carboxylate (24) (Schemes 2 and 3). Reduction of 24 with LiAlH4, followed by H2O and MeOH elimination gave the 3-methyl-idene-7-oxanorbornan-2-one derivative 26 which underwent 7-oxa ring opening through a SN2′ type of reaction with Me2CuLi (Scheme 4). Stereoselective hydrogenation and ketone reduction provided (1RS, 2SR,3RS,4RS,5RS,6RS,1′SR)-1- [1″-(3 ″,5″-dimethylfuran-2″-yl)]-c-3-ethyl-c-5-[(2-methoxyethoxy)m e-c-ethyl-c-c-5-(2-methoxyethoxy)methoxy]-t-4,t-6-dimethyl-cyclohexane-r-1,c-2-diol (32), the oxidative cleavage of which with Pb(OAc)4 generated a 6-oxo-aldehyde 33 (Schemes 4 and 5). Chemoselective protection of 33 and chemo- and stereoselective reductions generated (2RS,3RS,4SR,5SR,6SR,7RS)-7-(3′,5″-dimethylfuran-2′-yl)-2-ethyl-6-hydroxy-4-[(2-methoxyethoxy)methoxy]-3,5-dimethyloct-1-yl pivaloate (36) and its 4-hydroxy 6-epimer 40 (12 and 13 steps, resp., from adduct 8; Scheme 5). Oxidation of the furan ring of 36 led to a (2RS,3SR,4RS,5SR,6RS,7RS)-7-ethyl-3,5,8-trihydroxy-2,4,6-trimethyl-octanoic acid derivative 44, a polypropionate fragment with six contiguous stereogenic centres (Scheme 6).
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    Isomerisierung von (all-E)-Cycloviolaxanthin Herstellung und Charakterisierung von (9Z)-und (13Z)-Cycloviolaxanthin (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1444-1450 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Isomerization of (all-E)-Cycloviolaxanthin. Preparation and Characterization of (9Z)- and (13Z)-CycloviolaxanthinFrom (all-E)-cycloviolaxanthin (ex Capsicum annuum), (9Z)- and (13Z)-cycloviolaxanthin were prepared in a highly pure state. The configuration of both isomers was determined by means of UV/VIS, NMR, and CD spectroscopy.
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    URL: http://dx.doi.org/10.1002/hlca.19960790517
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  • 54
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    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1518-1518 
    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.19960790523
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  • 55
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    Intramolecular Vinyl Radical Cyclization Reactions of cyclohexadienes derived from sequential additions to (arene)(tricarbonyl)chromium complexes. Preliminary communicationDedicated to the memory of our late colleague Wolfgang Oppolzer (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1533-1536 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cyclohexa-1,3-dienes 7-10 with a 1,5,6-substitution pattern were prepared in a one-pot reaction sequence by sequential addition of MeLi and propargyl bromides to the tricarbonylchromium complex 1a. These products were subjected to radical cyclization procedures. Vinyl radical generation by Bu3Sn addition to the propargyl group in 7-10 was followed by regio- and diastereoselective intramolecular ring closure. Two different 5-exo-trig cyclizations are possible via different vinyl radical intermediates and cyclization to one or the other of the termini of the cyclohexadiene moiety. Internal alkynes reacted to yield exclusively the cis-fused hexahydroindene products 12-14, whereas the terminal alkyne yielded bicyclo[3.2.1]octenes 11 as sole products.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790603
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  • 56
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    The First Asymmetric Intramolecular Stetter Reaction. Preliminary Communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1899-1902 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first asymmetric intramolecular Stetter reaction is reported, using the chiral triazolium salt 1 as catalyst. Starting from the easily accessible 4-(2-formylphenoxy)but-2-enoates 2, this protocol opens up an enantioselective pathway to the benzo-annulated pyran-4-ones (chroman-4-ones) 3a-h with good yields and enantiomeric excesses of up to 74%.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790712
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  • 57
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    Reactions of 3-(Prop-2-enylidene)azetidin-2-ones with 4,5-Dihydro-3H-1,2,4-triazole-3,5-dionesDedicated to Prof. Gunther Seitz. Marburg. on the occasion of his 60th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1925-1929 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 3-(2-propenylidene)-β-lactams 1a-c react as semicyclic dienes with the dihydrotriazole-diones 2 in a stereoselective fashion leading to the cycloadducts 3. The 3′,3′-disubstituted derivative 4 gives no [4 + 2]-cycloaddition products but forms the adducts 5 and 6. Sterochemistry and reaction pathways are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790715
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  • 58
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    Mechanistic Details of the Fe+-Mediated C—C and C—H Bond Activations in Propane: A Theoretical Investigation (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1939-1956 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantum-chemical calculations employing a density-functional theory/Hartree-Fock hybrid method (B3LYP) have been used to explore the mechanistic details of the C—C and C—H bond-activation processes in propane mediated by a bare Fe+ ion. While the theoretically predicted results are in complete accord with all available experimental data, they give rise to a different mechanistic picture than envisaged previously. In contrast to earlier speculation, the activation barriers for the initial insertion steps of Fe+ into a C—H or C—C bond are found to be significantly below the Fe+ + C3H8 channel. The rate-determining steps for both, the C—C and the C—H bond activation branches of the [FeC3H8]+ potential-energy surface rather occur late on the respective reaction coordinates and are connected with saddle points of concerted rearrangement processes. The C—C bond activation, which leads to the exothermic reductive elimination of methane, occurs via the C—C inserted species and not as a side channel originating from a C—H inserted ion, as assumed hitherto. For the C—H bond-activation processes, which finally results in the exothermic expulsion of molecular hydrogen, two energetically similar reaction channels for an [1,2]-elimination exist. The results clearly show, that an [1,3]-H2-elimination mechanism cannot compete with the [1,2]-elimination paths, in line with the experimental findings. Overall, a lower energy demand for the reductive elimination of methane compared to the loss of H2 is obtained, straightforwardly explaining the preference of the former process observed experimentally.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790717
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  • 59
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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.19960790801
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  • 60
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    Synthesis of 1,2,5-Thiadiazepine Derivatives by Ring Enlargement of 1,2-Thiazetidin-3-one 1,1-Dioxides with 3-Amino-2H-azirines (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2067-2074 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: At 0° in MeCN, 2,2-disubstituted 3-amino-2H-azirines 1 and 4,4-disubstituted 1,2-thiazetidin-3-one 1,1-dioxides 7 react smoothly to give 1,2,5-thiadiazepin-6-one 1,1-dioxides of type 8 (Scheme 2). The reaction mechanism of this regiospecific ring enlargement to seven-membered heterocycles follows previously described pathways. The structures of 7a and 8b were established by X-ray crystallography (see Figs. 1 and 2).
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790803
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  • 61
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    Biosynthesis of δ-Jasmin Lactone ( = (Z)-Dec-7-eno-5-lactone) and (Z,Z)-Dodeca-6,9-dieno-4-lactone in the Yeast Sporobolomyces odorus (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2088-2099 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (all-Z)-(9,10,12,13,15,16-2H6)Octadeca-9,12,15-trienoic acid ( = α-linolenic acid; D6-4) was synthesized to investigate the biochemical formation of linolenic-acid-derived aroma compounds in cultures of the yeast Sporobolomyces odorus, using an established gas chromatographic/mass spectrometric (GC/MS) method. Three compounds were identified as labeled: (Z)-dec-7-eno-5-lactone (δ-jasmin lactone), (Z,Z)-dodeca-6,9-dieno-4-lactone, and (2E,4Z)-hepta-2,4-dienoic acid. Both lactones were biosynthesized mostly under conservation of the initial configuration from their corresponding oxygenated linolenic-acid intermediates. The application of (13S,9Z,11E,15Z)-13-hydroxy(9,10,12,13,15, 16-2H6)octadeca-9,11,15-trienoic acid (D6-7) as a OH-functionalized precursor of δ-jasmin lactone allowed to gain insight into the stereochemical course of the biosynthesis to both enantiomers of this lactone. In this experiment, 88.3% of the metabolized labeled precursor was transformed under retention of the original configuration of the (R)-enantiomer. This investigation is also a contribution to a better understanding of the C=C bond isomerization steps which took place during the β-oxidative degradation of the substrate.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790806
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  • 62
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    5,5′-Bi-5H-cyclopenta[2,1-b;3,4-b′]dipridinylidene, a New Bridging Ligand for Metal Complexes. Preliminary Communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2176-2179 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5,5′-Bi-5H-cyclopenta[2,1-b;3,4-b′]dipyridinylidene(1) was synthesized in three steps from 9,10-phenanthroline and characterized by UV/VIS and NMR spectroscopy, mass spectrometry, and cyclic voltammetry. Its ability to act as a bridging ligand is demonstrated by the synthesis of the complexes [Ru(bpy)2(1)](PF6)2 (6) and [{Ru(bpy)2}2(1)](PF6)4 (7) (bpy = 2,2′-bipyridine).
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790812
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  • 63
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    Synthesis and Structure of Functionalized Cyclododecadiynes and -dienes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2164-2175 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cyclododecadiynes 3b-d, 8b-d, and 10b-c with functionalities in two propargylic positions, as well as the monofunctionalized diyne 13b have been prepared from simple open-chain building blocks. In the DMPU ( = N,N'-dimethylpropyleneurea)-assisted ring-closing alkylation of 1,7-diynes, the twelve-membered ring compounds have been prepared in yields of 16-55%. The preparation of the diene-diyne 15 and the cyclododeca-5,11-diyne-1,4-dione 18 are described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790811
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  • 64
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    Synthesis of Enantiomerically Pure Carbohydrate-Derived Annulated Cyclopentadienes and Ferrocenes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2201-2218 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cyclopentadienes 3a/b/c, 8b/c, 12a/b/d, and 16a/b/d were prepared as mixtures of regioisomers from the D-mannitol-derivatives 1,6,10, and 14 and transformed into the ferrocenes 17, 18, and 19 (73%; 38:17:45), 23, 24, and 25 (70%; 6:42 :52), 26 (31%), and 27 (27%), respectively. Deprotection of 17-19 with HCl/MeOH gave the H2O-soluble ferrocenes 20-22; chloromercuration and iodination of 17 via 29 led to the C2-symmetric diiodo-ferrocene 30. The mono(chloromercurio) derivative 28, obtained as a by-product, was also transformed into 29. The structure of the ferrocenes 18 and 19, and of the bis(chloromercurio)ferrocene 29 has been established by X-ray analysis. The starting cyclopentadienes 3 were obtained in 50% yield from 1 by dialkylation of CpNa, followed by thermolysis of the spiro-annulated 2. Similarly, dimesylate 6 (from 4) gave the spiro-annulated diene 7 and the annulated dienes 8 in about equal amounts; thermolysis transformed 7 into 8 (62%). The dienes 12 were prepared in 15% yield from the ditriflate 10 via 11, the dimesylate 9 proving insufficiently reactive, and the dienes 16 (49%) from 14 via 15.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790815
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  • 65
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    Synthesis of Fused Triazoles as Probes for the Active Site of Retaining β-Glycosidases: From Which Direction Is the Glycoside Protonated? (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2190-2200 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure of the β-glycosidase inhibitors 1-7 and 10-13 suggests that protonation of O-C(1) (the glycosidic O-center) of the substrate by a carboxy group of the retaining β-glycosidases does not occur in the plane perpendicular to the ring of the glycon (β-side; ‘from the top’), but in the plane of the ring (‘from the side’). The triazoles 17 and 18 have been prepared in six steps from the L-xylofuranose 21. They possess a CH group instead of the N-center of the related tetrazoles 4 and 5, corresponding to the glycosidic O-atom, and a very similar structure, both in solution and in the solid state. Unlike the tetrazoles, however, which are good-to-medium inhibitors of retaining β-glycosidases, the triazoles do not inhibit the β-glycosidases from sweet almonds, snail, and bovine liver, and only slightly inhibit the β-glucosidase from Caldocellum saccharolyticum. This is in keeping with the proposed direction of protonation in the plane of the saccharide ring and with modelling studies, docking 4 into the active site of the white clover cyanogenic β-glucosidase and 6 into the E. coli β-galactosidase and the Lactococcus lactis 6-phospho-β-galactosidase.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790814
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  • 66
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    Double-Bond Shifts in [4n]Annulenes as a New Principle for Molecular Switches: First Results with Dimethyl Heptalene-1,2- and -4,5-dicarboxylatesDedicated to Vlado Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2282-2315 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new concept for molecular switches, based on thermal or photochemical double-bond shifts (DBS) in [4n]annulenes such as heptalenes or cyclooctatetraenes, is introduced (cf. Scheme 2). Several heptalene-1,2- and -4,5-dicarboxylates (cf. Scheme 4) with (E)-styryl and Ph groups at C(5) and C(1), or C(4) and C(2), respectively, have been investigated. Several X-ray crystal-structure analyses (cf. Figs. 1-5) showed that the (E)-styryl group occupies in the crystals an almost perfect s-trans-conformation with respect to the C=C bond of the (E)-styryl moiety and the adjacent C=C bond of the heptalene core. Supplementary 1H-NOE measurements showed that the s-trans-conformations are also adopted in solution (cf. Schemes 6 and 9). Therefore, the DBS process in heptalenes (cf. Schemes 5 and 8) is always accompanied by a 180° torsion of the (E)-styryl group with respect to its adjacent C=C bond of the heptalene core. The UV/VIS spectra of the heptalene-1,2- and -4,5-dicarboxylates illustrated that it can indeed be differentiated between an ‘off-state’, which possesses no ‘through-conjugation’ of the π-donor substituent and the corresponding MeOCO group and an ‘on-state’ where this ‘through-conjugation’ is realized. The ‘through-conjugation’, i.e., conjugative interaction via the involved s-cis-butadiene substructure of the heptalene skeleton, is indicated by a strong enhancement of the intensities of the heptalene absorption bands I and II (cf. Tables 3-6). The most impressive examples are the heptalene-dicarboxylates 11a, representing the off-state, and 11b which stands for the on-state (cf. Fig.8).
    Additional Material: 12 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790819
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  • 67
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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.19960790101
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    Instructions to Authors (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1-19 
    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.19960790131
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  • 69
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    Effect of non-planarity on the spin distribution in the radical anions containing the stilbene or the azobenzene π-system: An ESR and ENDOR Study (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 307-318 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The radical anions of 5H-dibenzo[a,d]cycloheptene (9), 5 H-dibenzo[c,f][1,2]diazepine (10), 5,6-dihydrodibenzo[a,e]cyclooctene (11), 5,6-dihydrodibenzo[c,g][1,2]diazocine (12), and (E)-2,2,5,5-tetramethyl-3,4-diphenylhex-3-ene (13) were characterized by ESR and ENDOR spectroscopy. Their hyperfine data were compared with those previously reported for radical anions also containing the stilbene or the azobenzene π-system. Whereas the π-spin distribution in the radical anions of the stilbene series is only moderately sensitive to deviations of the π-system from planarity, the radical anions of the azobenzene series respond to steric strain by shifting the π-spin population from the benzene rings to the azo group. This finding is impressively demonstrated by the similar hyperfine data for 9· - and 11· - which contrast with the strongly highly hindered 13· -. A. plausible interpretation is readily provided by the electron affinities of the constituent π-moieties in stilbene and azobenzene. While those of benzene and ethene are both comparatively low, the azo group has a considerably higher electron affinity.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790130
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    Theoretical Calculations of β-Lactam Antibiotics. Part VII. Influence of the solvent on the basic hydrolysis of the β-lactam ring (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 353-362 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We used semi-empirical and ab initio calculations to investigate the nucleophilic attack of the OH- ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions rather than the studied reaction. We also used the PM3 semi-empirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential barrier of 20.7 kcal/mol due to the desolvation of the OH- ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a H2O solvation sphere of 20 molecules around the solute, the potential barrier is lowered to 17.5 kcal/mol, which is very close to the experimental value (16.7 kcal/mol).
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790204
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  • 71
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    New Triterpene Saponins from Herniaria glabraDedicated to Prof. K. Hiller on the occasion of his 65th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 385-390 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three new saponins 1-3 were isolated from Herniaria glabra by means of prep. HPLC and TLC. The structures were established mainly by a combination of 2D-NMR techniques (COSY, TOCSY, ROESY, HMQC, and HMBC) as O-α-L-rhamnopyranosyl-(1→4)-O-β-D-glucopyranosyl-(1-→6)-O-[β-D-glucopyranosyl-(1→2)]-β-D-glucopyranosyl medicagen-28-ate (herniaria saponin 4; 1), O-β-D-glucopyranosyl-(1→3)-O-α-L-rhamnopyranosyl-(1→2)-O-[β-(3R)-D-apiofuranosyl-(1→3)]-β-D-4-O-acetylfucopyranosyl 3-O-(β-D-glucuronopyranosyl)-16α-hydroxymedicagen-28-ate (herniaria saponin 5; 2), and O-α-L-rhamnopyranosyl-(1→4)-O-β-D-glucopyranosyl-(1→6)-O-[β-D-6-O-acetylglucopyra nosyl-(1→2)]-β-D-glucopyranosyl medicagen-28-ate (herniaria saponin 6; 3).
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790207
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    Nucleotides. Part XLVIPart XLV: [1].. The synthesis of phospholipid conjugates of antivirally active nucleosides by the improved phosphoramidite methodology (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 426-438 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The application of the improved phosphoramidite strategy for the synthese of oligonucleotides using β-eliminating protecting groups to phospholipid chemistry offers the possibility to synthesize phospholipid conjugates of AZT (6) and cordycepin. The synthesis of 3′-azido-3′-deoxythymidine (6) was achieved by a new isolation procedure without chromatographic purification steps in an overall yield of 50%. Protected cordycepin ( = 3′-de-oxyadenosine) derivatives, the N6,2′-bis[2-(4-nitrophenyl)ethoxycarbonyl]cordycepin (12) and the N6,5′-bis[2-(4-nitrophenyl)ethoxycarbonyl]cordycepin (13) wre prepared by known methods and direct acylation of N6-[2-(4-nitrophenyl)ethoxycarbonyl]cordycepin (9), respectively. These protected nucleosides and the 3′-azido-3′-de-oxythymidine (6) reacted with newly synthesized and properly characterized lipid-phosphoramidites 21-25, catalyzed by 1H-tetrazole, to the corresponding nucleoside-phospholipid conjugates 26-38 in high yield. The deprotection was accomplished via β-elimination with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in aprotic solvents to give analytically pure nucleoside-phospholipid diesters 39-51 as triethylammonium or sodium salts. The newly synthesized compounds were characterized by elemental analyses and UV and 1H-NMR spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790210
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  • 73
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    Ring-Opening Reaction of a Dihydrozirconafuran (= 2,5-Dihydro-1,2-oxazirconole): An equilibrium studied by NMR methods, and its chemical consequences (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 454-460 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The dihydrozirconafuran 3 (2,2-di(cyclopenta-2,4-dien-l-yl)-2,5-dihydro-5,5-dimethyl-3,4-bis(trimethylsilyl)-1,2-oxazirconole) is a valuable synthetic equivalent to the 14-electron fragment Cp2ZrThroughout this paper, Cp=η5-C5H5. The reactions of this metallacycle suppose an equilibrium with an ‘opened’ form [ZrCp2(RC≡CR)(acetone)], the latter paving the way to numerous ligand replacements. In a solution containing the dihydrozirconafuran and acetone, these reactions are reversible and degenerate, and can be studied by NMR methods: by two-dimensional spectroscopy and by observation of the isotope-induced chemical shifts that occur upon incorporation of (D6)acetone into the metallacycle. The findings give an indirect proof of the described equilibrium.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790213
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    Synthesis of (all-E,2R,2′R)-oscillol (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 499-506 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optically active (all-E,2R,2′R)-oscillol (= (all-E,2R,2′R)-3,4,3′,4′-tetradehydro-1,2,1′,2′-tetrahydro-ψ,ψ-carotene-1,2,1′,2′-tetrol; 1) was synthesized according to the C10 + C20 + C10 = C40 strategy, applying the Wittig reaction to couple the synthons 4 and 6. The chiral centre was introduced by a Sharpless dihydroxylation of 3-methylbut-2-enyl 4-nitrobenzoate (8).
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790217
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  • 75
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    Synthesis of Tenuilobine, a Bis-polyamine Alkaloid from Oncinotis tenuiloba, and Its Transamidation to Isotenuilobine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 541-547 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: From the leaves of Oncinotis tenuiloba STAPF, a novel polyamine alkaloid, tenuilobine (9), was isolated. This paper presents the synthesis of 9, as well as the base-catalyzed Zip reaction of 9, leading to the transamidation product isotenuilobine (10). The structure of 10 was further confirmed by 2D-NMR correlation spectroscopy. For analytical purposes, the bis-polyamines 9 and 10 were converted into their pentaacetyl derivatives 12 and 11, respectively, which were readily separable by reverse-phase HPLC.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790221
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  • 76
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    Synthesis of 1,6-Dideoxynojirimycin, 1,6-Dideoxy-D-allo-nojirimycin, and 1,6-Dideoxy-D-gulo-nojirimycin via Asymmetric Hetero-Diels-Alder Reactions (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 560-567 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Asymmetric Diels-Alder reaction of sorbaldehyde O-methyloxime 1d with chiral chloronitroso derivative 2 of D-mannose, followed by osmylation of the primary cycloadduct, led to diol 6a with excellent enantioselectivity (ee 〉 99%; Scheme 1). Catalytic hydrogenolysis of 6a gave 1,6-dideoxy-D-allo-nojirimycin (7a). Nucleophilic ring opening of cyclic sulfate 8 allowed a straightforward synthesis of 1,6-dideoxynojirimycin (11) and of 1,6-dideoxy-D-gulo-nojirimycin (12; Scheme 2).
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790223
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  • 77
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    Nucleosides. Part LX.Part LIX: [1]. Synthesis and Characterization of Monomeric Cordycepin-Vitamin and Cordycepin-Lipid Conjugates Model Substances for Biodegradable Ester and Carbonate Linkages in Conjugates and Potential Inhibitors of HIV-1 ReplicationDedicated to Prof. Dr. h. c. Hans-Jürgen Bestmann on the occassion of his 70th birthday and in admiration to his interesting contribution to organic chemistry (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 609-618 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Monomeric 3′-deoxyadenosine (cordycepin) was modified at the 2′-O- (13-18) and 5′-O-position (25-29) by the vitamins E, D2, and A and by the two lipids 1,2-di-O-palmitoylglycerol and 1,2-di-O-hexadecylglycerol via succinate or carbonate linkages. The base-labile conjugates afforded protection groups like the 2-(4-nitro-phenyl)ethoxycarbonyl (npeoc) and monomethoxytrityl group (MeOTr) that are cleavable without harming the ester and carbonate bonds, respectively. Monomeric conjugates of cordycepin and vitamin E, vitamin D2, 1,2-di-O-palmitoylglycerol, and 1,2-di-O-hexadecylglycerol (see 13, 14, 17, 18, 25, 26, 28, and 29) inhibited HIV-1-induced syncytia formation 1.7 to 6.2 fold compared to 1.5-fold for cordycepin (see Table); IC50 values for 25 and 28 were 257 and 267 m̈M, respectively. In addition, the monomeric cordycepin-vitamin and -lipid conjugates inhibited HIV-1 RT activity 28-49% which compares with a 13% inhibition of HIV-1 RT observed for cordycepin. The minimal inhibition of HIV-1-induced syncytia formation and HIV-1 RT activity did not proceed by the activation of RNase L. The monomeric conjugates tested (13, 14) increased PKR expression.
    Additional Material: 1 Tab.
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  • 78
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    Anion Cryptates: Synthesis, Crystal Structures, and Complexation Constants of Fluoride and Chloride Inclusion Complexes of Polyammonium Macrobicyclic LigandsDedicated to the memory of Willy Simon (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 569-587 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three macrobicyclic octamines 1-3 and the macrotricyclic hexadecamine 14 have been synthesized. The octamines 1-3 bind anionic substrates when protonated. The stability constants of the complexes between the protonated forms of the macrobicyclic polyamines and halide anions have been determined by pH-metric measurements. The stability constants in H2O are very high; 1 in its hexaprotonated form binds F- with high selectivity (selectivity F-/Cl- 〉 108), while 3 exhibits strong stability constants for both F- and Cl-. Three X-ray structures have been obtained, one where F- is held inside the cavity of 1 · 6H+, one where Cl- is included in 3 · 6H+, and 3 · 6H+ where the cavity is empty.
    Additional Material: 5 Ill.
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  • 79
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    Perrhenat-katalysierte Umlagerung von Ethinyl-β-ionol (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 646-650 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Perrhenate-Catalyzed Rearrangement of Ethinyl-β-ionolThe rearrangement of 2-ethinyl-β-ionol (1) to α,β-unsaturated carbonyl compounds using tetraalkylammonium perrhenate catalysts was studied. It was found that the Rupe-Kambli product, ketone 2, is the main product (ca. 70%) of this rearrangement. The by-product 4 (ca. 15%) is formed in a ring-closure reaction.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790307
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  • 80
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    ESR Study of Nitroxide Radicals Generated from Triaz-2-en-1-ols (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 663-669 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The triazenols 4-R1—C6H4—N=N—N(OH)—R2 (1), oxidized with t-BuO⋅2 radicals, produced nitroxide radicals R1—C6H4—N(O⋅)—N=N(R2) +O- (5). The suggested radical structure was confirmed by 15N-labeling. The reaction of triazenols 1 with PbO2 proceeded under N2 elimination, in which case nitroxides R1—C6H4—N(R2)—O⋅(2) were observed as the final radical products. The intermediate R1—C6H4⋅ radicals were identified by spin-trapping.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790310
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  • 81
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    Nucleotides. Part XLVII.Part XLVI: [1]. Synthesis of 3′-Deoxyadenylyl-(2′-5′)-3′-deoxyadenylyl-(2′-5′)-3′-O-(2-hydroxyethyl)adenosine and 3′-Deoxyadenylyl-(2′-5′)-3′-deoxyadenylyl-(2′-5′)-3′-O-{2-[(cholest-5-en-3β-yloxy)carbonyloxy]ethyl}adenosine: A New Type of (2′-5′)Adenylate Trimer Conjugate (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 718-726 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A suitably protected adenosine derivative bearing an acetyl-protected 2-hydroxyethyl moiety in 3′-O-position was attached to the 2′-terminus of a cordycepin (3′-deoxyadenosine) dimer. Coupling was performed by phosphoramidite chemistry using two alternative approaches - condensation of 5′-phosphoramidite 16 with 2′-OH cordycepin dimer 18 and condensation of dimeric cordycepin phosphoramidite 19 with 5′-OH adenosine derivative 15 - of which the latter synthesis worked best (→20). After cleavage of the acetyl protecting group (→21), cholesteryl carbonate was introduced into the OH function of the spacer (→24). Final deblocking of trimer 24 with, or trimer 21 without the cholesterol moiety afforded the modified cordycepin conjugates 25 and 23, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790314
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  • 82
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    Computations of 57Fe-NMR Chemical Shifts with the SOS-DFPT Method (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 742-754 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 57Fe Shielding tensors of substituted iron-carbonyl complexes have been computed employing the density-functional-based SOS-DFPT method (sum-over-states density-functional perturbation theory) with the IGLO (individual gauge for localized orbitals) choice of gauge origins and with large basis sets. The shieldings computed for [Fe(CO)5], [Fe(CO)3(H2C=CHCH=CH2)], [Fe(CO)3(cyclo-C4H4)], [Fe(CO)4(H2C=CHOMe)], [Fe(CO)4(H2C=CHCN)], [Fe(CO)3(H2C=CHCH=O)], and [Fe(CO)2(C5H5)R] (R = Me, Bu, i-Pr) correlate with the experimental δ(57Fe) values. However, the slope of the correlation line is 0.55 instead of 1, i.e., only about one half of the substituent effects on σ(Fe) is recovered in the calculations. Nearest-neighbor effects appear to be described qualitatively (cf. in the [Fe(CO)2(C5H5)R] series, whereas effects of more remote substituents, e.g., for [Fe(CO)4(H2C=CHX)] (X = MeO and CN)) are not reproduced. Dissociation energies of these species are discussed because of their relevance to experimental rate constants for substitution processes which are known to correlate with δ(57Fe). Even though the δ(13C) and δ(1H) data of ferrocene (7) are well reproduced theoretically, the computed σ(Fe) shielding of 7 deviates substantially from the σ(calc.)/δ(expt.) correlation, possibly indicating additional shortcomings in the theoretical description of this molecule.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790317
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  • 83
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    Syntheses of New Isocephems and Isodethiaoxacephems as Antimicrobial Agents (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 813-819 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cis-configurated isocephem 26 as well as isodethiaoxacephems 21 and 33 were synthesized (Schemes 4 and 5). The key step involves chlorination of the corresponding carbanions of 23, 18, and 31 with CF3SO2Cl followed by internal alkylation. β-Lactams 3, 21, 26, and 33 were found to possess biological activity against several pathogenic microorganisms in vitro. Electronic activation of the lactam moiety in isodethiaoxacephem 33 remarkably enhanced its biological activity.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790323
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  • 84
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    An Analysis of the Bonding Properties of Benz[a]azulene by X-Ray, NMR, and Computational StudiesDedicated to Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 837-854 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The X-ray crystal structures of 9-phenylbenz[a]azulene (4) and the corresponding non-benzannelated form, 4-phenylazulene (5), have been determined (cf. Fig.2). In contrast to 5, the skeleton of which shows nearly equal C,C bond lengths (cf. Table 1), the seven-membered ring of 4 exhibits clearly alternating C,C bond lengths (cf. Table 1). This is in agreement with a strong accentuation of the heptafulvene substructure in 4 by the [a] benzannelation. The alternating bond lengths of 4 and of its parent structure 3 are also reflected in the corresponding variations of the 3J(H,H) and 1J(13C,13C) values of these benz[a]azulenes (cf. Tables 4 and 5). Computations on the MP2/6-31G* level as well as on the BP86/6-31G* level for azulene (6), benz[a]azulene (3), and heptafulvene (7) are in good agreement with the experimental values (cf. Tables 6-8).
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790326
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  • 85
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    Synthesen von 2,3-Dioxoalkylphosphonaten und anderer neuartiger β-Ketophosphonate sowie eines Phosphinopyruvamids ( = (Alkyloxyphosphinyl)pyruvamids)Herrn Kollegen H.W. Roesky, Göttingen, mit den besten Wünschen zum 60. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 895-912 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Syntheses of 2,3-Dioxoalkylphonates and Other Novel β-Ketophosphonates As Well As of a Phosphinopyruvamide ( = (Alkyloxyphosphinyl)pyruvamide)The new 3-(diethoxyphosphoryl)-2-oxopropanoates 5 and 6 and -propanamides 1-4 with various amino substituents at C(3) were prepared (Scheme 2). These compounds exist, depending on N-substitution, as pure (E)-enols (in the case of1 and 5) or as a mixture of three tautomeric forms (in the case of 1-4 and 6). The configuration could be unambiguously assigned from the 1H-, 13C-, and 31P-NMR spectra. Phosphinopyruvamide ( = (alkyloxyphosphinyl)pyruvamide) 9 was prepared in a similar manner in spite of the instability of phosphinate derived carbanions. Some 3-(ethoxyimino)-2-oxobutylphosphonates, 11-13 (Scheme 5), and various 3,3-dimethoxy-2-oxoalkylphosphonates, 19-23 and 26-33 (Scheme 6), were available from the reaction of lithioalkylphosphonates with 2-(ethoxyimino)propanoates and 2,2-dimethoxyalkanoates, respectively. The 3,3-dimethoxybutylphosphonates 20, and 26-30 were cleaved to give 2,3-dioxobutylphosphonates 34-39 (Scheme 6). This method provides easy access to a new class of potentially pharmaceutically useful compounds.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790329
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  • 86
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    Oncinotine-Type Spermidine Alkaloids from Oncinotis tenuiloba. Transformation of N-acetyloncinotin-12-one to N,N′-diacetylinandenin-12-one (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 973-981 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: From extracts of Oncinotis tenuiloba STAPF, two novel polyamine alkaloids, oncinotin-11-one (5) and oncinotin-12-one (6), were isolated. Peracetylation of 6 provided the N-acetyl derivative 11 as well as N,N′-diacetylinandenin-10-en-12-one (12) due to a β-elimination-type side reaction resulting in ring enlargement of 11 (Scheme 1). Deuteration of 12 yielded 13, showing the same retention time as N,N′-diacetylinandenin-12-one (14), when co-HPLC was performed together with different keto-isomeric N,N′-diacetylinandeninones. Structure elucidation was extended by Schmidt degradation of 6 and N,N′-diacetyl(10,11-2H2)inandenin-12-one (13); the degradation products were identified by GC and ESI-MS. The structure of 5 was proposed on the basis of spectroscopic means. Comparison of the spectroscopic data of 5 with those obtained from synthetic material as well as co-HPLC of the N-acetyl derivative 20 together with the corresponding synthetic compound revealed the identity of the substances and confirmed the structure of 5. Additionally, oncinotine (2) and neooncinotine (3) were isolated, separated, and identified with authentic samples by co-HPLC of their N-acetyl derivatives 8 and 9, respectively.
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    URL: http://dx.doi.org/10.1002/hlca.19960790405
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  • 87
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    Metal Complexes with Macrocyclic Ligands. Part XLIIPart XLI: [1].. Tetraazamacrocyclic nickel(II) complexes with a methylthio or a methoxy pendant chain as model for cofactor F430 (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1011-1020 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 14-membered macrocyclic Ni2+ complexes of 1 and 2, with a methylthio pendant chain, and those of 3 and 4, with a methoxy pendant chain, have been synthesized and their chemistry has been studied. Solution spectra in H2O, MeCN, and DMF indicate no participation of the side-chain donor group in metal coordination. This is also the case in the solid state as shown by the X-ray structures of the Ni2+ complexes with 1 and 2, in which a tetrahedrally distorted square-planar geometry around the Ni2+ results by the coordination of the four N-atoms of the macrocycle. Cyclic voltammetry of these complexes in MeCN reveals that Ni2+ is reversibly reduced to Ni+ between -0.7 and -0.8 V vs. SCE. For the complexes with 1 and 2, the thioether bond is cleaved at more negative potentials, whereby a thiol group is formed. This thiol group is then oxidized at ca. +0.7 V vs. SCE, when a glassy C electrode is used, or at ca. 0 V vs. SCE at a dropping Hg electrode. No cleavage of the ether bond in the complexes with 3 and 4 is observed under similar conditions. Reduction of the Ni2+ complexes of 1 and 2 with Na-amalgam in DMF produces small amounts of methane only in the case of 1, indicating the importance of the proximity between the NiI centre and the MeS group.
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  • 88
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    Preparation of α,α-Dibromocyclobutanones from Olefins: A simple procedure for the regioselective functionalization of olefinsFor a preliminary communication, see [1]. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1075-1084 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dibromoketene is prepared by in-situ Zn dehalogenation of tribromoacetyl bromide in the presence of POCl3. Under these conditions, the cycloaddition of dibromoketene with olefins 5 yields α,α-dibromocyclobutanones 2, which are cleaved with alkoxides to yield β-dibromomethyl esters 4. Several examples are given. For monosubstituted olefins, the functionalization is regioselective, the methoxycarbonyl group being introduced at the terminal C-atom.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790416
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  • 89
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    Theory-enforced Re-investigation of the Origin of the Large Metal-Carbon Bond Strength in PdCH2I+ and its reactions with unsaturated hydrocarbonsIn [1], a preliminary account of the reaction of ground-state Pd+ with CH3I is given.Dedicated to Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1110-1120 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: State-of-the-art ab initio studies demonstrate that the reaction Pd+ + CH3I → PdCH2I+ + H. is endothermic by ca. 20 kcal/mol, which translates into a bond dissociation energy (BDE) of ca. 83 kcal/mol for the Pd+—CH2I bond. This figure is in agreement with an experimental bracket of 68 kcal/mol 〈 BDE(Pd+—CH2I) 〈 92 kcal/mol. Based on these findings, the previously studied Pd+/CH3I system was re-investigated, and double-resonance experiments demonstrate that the formation of PdCH2I+ occurs stepwise via PdCH3+ as a reactive intermediate. Further, ion/molecule reactions of PdCH2I+ with unsaturated hydrocarbons are studied, which reveal the formation of carbon-carbon bonds in the gas phase.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790419
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  • 90
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    The Synthesis and Characterization of New Optically Active ‘dimeric’ ‘pineno’-[4,5]-fused 2,2′-bipyridines linked without spacer or by small spacer groups (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1192-1202 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Linked chiral bipyridines 2-4 are prepared by combining two optically active ‘pineno’-[4,5]-fused 2,2′-bipyridines in a stereoselective reaction (Scheme 1). These potential ligands are new members of the ‘chiragen’ family, and are characterized by NMR spectroscopy and, in the case of 2 and 3 by single-crystal X-ray analysis. A new synthesis of ‘dipineno’-[4,5;4′,5′]-fused 2,2′ -bipyridine 8 is described, which, when coupled, gives additional four chiral centres to the analogous ‘chiragen’ series ( → 9). Analysis of the CD spectra allowed conformational information about the solution species to be determined.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790425
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    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1240-1240 
    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.19960790430
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  • 92
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    Singlet-Oxygen (1Δg) Production by Ruthenium(II) complexes containing polyazaheterocyclic ligands in methanol and in water (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1222-1238 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the context of our studies on ruthenium(II) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (kq) of the metal-to-ligand charge-transfer-excited state of a series of homoleptic [RuL3] complexes (where L stands for 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2′-bipyrazine (bpz), 4,7-diphenyl-1,10-phenanthroline (dip), diphenyl-1,10-phenanthroline-4,7-disulfonate (dpds), and 1, 10-phenanthroline-5-octadecanamide (poda)) in H2O and in MeOH. These compounds are singlet-oxygen (O2(1Δg)) sensitizers, and quantum yields of singlet-oxygen production (ΨΔ) in both solvents are also reported. Values of kq and ΨΔ depend on the nature of the ligand L and on the solvent, ΨΔ values showing a large range of variation (0.2 to 1.0). In MeOH, the only pathway for quenching of the excited [RuL3] complexes by molecular oxygen is energy transfer: the fraction of quenched excited states yielding singlet oxygen (ƒΔT) is unity for all compounds in the series investigated. Changing from MeOH to H2O has several remarkable effects: higher kq and lower ΨΔ values are observed; ƒΔT drops to ca. 0.5 except for [Ru(bpz)3]2+. In fact, [Ru(bpz)3]2+ is by far the weakest reductant in the series and behaves differently from the other complexes, with lowest kq and ΨΔ values and a ƒΔT equal to 1 in both solvents. Results are interpreted on the basis of the role played by charge-transfer interactions between the sensitizer excited state and molecular oxygen in the quenching mechanism. RuII Complexes based on the 4,7-diphenyl-1, 10-phenanthroline (dip) ligand are very efficient and stable singlet-oxygen sensitizers with ΨΔ values close to unity in air-saturated MeOH.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790428
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  • 93
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    Three-Component Reactions with Sterically Crowded 2,2,4,4-Tetramethyl-3-thioxocyclobutanone, Phenyl Azide, and Electron-Deficient C,C-DipolarophilesPresented by G.M. at the ‘17th International Symposium on the Organic Chemistry of Sulfur’, Tsukuba, Japan, July 1996.Dedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1305-1314 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In order to trap ‘thiocarbonyl-aminides’ A, formed as intermediates in the reaction of thiocarbonyl compounds with phenyl azide, a mixture of 2,2,4,4-tetramethyl-3-thioxocyclobutanone (1), phenyl azide, and fumarodinitrile (8) was heated to 80° until evolution of N2 ceased. Two interception products of the ‘thiocarbonylaminide’ A (Ar=Ph) were formed: the known 1,4,2-dithiazolidine 3 (cf. Scheme 1) and the new 1,2-thiazolidine 12 (Scheme 2). The structure of the latter was established by X-ray crystallography (Fig.1). The analogous ‘three-component reaction’ with dimethyl fumarate (9) yielded, instead of 8, in addition to the known interception products 3 and 6 (Scheme 1), two unexpected products 15 and 16 (Scheme 3), of which the structures were elucidated by X-ray crystallography (Fig.2). Their formation is rationalized by a primary [2 + 3] cycloaddition of diazo compound 18 with 1 to give 19, followed by a cascade of further reactions (Scheme 4).
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790507
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  • 94
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    Synthesis of Oncinotin-11-one, a Macrocyclic Polyamine Alkaloid from Oncinotis tenuiloba (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1379-1386 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper presents a short synthesis of oncinotin-11-one (11), a minor alkaloid of Oncinotis tenuiloba (Apocynaceae). Based on a disconnection approach, the spermidine portion of the key intermediate 6 was constructed consecutively by simple N-alkylations starting from ethyl piperidine-2-carboxylate (1). Treatment of 6 with in situ lithiated 2-[(10-bromodecyl)oxy]tetrahydropyran resulted in the formation of the keto moiety under simultanous deprotection of the lactam N-atom to give the amino ketone 7 in 71% yield. Cleavage of the tetrahydro-2H-pyran-2-yl(Thp) portion and Jones oxidation of the resulting alcohol 8 gave the amino acid 9 which was cyclized. Final N-debenzylation of 10 provided the natural alkaloid 11. Only two protective groups were needed in this synthesis. The reaction of N-alkyl-lactams with organometallic reagents is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790511
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  • 95
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    The Diels-Alder Reactivity of 2,2′-Ethylidenebis[3,5-dimethylfuran] and Exploratory Chemistry of the Mono-adducts (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1415-1427 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the presence of Me3Al, 1-cyanovinyl acetate added to 2,2′-ethylidenebis[3,5-dimethylfuran] (1) to give a 20:10:1:1 mixture of mono-adducts 4,5,6, and 7 resulting from the same regiocontrol (‘para’ orienting effect of the 5-methyl substituent in 1). The additions of a second equiv. of dienophile to 4-7 were very slow reactions. The major mono-adducts 4 (solid) and 5 (liquid) have 2-exo-carbonitrile groups. The molecular structure of 4(1RS,1′RS,2SR,4SR)-2-exo-cyano-4-[1-(3,5-dimethylfuran-2-yl)ethyl-7-oxabicyclo[2.2.1]hept-5-en-2-endo-yl acetate) was determined by X-ray single-crystal radiocrystallography. Mono-adducts 4 and 5 were saponified into the corresponding 7-oxanorbornenones 8 and 9 which were converted with high stereoselectivity into (1RS,1′SR,4RS,5RS,6RS)-4-[1-(3,5-dimethyl furan-2-yl)ethyl]-6-exo-methoxy-1,5-endo-dimethyl-7-oxabicyclo [2.2.1]heptan-2-one dimethyl acetal (12) and its (1′RS-stereoisomer 12a, respectively. Acetal hydrolysis of 12a followed by treatment with (t-Bu)Me2SiOSO2CF3 led to silylation and pinacol rearrangement with the formation of (1RS,1′RS,5RS,6RS)-4-[(tert-butyl)dimethy lsilyloxy]-1-(3,5-dimethylfuran-2-yl)ethyl]-5-methoxy-6-methyl-3-methylidene- 2-oxabicyclo[2.2.1]heptane (16). In the presence of Me3Al, dimethyl acetylenedicarboxylate added to 12 giving a major adduct 19 which was hydroborated and oxidized into (1RS,1′RS,2″RS,3″RS,4SR,4″RS,5 SR,6SR)-dimethyl 5-exo-hydroxy-4,6-endo-dimethyl-1-[1-(3-exo,5,5-trimeth oxy-2-endo,4-dimethyl-7-oxabicyclo[2.2.1]hept-2-yl)ethyl]-7-oxabicyclo [2.2.1]hept-2-ene-2,3-dicarboxylate (20). Acetylation of alcohol 20 followed by C=C bond cleavage afforded (1′RS,1″SR,2RS,2′″SR,3RS, 3″SR,4RS,4″SR,5RS)-dimethyl {3-acetoxy-2,3,4,5-tetrahydro-2,4-dimethyl-5-[1-(3-exo,5,5-trimethoxy -2-endo,4-dimethyl-7-oxabicyclo[2.2.1]hept-1-yl)-ethyl]furan-2,5-diyl} bis[glyoxylate] (24).
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790514
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  • 96
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    Lipase-Catalysed Regioselective Deacetylation of androstane derivatives (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 999-1004 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of acetoxy derivatives of androstane was deacetylated in organic solvents by several lipases. The most satisfactory results were obtained with lipase from Candida cylindracea (CCL) and Candida antarctica (CAL). In some derivatives, CCL and CAL showed an overwhelming regioselectivity towards the removal of the 3β- or the 17β-acetyl group (see Table 2). Three new steroid derivatives were obtained through this approach. A hypothetical rationale for the behaviour of these enzymes is given.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790408
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  • 97
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    Synthesis and Circular Dichroism of Both Enantiomers of 2-deuterofluoroacetic acid (= Fluoro[2H1]acetic acid) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1021-1025 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Sharpless epoxidation of (E)-1-(trimethylsilyl)[1-2H1]oct-1-en-3-o1 (3a) yielded (1S,2S,3S)- and (1R,2R,3R)-1-(trimethylsilyl)-1,2-epoxy[1-2H1]octan-3-ols (4a and 4b, resp.) which were converted in three steps into (S)- and (R)-fluoro[ 2H1]acetic acid (7a and 7b, resp.) in good yields. Their high isotopic and optical purity was established by 1H- and 19F-NMR, mass, and circular-dichroism spectroscopy.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790411
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  • 98
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    Nucleotides. Part XLVIIIPart XLVII: [1].. Synthesis of 2′-amino-2′-deoxyarabinonucleoside phosphoramidite building blocks (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1067-1074 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chemical syntheses of 2′-amino-2′-deoxyarabinonucleosides of uracil, thymine, cytosine, adenine, and guanine and their conversion into suitably protected 3′-phosphoramidite building blocks 24-28 for oligonucleotide synthesis are described. The 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) group was used for protection of the aglycon and the 2′-amino functions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790415
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  • 99
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    Synthesis of 1,3,4,5-Tetrahydro-2-benzoxepin Derivatives as Conformationally Restricted Analogues of cyclamenaldehyde-type compounds and as intermediates for highly odour-active homologues (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1095-1109 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nine 1,3,4,5-tetrahydro-2-benzoxepin derivatives have been prepared as mimics of the folded (gauche) conformation of cyclamenaldehyde (1) and related compounds, but none of them showed the typical lily-of-the valley (muguet) odour activity. However, conversion of these substances to previously unknown analogues of 1 having a Me substituent on the aromatic ring in an ortho position to the side chain led to new fragrance substances with remarkable properties. The results indicate that the extended (anti) conformation is more likely to be the ‘bioactive’ one at the receptor site(s).
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790418
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  • 100
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    The Degradation Mechanism of an Oral Cephalosporin: Cefaclor (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1793-1802 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The degradation of cefaclor (1), an oral cephalosporin antibiotic, was studied at 37° in a neutral aqueous medium by HPLC and 1H-NMR. Under these conditions, 1 underwent intramolecular aminolysis by the 7-side-chain NH2 group on the β-lactam moiety to give a piperazine-2,5-dione. The most prominent peak in the HPLC profile of a degradation solution from 1 was isolated by prep. HPLC. Mechanistically, the formation of this degradation product cis-11 from 1 involves the contraction from a six-membered cephem ring to a five-membered ring, which presumably takes place via a common episulfonium ion intermediate 9 (see Scheme). Loss of the Cl-atom from 3-chloro-3-cephem is a general reaction subsequent to β-lactam ring opening.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790705
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