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  • 1990-1994  (822)
  • 1960-1964
  • 1920-1924
  • 1991  (822)
  • Organic Chemistry  (822)
  • 1
    Electronic Resource
    Electronic Resource
    A New General Synthesis of 2,2-Dialkyl-2,3-dihydro-4H-pyran-4-ones and Their Application for the in situ Preparation of Electron-Rich Dienes in Carbonyl-Alkyne Exchange Reactions with AcetylenesPresented in part at the autumn meeting of the Swiss Chemical Society in Bern on October 20, 1989. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 27-46 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The substituted 2,2-dialkyl-2,3-dihydro-4H-pyran-4-ones of type II and III have been prepared by acid-catalyzed cyclization of the corresponding substituted acetylenic ketones I in good to excellent yields (Scheme 1). These 2,2-dialkyl-2,3-dihydro-4H-pyran-4-ones II and III have been used for the in situ preparation of highly reactive dienes of type IV-VI (Scheme 2) in carbonyl-alkyne exchange reactions with electron-poor alkynes VII to yield the highly substituted aromatic compounds VIII and IX. These reactions proceed in good yields and with excellent degree of regioselectivity. Aryl-substituted 2,2-dialkyl-2,3-dihydro-4H-pyran-4-ones III (R1 = Ar) subsequently yield highly substituted biaryls. Reaction mechanisms are presented for the formation of the 2,2-dialkyl-2,3-dihydro-4H-pyran-4-ones as well as for the carbonyl-alkyne exchange reactions with electron-poor acetylenes.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740106
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  • 2
    Electronic Resource
    Electronic Resource
    Structural and Luminescence Study of the 3:2 Complex between Europium Nitrate and the B Isomer of Dicyclohexyl-18-crown-6Part 40 of the series ‘Complexes of Lanthanoid Salts with Macrocyclic Ligands’; Part 39 [1]. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 65-78 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal and molecular structure of bis[dinitrato-(2,4,8,15,18,21-hexaoxatricyclo[20.4.0.09,14]hexacosane)europium(III)]pentakis(nitrato)europiate(III) [Eu(NO3)2LB]2([Eu(NO3)5]), has been determined at 170 K from single-crystal X-ray diffraction. The complex crystallizes in the monoclinic space group P21/n (ITC No. 14): a = 16.338(3) Å, b = 15.704(3) Å, c = 24.474(4) Å, β = 97.73(1)°, Z = 4. The structure was refined to a final R value of 0.058 (Rw = 0.060). The asymmetric unit contains three independent ions lying on general positions: [Eu(NO3)5]2- and two distinct [Eu(NO3)2LB]+ cations with the macrocyclic ligand in the cis-anti-cis conformation (B-isomer). The EuIIIions are 10-coordinate with the following mean bond lengths: Eu-O(nitrate) = 2.48(2) Å in the anion and 2.45(2) Å in the two cations, Eu-O(ether) = 2.56(8) and 2.55(5) Å. Small but significant differences are observed between the two complex cations, especially with respect to the positions of the cyclohexyl substituent. A conformational analysis performed on the six O-atoms of the complex cations confirms the predictions of a simple model. The metal ion sites of the complex have been probed by high-resolution excitation and emission spectra at 296 and 77 K. The 5D0←7F0 excitation spectrum displays two main bands along with several other minor components. A detailed analysis of the corresponding and selectively excited emission spectra leads to the observation of three types of spectra corresponding to the three crystallographically different EuIII ions. Moreover, three minor sites are identified, one anionic and two cationic, with a population equal to ca. 10% of the population of the main sites. We interpret this finding as reflecting the presence of molecules with slightly different conformations.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740109
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  • 3
    Electronic Resource
    Electronic Resource
    Asymmetric Diels-Alder Cycloadditions with C2-Symmetrical Chiral Carbamoylnitroso Dienophiles (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 103-109 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The C2-symmetrical chiral pyrrolidines 2 and 3 are of opposite helicity. The corresponding N-acylnitroso dienophiles 6 and 7 react in good yield with cyclohexadiene, leading thereby with excellent diastereoisomeric excess to the expected Diels-Alder cycloadducts (see Scheme). The [2.2.2] bicyclic moieties of the major diastereoisomers 9 and 11 proved to be of opposite configuration, as expected. Their configuration is best explained by assuming the acylnitroso dienophile to be in the s-cis conformation in the transition state, the approach of the diene being endo (see Fig.).
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740112
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  • 4
    Electronic Resource
    Electronic Resource
    2,3- and 4,5-Secodopa, the Biosynthetic Intermediates Generated from L-Dopa by an Enzyme System Extracted from the Fly Agaric, Amanita muscaria L., and Their Spontaneous Conversion to Muscaflavin and Betalamic Acid, Respectively, and Betalains (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 124-140 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An enzyme is extracted from the red peel of Amanita muscaria which cleaves the C(2)-C(3) and the C(4)-C(5) bond of the aromatic ring of L-dopa (1) to form a mixture of 4,5-secodopa (= salt of 6-amino-2-hydroxy-4-(2′-oxoethylidene)hept-2-enedioic acid; 2) and 2,3-secodopa ( = salt of 7-amino-5-formyl-2-hydroxyocta-2,4-dienedioic acid; 3), two hitherto hypothetical biosynthetic intermediates (see Scheme). Though isolation of these products has not been possible, structural evidence is inferred from reaction products, kinetics, and spectroscopical characteristics in comparison with known compounds. Secodopas 2 and 3 are characterized in dilute solution by HPLC and UV/VIS spectroscopy (anions; λmax 424 and 414 nm, resp., ∊420 = 25500; on acidification, shift to 380 and 372 nm, resp.). They cyclize without enzyme catalysis, optimally at pH 4.5-5; 3 produces muscaflavin (5) and 2 betalamic acid (4). The products arc identified by direct comparison with authentic samples in HPLC, by 1H-NMR of 5, and by condensation of 4 with L-proline to form the well known betalain indicaxanthin (7). The enzymatic conversion of L-dopa (1) via 2 to betalamic acid (4; (S)) and its condensation with L-proline leads to pure natural indicaxanthin (7; (2S,115)); correspondingly, the enzymatic conversion of D-dopa to (R)-betalamic acid and its condensation with L-proline produces isoindicaxanthin ((2S,11R)) which is unknown in nature. Particularly relevant is the fact that the same enzyme cleaves pyrocatechol to produce a solution of the enolate form of the known 2-hydroxy-6-oxohexa-2,4-dienoate (secopyrocatechol; 9; see Fig. 5). Dissociation constants of the corresponding enolic functions in the cleavage products are determined by spectrometric titration and compared to those of known systems.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740115
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  • 5
    Electronic Resource
    Electronic Resource
    Photochemical Reactions of Tetrahydroquinoxalin-2(1H)-ones and Related Compound (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 225-231 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Photochemical behaviors of the pyrazinone derivatives 5,6,7,8-tetrahydroquinoxalin-2(1H)-ones 1a-c and 1,5,6,7,8,9-hexahydro-2H-cyclohepta[b]pyrazin-2-one 1d were investigated. Dye-sensitized photo-oxygenation of 1a-c gave the 1:1 adducts 5a-c of the corresponding 3,8a-epidioxy-3,5,6,7,8,8a-hexahydroquinoxalin-2(1H)-one 4 and H2O, whereas 1d gave 3,9a-epidioxy-1,3,5,6,7,8,9,9a-octahydro-2H-cyclohepta[b]pyrazin-2-one 4d (Scheme 2). The different kind of products was interpreted as being the result of the ring strain and steric hindrance of endoperoxides produced from 1a-d with singlet oxygen. Irradiation of 1a-b in the presence of alkenes gave tricyclic azetidine derivatives 9 by [2 + 2] cycloaddition of the C=N bond of 1 to the alkene.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740122
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  • 6
    Electronic Resource
    Electronic Resource
    Analogues of Sialic Acids as Potential Sialidase Inhibitors. Synthesis of C6 and C7 Analogues of N-Acetyl-6-amino-2,6-dideoxyneuraminic Acid (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 343-369 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The piperidines 12-18, piperidmose analogues of Neu5Ac (1) with a shortened side chain, were synthesized from N-acetyl-D-glucosamine via the azidoalkene 32 and tested as inhibitors of Vibrio cholerae sialidase. Deoxygenation at C(4) of the uronate 22, obtained from the known D-GlcNAc derivative 20, was effected by β-elimination (→ 23), exchange of the AcO at C(3) with a (t-Bu)Me2SiO group and hydrogenation (→ 26; Scheme 1). Chain extension of 26 by reaction with Me3SiCH2MgCl gave the D-ido-dihydroxysilane 28, which was transformed into the unsaturated L-xylo-mesylate 29 and further into the L-lyxo-alcohol 30, the mesylate 31, and the L-xylo-azide 32. The derivatives 29-31 prefer a sickle zig-zag and 32 mainly an extended zig-zag conformation (Fig. 2). The piperidinecarboxylate 15 was obtained from 32 by ozonolysis (→ 33), intramolecular reductive animation (→ 34), and deprotection, while reductive animation of 34 with glycolaldehyde (→ 35) and deprotection gave 16 (Scheme 2). An intramolecular azide-olefin cycloaddition of 32 yielded exclusively the fused dihydrotriazole 36, while the lactone 39 did not cyclize (Scheme 3). Treatment of 36 with AcOH (→ 37) followed by hydrolysis (→ 38) and deprotection led to the amino acid 18. To prepare the (hydroxymethyl)piperidinecarboxylates 12 and 17, 32 was first dihydroxylated (Scheme 4). The L-gluco-diol 40 was obtained as the major product, in agreement with Kishi's rule. Silylation of 40 (→ 42), oxidation with periodinane (→ 44), and reductive animation gave the L-gluco-piperidine 45. It was, on the one hand, deprotected to the amino acid 12 and, on the other hand, N-phenylated (→ 46) and deprotected to 17. While 45 and 12 adopt a 2C5 conformation, the analogous N-Ph derivatives 46 and 17 adopt a 5C2 and a B3,6 conformation, respectively, on account of the allylic 1,3-strain. The conformational effects of this 1,3-strain are also evident in the carbamate 47, obtained from 45 (Scheme 5), and in the C(2)-epimerized bicyclic ether 48, which was formed upon treatment of 47 with (diethylamino)sulfur trifluoride (DAST). Fluorination of 40 with DAST (→ 49) followed by treatment with AcOH led to the D-ido-fluorohydrin 50. Oxidation of 50 (→ 51) followed by a Staudinger reaction and reduction with NaBH3CN afforded the (fluoromethyl)piperidine 52, while reductive amination of 51 with H2/Pd led to the methylpiperidine 55, which was similarly obtained from the keto tosylate 54 and from the dihydrotriazole 36. Deprotection of 52 and 55 gave the amino acids 13 and 14, respectively. The aniline 17 does not inhibit V. cholerae sialidase; the piperidines 12-16 and 18 are weak inhibitors, evidencing the importance of an intact 1,2,3-trihydroxypropyl side chain.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740214
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  • 7
    Electronic Resource
    Electronic Resource
    Synthese von Monomorin I, einem Spurpheromon der Pharao-Ameise (Monomorium pharaonis) (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 438-444 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Monomorin I, a Trail Pheromone of the Pharao Ant (Monomorium pharaonis)Katalytic hydrogenation of the 4-nitroalkanone 5 yielded the cis-pyrrolidine ester 6 (Scheme). The preparation of 5 was accomplished by ethanolysis of the nitrocyclohexanone derivative 4. Iodination at C(α) of 6, followed by cyclization, gave a nearly 5:6 mixture of the two C(5)-epimers 10/9 of octahydroindolizine. Epimerisation of the undesired 10 to 9 and subsequent transformation of the latter gave racemic monomorine I (1; 34% yield from 5).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740221
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  • 8
    Electronic Resource
    Electronic Resource
    Masthead (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991) 
    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.19910740301
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  • 9
    Electronic Resource
    Electronic Resource
    Structure cristalline d'une thiazolorifamycine: rifamycine P (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 326-330 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Crystal Structure of a Thiazolorifamycine: Rifamycine PThe crystal structure of a natural thiazolorifamycin, rifamycin P, as an adduct with EtOH and H2O, has been determined by X-ray crystallography by a combination of direct methods and Patterson techniques. Block-diagonal least-squares refinement using 5996 independent reflections and 1475 parameters led to an R factor of 0.092 (two independent molecules of rifamycin P). The middle part of the ansa chain, essential for activity against the bacterial DNA-dependent RNA polymerase (DDRP) resembles that of active rifamycins. The four OH groups lie on the same side of the molecule and are almost in the same plane. Interatomic distances between the O-atoms agree well with a spatial model derived from X-ray studies, performed on semisynthetic active rifamycins.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740211
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  • 10
    Electronic Resource
    Electronic Resource
    Synthesis of 4-{{[(Isopropyloxy)methylphosphoryloxy]imino}methyl}-1-methylpyridinium Iodide and Its Characterisation (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 331-335 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oximes are used in the therapy of organophosphate poisoning. They form an interaction product with the organophosphate which is claimed to be more toxic than the organophosphate itself. We have synthesized one of these compounds which would be formed by the reaction of pyridine-4-carbaldehyde oxime methiodide with sarin (GB). Thus, reaction of oxime 1 with sarin chloridate (2), yielded 4-{{[(isopropyloxy)methylphosphoryloxy]-imino}methyl}-1-methylpyridinium iodide (4) which was characterized by NMR spectroscopy and X-ray crystallography.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740212
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  • 11
    Electronic Resource
    Electronic Resource
    Transformations in the Isoxazole Series: Synthesis of Substituted 2-AminothiazolesDedicated to Prof. Dr. Oskar Jeger (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 531-542 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Substitued N-(isoxazol-4-yl)thioureas 1 undergo a transformation in the presence of hexacarbonylmolybdenum and acid to yield functionalized thiazoles 3 in a one-pot reaction. In a few cases, 1,4,5-trisubstituted dihydroimidazolethiones 4 are also isolated as side products. Mechanistic considerations are outlined and scope and limitations of this new methodology discussed.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740309
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  • 12
    Electronic Resource
    Electronic Resource
    Biosynthesis of the Spermine Alkaloid Aphelandrine (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 565-571 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Aphelandrine (1) is shown to be biosynthesized in the root cells of Aphelandra tetragona (VAHL) NEES from labelled putrescine (4), spermidine (5), and cinnamic acid (3). Whether spermine (6) and the (p-hydroxycinnamoyl)spermidine 8 are precursors of 1 is uncertain, since the latter is hydrolysed to a large extent before incorporation, and the former is metabolized to 4 and 5. Methionine (7) is the source of the 3-aminopropyl unit of 5 and 6.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740313
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  • 13
    Electronic Resource
    Electronic Resource
    Glycosylidene Carbenes. Part 3. Synthesis of SpirocyclopropanesPresented at the ‘XVIth International Carbohydrate Symposium’, Yokohama, Japan, 12th-17th August 1990, Abstr. PL-05. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 585-593 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermolysis of the glycosylidene-derived O-benzylated diazirine 1 in the presence of N-phenylmaleimide (2), acrylonitrile (3), dimethyl fumarate (4), or dimethyl maleate (5) led in good yields to mixtures of the spirocyclopropanes 6/7, 8-11, 12/13, and 12/13/16/17. The diastereoselectivity depends upon the alkene. The cycloaddition of 1 to 5 is not diastereospecific, in keeping with previous results. Deprotection of 12, 13, 16, and 17 yielded the tetrols 14,15, 18, and 19, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740316
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  • 14
    Electronic Resource
    Electronic Resource
    Lithium-Salt Effects in Peptide Synthesis. Part II. Improvement of Degree of Resin Swelling and of Efficiency of Coupling in Solid-Phase Synthesis (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 628-643 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The course of solid-phase peptide-coupling reactions as well as the swelling properties of a peptide-resin are influenced by the addition of inorganic salts (LiCl, LiBr, LiClO4, KSCN). Used as additives, these salts can (i) improve coupling yields (e.g., for Fmoc-(Ala)5-Phe-resin → Fmoc-(AIa)6-Phe-resin in DMF/CH2Cl2 1:1 from 89.4 to 97.1% (for polyethylene oxide) on polystyrene (= PEO-PS) resin) or from 77.5 to 93.8% (for poly-(N,N′-dimethylacrylamide) on ‘Kieselgur’ (=PDMAA-KG) resin) without and with 0.4M LiCl, respectively), (ii) increase resin swelling (e.g. for Fmoc-(Ala)5-Phe-(polystyrene resin) from 2.42- to 5.73-fold in 1-methylpyrrolidin-2-one (=NMP) without and with LiCl, (respectively), and (iii) change coupling rates. Example;; of coupling reactions and swelling behaviour (degree and rate) in different solvents (DMF, DMF/CH2Cl2 1:1, THF, NMP, N,N-dimethylpropyleneurea (= DMPU) with and without salts) using different resins (polystyrene (PS); PEO-PS, and PDMAA-KG) and an improved analysis of alanine oligomers up to Ala12-Phe by HPLC and FAB-MS are reported.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740320
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  • 15
    Electronic Resource
    Electronic Resource
    On the Methyl-Transfer Reaction in Crystalline Methyl 2-(Methylthio)benzenesulfonate: a Thermally Induced Non-Topochemical Solid-State Reaction (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 662-669 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rearrangement of methyl 2-(methylthio)benzenesulfonate (1) to the zwitterionic 2-(dimethyl-sulfonium)benzenesulfonate (2) is known to proceed in solution by intermolecular Me transfers. The same rearrangement has been observed to occur in crystalline 1, but the crystal structure shows that the molecular packing is not conducive to intermolecular Me transfer. The reaction has been carried out with mixed crystals composed of 1 and deuteriomethylated (D6)-l. By fast-atom-bombardment mass spectroscopy, it has been shown that the product consists of a 1:2:1 mixture of the non-, tri-, and hexadeuterated species, the mixture expected, if the solid-state reaction proceeds by intermolecular Me transfers. From this result, together with the slower rates of conversion in the single crystal compared with the melt, it can be concluded that the reaction must occur not topochemically but rather at defects such as microcavities, surfaces, and other irregularities in the ordered crystal arrangement.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740323
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  • 16
    Electronic Resource
    Electronic Resource
    Syntheses and Crystal Structures of 4,5-Dihydro-2-(2′-hydroxyphenyl)oxazole-Containing Metal Complexes (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 717-726 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the complexes 3 of various metals ligated to chiral 4,5-dihydro-2-(2′-oxidophenyl-ϰO)oxazoles-ϰN is described (Scheme). Three of them, i.e. 3a, 3e, and 3f containing CuII, ZnII and NiII, respectively, were analyzed by X-ray diffraction studies. A series of CuII complexes (6a-d) with differently substituted dihydrooxazoles have been synthesized.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740405
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  • 17
    Electronic Resource
    Electronic Resource
    1-(2′-Deoxy-β-D-xylofuranosyl)thymine Building Blocks for Solid-Phase Synthesis and Properties of Oligo(2′-Deoxyxylonucleotides) (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 748-760 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1-(2′-Deoxy-β-D-threo-pentofuranosyl)thymine (= 1-(2′-deoxy-β-D-xylofuranosyl)thymine; xTd; 2) was converted into its phosphonate 3b as well as its 2-cyanoethyl phosphoramidite 3c. Both compounds were used for solid-phase synthesis of d[(xT)12-T] (5), representing the first DNA fragment build up from 3′-5′-linked 2′-deoxy--β-D-xylonucleosides. Moreover, xTd was introduced into the innermost part of the self-complementary dodecamer d(G-T-A-G-A-A-xT-xT-C-T-A-C)2 (9). The CD spectrum of d[(xT)12-T] (5) exhibits reversed Cotton effects compared to d(T12) (6; see Fig. 1), implying a left-handed single strand. With d(A12) (7) it could be hybridized to form a propably Left-handed double strand d(A12) · d[(xT)12-T] (7·5) which was confirmed by melting experiments in combination with temperature-dependent CD spectroscopy. While 5 was hydrolyzed by snake-venom phosphodiesterase, it was resistant towards calf-spleen phosphodiesterase. The modified, self-complementary duplex 9 was hydrolyzed completely by snake-venom phosphodiesterase, at a twelvefold slower rate compared to unmodified 8; calf-spleen phosphodiesterase hydrolyzed 9 only partially.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740408
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  • 18
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    Conformational Analysis of Marine Polyhalogenated β-Chamigrenes Through Temperature-Dependent NMR SpectraPresented in part by G.G. at both CISCI 90, S. Benedetto del Tronto, 30 September-5 October 1990, and the Congresso Nazionale di Risonanze Magnetiche, Pisa, 22-24 October 1990. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 774-786 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Temperature-dependent 1H- and 13C-NMR spectra reveal that polyhalogenated marine β-chamigrenes or synthetic derivatives thereof which are trans-diequatorially, substituted at C(8) and C(9), such as rogiolol ((-)-2), obtusol ((+)-3), and their acetates (+)-1 and (-)-4, undergo slow ring-A chair-chair inversion. Conformational equilibria and kinetics are investigated with the aid of synthetic model compounds and molecular-mechanics calculations. Thus, steric repulsions between Brax-C(2) and Heq-C(7) are seen to disfavour thermodynamically conformers 1b, 2b, 3b, and 4b, which can only be detected through cross-saturation transfer, while additional steric repulsions between Meax-C(1) and OHax-C(3) make conformer 8b of obtusol epimer so scarcely populated that it can not be detected. In agreement, with (+)-9 and (+)-10, which have a trigonal C(2), two conformers can be directly observed by NMR. The kinetic barriers, which are seen to arise mainly from steric repulsions between Hax-C(14) and the axial H or halogen atoms at C(8) and C(10), are calculated and discussed with respect to well documented exocycliemethylidene-substituted cyclohexane(ene) systems. This helps to rationalize why in rogiolol acetate ((+)-1) ring B is unusually inert towards Zn/Et2O/AcOH which causes bromohydrine-group elimination from ring A.
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  • 19
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    Cycloviolaxanthin (= (3S,5R,6R,3′S,5′R,6′R)-3,6:3′,6′-Diepoxy-5,6,5′,6′-tetrahydro-β,β-carotin-5,5′-diol), ein neues Carotinoid aus Paprika (Capsicum annuum) (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 819-824 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cycloviolaxanthin (= (3S,5R,6R,3′S,5′R,6′R)-3.6:3′,6′-Diepoxy-5,6,5′,6′-tetrahydro-β,β-carotene-5,5′-diol), a Novel Carotenoid from Red Paprika (Capsicum annuum)From red paprika (Capsicum annuum var. longum nigrum) cycloviolaxanthin was isolated as a minor carotenoid and, based on spectral data, assigned the symmetrical structure 8.
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  • 20
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    Studies Directed Towards the Biosynthesis of the C7N Unit of Rifamycin B: A New Synthesis of Quinic Acid from Shikimic Acid (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 807-818 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of quinic acid (4) via epoxide 13, starting from shikimic acid (5), is described (Scheme 1). Treatment of 13 with thiophenol yielded not only 17, but also the γ-lactones 18 and 19 as result of migration of silyl groups within a cis- and trans-diol system. The conversion provides a direct stereoselective epoxidation of a shikimic-acid derivative as well as an alternative pathway for the preparation of 4. A shorter approach via the disilylated epoxide 22 was unsuccessful because the γ-lactone 25 was obtained in place of the desired α-hydroxy ester 24 (Scheme 2).
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  • 21
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    Nucleotides. Part XXXVIPart XXXV:[1]. Syntheses and biological characterization of phosphorothioate analogues of (3′-5′)adenylate trimer (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 892-898 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The four protected diastereoisomcrs 7a/7b and 8a/8b P-thioadenylyl-(3′-5′)-P-thioadenylyI-(3′-5′)-adenosine were synthesized, separated, and deblocked to the free oligonucleotides (Scheme). Biochemical characterization of these (3′-5′)phosphorothioate analogues of adenyiate trimer indicate that these compounds, and the corresponding 5′-monophosphates, neither bind to nor activate RNase L, and are considered to be valuable control compounds in screening experiments.
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  • 22
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    Double-Diamond Inclusion Compounds of 2,6-Dimethylydeneadamantane-1,3,5,7-tetracarboxylic Acid (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 825-877 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structures of four inclusion compounds of 2,6-dimethylideneadamantane-1,3,5,7-tetracarboxylic acid (4) are described, which involve the following guest species: (a) a mixture of 4-methylpent-3-en-2-one, 2,6-dimethylhepta-2,5-dien-4-one, and mesitylene (condensation products of acetone); (b) mesitylene; (c) a mixture of 4-methylpent-3-en-2-one and mesitylene; (d) (tert-butyl)benzene. In all four cases, the host architectures consist of two interpenetrating super-diamond networks built up by the tetra-acid molecules via pairwise H-bonds between the tetrahedrally directed COOH groups. In the first three cases (tetragonal crystal symmetry), the two diamond-like host lattices interpenetrate symmetrically, in the fourth case (monoclinic) asymmetrically. This asymmetry is brought about by the increased steric bulk of the (tert-butyl)benzene guest molecules. Attempts to enforce an inclusion compound of 4 with a single, extremely hollow diamond-like host lattice by offering still bulkier guest molecules have as yet not been met with success. The generally very high propensity of 4 to form inclusion compounds was envisaged and designed beforehand by appropriate evaluation and modulation of the crystal structure of the parent adamantane-1,3,5,7-tetracarboxylic acid, which represents a fivefold diamond-like self-inclusion compound. Crystals of the free, uncomplexed 4 appear to be extremely unstable and could so far not be obtained. On the other hand, from aqueous solution a very stable monohydrate of 4 may be crystallized (4-H2O), which was also subjected to X-ray analysis. The (triclinic) crystal structure of 4·H2O involves an interesting dichotomy inasmuch its pattern of H-bonding may be rationalized either in terms of a double, cross-linked super-zincblende (sphalerite) architecture, or as a system of porous, puckered 4-connected sheets, which interpenetrate each other pairwise and are cross-linked by the H2O molecules. Various structure and (space group) symmetry characteristics of the supramolecular solid-state complexes reported here are highlighted by pointing out analogies with comparable structures retrieved from the literature.
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  • 23
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    Masthead (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991) 
    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.19910740501
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  • 24
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    Novel Antifungal Tetracyclic Compounds from Bauhinia rufescens LAM.Presented in part at the Meeting of the Swiss Chemical Society, Bern, 19th October 1990. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 791-799 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: As the root bark CH2Cl2 extract of Bauhinia rufescens showed antifungal activity in a bioassay with the plant pathogenic fungus Cladosporium cucumerinum, a phytochemical investigation was undertaken on material collected in Niger. Activity-guided fractionation of this extract, using different preparative chromatographic methods, allowed the isolation of the four new antifungal tetracyclic compounds 1-4. Their structures were established by 1H- and 13C-NMR data, and single-crystal X-ray analyses were used to confirm the structures of 1 and 3. The isolated compounds seem to be biogenetically related to a stilbene.
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    URL: http://dx.doi.org/10.1002/hlca.19910740413
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  • 25
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    Versatile Stereoselective Synthesis of Completely Protected Trifunctional α-Methylated α-Amino Acids Starting from Alanine (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 800-806 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new route to completely protected α-methylated α-amino acids starting from alanine is described (see Scheme). These derivatives, which are obtained via base-catalyzed opening of the oxazolidinones (2S,4R)- and (2R,4S)-2, can be directly employed in peptide synthesis. The synthesis of both enantiomers of Z-protected α-methylaspartic acid β-(tert-butyl)ester (O4-(tert-butyl) hydrogen 2-methylaspartates (R) or (S)-4a), α-methyl-glutamic acid γ-(tert-butyl) ester (O5-(tert-butyl) hydrogen 2-methylglutamate (R)- or (S)-4b), and of Nε-bis-Boc-protected α-methyllysine (N6,N6-bis[(tert-butyloxy)carbonyl]-2-methyllysine (R)- or (S)-4c) is described in full detail.
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    URL: http://dx.doi.org/10.1002/hlca.19910740414
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  • 26
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    Nucleotides. Part XXXVPart XXXV:[1]. Synthesis of 3′-deoxyadenylyl-(2′-5′)-3′-deoxyadenyIyl-(2′-ω)-9-(ω-hydroxyalkyl)adenines (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 887-891 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Via the phosphotriester approach, new structural analogs of (2′-5′)oligoadenyiates, namely 3′-deoxyadenylyl-(2′-5′)-3′-dcoxyadenylyl-(2′-ω)-9-(ω-hydroxyalkyl)adenines 18-21, have been synthesized (see Scheme) which should preserve biological activity and show higher stability towards phosphodiesterases. The newly synthesized oligonucleotides 18-21 have been characterized by 1H-NMR spectra, TLC, and HPLC analysis.
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    URL: http://dx.doi.org/10.1002/hlca.19910740419
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  • 27
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    Mitteilungen (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 905-906 
    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.19910740423
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  • 28
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    [5-Leucin]enkephalin-Related Glycoconjugates: Structurally Novel Agents Effective against HIV-1 (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 951-955 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of [Leu5]enkephalin-related glycoconjugates with an ester-, ether-, or amide-type linkage were synthesized and evaluated for antiviral activity against HIV-1 in a cell-culture system using peripheral blood lymphocytes. All tested glycoconjugates exhibited a certain antiviral activity which was significantly higher than the activity of the parent peptide compound itself. These results indicate that synthetic glycoconjugates of opioid peptides are good candidates for the development of anti-HIV agents.
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  • 29
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    Synthesis, Isolation, and Full Spectroscopic Characterization of Eleven (Z)-Isomers of (3R,3′R)-Zeaxanthin (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 969-982 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Developmental efforts to improve the yield of the chemical synthesis of (3R,3′R)-zeaxanthin resulted in the isolation, partly by chromatography from reaction mixtures, and full spectroscopic characterization by 1H-NMR, UV/VIS, and CD spectrosocpy of eleven (Z/E)-isomers of zeaxanthin: (7Z)-, (9Z), (13Z)-, (15Z)-, (7Z,7′Z)-, (9Z,9′Z)- (7Z,9Z,7Z)-, (7Z,11Z,7′Z)-, (9Z,13Z,9′Z)-, (7Z,9Z,7′Z,9′Z)-, and (7Z,9Z,11Z,7′Z,9′Z)-zeaxanthin. Five of these isomers were obtained by specific synthesis, namely the (7Z)-, (7Z,7′Z)-, (9Z,9′Z)-, (7Z,9Z,7′Z)-, and (7Z,9Z,7′Z-9Z)-isomers.
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  • 30
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    Ringerweiterung von 1,2-Thiazol-3(2H)-on-1,1-dioxiden und 3-Amino-2H-azirinen zu 4H-1,2,5-Thiadiazocin-6-on-1,1-dioxiden (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1002-1010 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ring Enlargement of 1,2-Thiazol-3(2H)-one-1,1-dioxides and 3-Amino-2H-azirines to 4H-1,2,5-Thiadiazocin-6-one-1,1-dioxidesReaction of 3-amino-2H-azirines 2 with the 1,1-dioxides 4 and 7 of 1,2-thiazol-3(2H)-ones and 1,2-thiazoli-din-3-ones, respectively, in i-PrOH at room temperature leads to 4H-1,2,5-thiadiazocin-6(5H)-one-1,1-dioxides 5 (Scheme 2, Table) and the corresponding 7,8-dihydro derivatives 8 (Scheme 4), respectively. The structure of some of the new 8-membered heterocycles as well as the structure of the minor by-product 6 (Scheme 3) have been established by X-ray crystallography (Chapt. 4). The proposed reaction mechanism for the ring expansion to 5 and 8 (Scheme 2) is in accordance with previously published results of reactions of 2 and NH-acidic heterocycles and is further supported by the results of the reaction of 4a and the (1-15N)-labelled aminoazirine 2a*.
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  • 31
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    Zur Oxidation von 1,2-Thiazolen: Ein einfacher Zugang zu 1,2-Thiazol-3(2H)-on-1,1-dioxidenHerrn Prof. Dr. Siegfried Hauptmann zum 60. Geburtstag gewidmet (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1059-1070 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation of 1,2-Thiazoles; A Convenient Approach to 1,2-Thiazol-3(2H)-one 1,1-DioxidesThe 1,2-thiazoles obtained from 3-chloroalk-2-enals and ammonium thiocyanate (7 → 9, Scheme 1) are easily transformed to 1,2-thiazol-3(2H)-one 1,1-dioxidcs 10 on treatment with H2O2 in AcOH at 80°. Hydrogenation of 10 in AcOH yields the corresponding saturated 1,2-thiazolidin-3-one 1,1-dioxides 16 (Scheme 3). Cycloalka[c]-1,2-thiazoles 18 are prepared from 2-[(thiocyanato)methyliden]cycloalkan-1-ones and ammonia (Scheme 4). Surprisingly, oxidation of 18a with H2O2 in AcOH yields the tricyclic oxaziridine 19.
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  • 32
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    Zur Synthese von Derivaten des Pyrido[1,2-a]azepinsProf. Horst Prinzbach zum 60. Geburtstag gewidmet (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1095-1101 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the Synthesis of Derivatives of Pyrido[1,2-a]lazepineThe conjugated pyridinium ylides of type 4 undergo 8π -electrocyclisation affording the heterobicyclic allenes 5 which, in the presence of H2O and H2O2, are subsequently transformed into the substituted pyrido[1,2-a]-azepinones 6, a novel heterocyclic system. The chemical behaviour of 6a,b is dominated by cleavage of the lactam bond and subsequent reaction to pyridinc and isoquinoline derivatives, respectively; e.g. reaction of 6a, b with 4,5-dihydro-4-methyl-3H-1,2,4-triazole-3,5-dione affords the cycloadducts 11 and 12 almost quantitatively. The successful transformation of 6b into 15 is described, the latter being the first representative of the presently unknown, potentially antiaromatic pyrido[1,2-a]azepine (1).
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    Mitteilungen (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1156-1156 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740525
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    Regioselectivity, Stereoselectivity, and Isotopically Sensitive Branching in the Fe(I)-Mediated Dehydrogenation of Octane-1,8-diol in the Gas PhaseDedicated to Prof. Jürgen Sauer on the occasion of his 60th birthday (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1127-1134 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regio- and stereospecific labeling experiments are conducted to unravel the mechanistic features of the Fe+-induced dehydrogenation of octane-1,8-diol in the gas phase. With regard to the regioselectivity, ca. 20% of molecular hydrogen originates from the C(3)/C(4) or the equivalent C(5)/C(6) positions. The remaining 80% are, provided by the C(4)/C(5) methylene units. The steps, preceding the reductive elimination of hydrogen, are irreversible, and the overall reaction follows a 1,2-elimination mode. The loss of HD from C(3)/C(4) is associated with a kinetic isotope effect KH2/KHD = 1.68. Formation of D2 from the positions C(4)/C(5) has an isotope effect of KH2/KD2 = 4.7; this figure is slightly dependent on the configuration at C(4)/C(5). Most interesting is the finding that the configuration at C(4)/C(5) in [4,5-D2]octane-1,8-diol, i.e. 5c vs. 5d, plays a pivotal role in the dehydrogenation of the central C(4)/C(5) part. This unexpected and unprecedented result is explained in terms of conformational analysis. A staggered-like conformation serves as a precursor to generate a trans-fused bicyclic intermediate 6. It is this very intermediate from which most of the molecular hydrogen is eliminated. Of minor importance is the as-fused chelate 7, which is formed from an eclipsed-like conformation of the octane-1,8-diol/Fe+ complex. The contribution of 6 and 7 to the product formation is controlled by the relative configuration at the labeled positions C(4)/C(5). For the D,L-form 5c, we estimate a ratio of ca. 9:1 for the contribution of 6 vs. 7; due to an isotope effect, this ratio drops to 1:85:1 for the meso-form 5d. This finding constitutes the first example for the existence of isotopically sensitive branching (‘metabolic switching’) in gas-phase organometallic chemistry.
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    Crystal Structures of the Lanthanum(III), Europium(III), and Terbium(III) Cryptates of Tris(bipyridine) Macrobicyclic Ligands (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1163-1170 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structures of the LaIII, EuIII, and TbIII complexes of macrobicyclic [bpy.bpy.bpy] ligands, [La3+ ⊂ 1]3 Cl- ( = 3-La), [Tb3+ ⊂ 1]3 Cl- ( = 3-Tb), and [Eu3+ ⊂ 2]3 C1- ( = 3-Eu), have been determined. They confirm the cryptate nature of these species, the cations being bound to the eight N-sites of the ligand. The macrobicycle presents two open faces, thus allowing additional coordination of two species, Cl- ions or H2O molecules, to the bound cations. These data provide structural support for the photophysical studies of the luminescent properties of the EuIII and TbIII cryptates, which indicated residual coordination of H2O molecules.
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  • 36
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    Biphenyltetrols and Dibenzofuranones from Oxidative Coupling of Resorcinols with 4-Alkylpyrocatechols: New Ciues to the Mechanism of Insect Cuticle Sclerotization (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1205-1212 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation of 4-alkylpyrocatechols 2 by means of an insect diphenoloxidase (laccase) or K3 [Fe(CN)6] yields, in the presence of resorcinols 1 (R2 = H), complex mixtures of products from which biphenyltetrols 3 (R3 = H) and dibenzofuranones 5 and 6 were isolated. It is suggested that similar homo-coupling products are formed from pyrocatechols 2b and 2c in insects during cuticle sclerotization.
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  • 37
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    Quantitative Formation in Water of [Ru(CO)(H2O)5]2+ from Hexaaquaruthenium(II) and Carbon Monoxide (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1236-1238 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: High-pressure 17O- and 13C-NMR show that [Ru(H2O)6]2+ reacts quantitatively with carbon monoxide (50 bar) in water to form [Ru(CO)(H2O)5]2+.
    Additional Material: 1 Ill.
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    Synthese von ‘Push-Pull’-Oligoacetylenen24. Miteilung über Aminoacryl-Derivate. 23. Mitteilung: [1]Kurzmitteilung: [2] (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1264-1272 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of ‘Push-Pull’-OligoAcetylenes‘Push-pull’ triacetylenes 11a, b, c, as well as ‘push-pull’ tetraacetylene 13b have been prepared by reaction of the corresponding trichloroene(oligoinyl)amines 9 and 10 with 2 mol-equiv. of BuLi followed by acylation. The sequences (Schemes 3 and 4) are very simple and straightforward, they could in principle be applied to the synthesis of ‘push-pull’ pentaAcetylenes 15 and hexaacetylenes 17 (Scheme 5). Main limitations are the moderate yields as well as the low thermal stability of push-pull oligoacetylenes.
    Additional Material: 1 Tab.
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  • 39
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    Metabolic products of microorganisms. Part 262Part 261: [1].. The absolute configuration of sphydrofuran, a widespread metabolite from streptomycetes (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1281-1286 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Sphydrofuran (1) was isolated from the culture filtrate of Streptomyces sp. (strain Gö 28 and Tü 3616) by chemical screening methods. Metabolite 1 is an anomeric and ring-chain tautomeric mixture and could easily be transformed into the stable furan derivative 2 under acidic conditions. The constitution and relative configuration of 1 was established by X-ray crystallography of its 8-O-methyl derivative 3a. The absolute configuration at C(4) of 1 and thus of the whole molecule (3R,4S,5S) could be determined starting from 2 using the Helmchen method.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740617
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  • 40
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    Structure and reactivity of xanthocorrinoids. Part VPart IV, [1].. Formation of trans-diol derivatives of 5,6-dihydrocobyrinic acid from xanthocorrinoids under acidic conditions (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1287-1295 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On treatment with H2SO4/MeOH, epimerization of hexamethyl cis-5,6-dihydroxycobyrinate c,8-lactam (3) takes place quantitatively at C(6), yielding the corresponding trans-diol 4. The corresponding lactone 7, whose structure has been established by X-ray analysis, is obtained from xanthocorrinoids 5 and 6 under similar conditions.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740618
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  • 41
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    Alternative Strategies for the Fmoc Solid-Phase Synthesis of O4-Phospho-L-tyrosine-Containing Peptides (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1314-1328 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of biologically relevant O4-phospho-L-tyrosine-containing peptides have been synthesized by either the global phosphorylation of the side-chain-unprotected L-tyrosine moiety in presynthesized resin-bound peptides or alternatively by the incorporation of suitably protected O4-phospho-L-tyrosine building blocks in the continuous-flow method of Fmoc solid-phase peptide synthesis. Different phosphate-protecting groups have been applied.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740621
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  • 42
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    Stereoselektive Synthese von Pyrrolizidin-Alkaloiden aus NitroalkanonenHerrn Professor Dr. C. H. Eugster zurn 70. Geburtstag gewidmet (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1352-1361 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective Synthesis von Pyrrolizidin Alkaloids from NitroalkanonesReduction of 5-nitropentadecane-2,8-dione (11), synthesized by a Michael reaction from nitromethane, methyl vinyl ketone, and dec-1-en-3-one, gave, depending on the conditions, two epimeric 3-heptyl-2,3,5,6,7,7a-hexahydro-5-methyl-1H-pyrrolizines as the main products: Catalytic hydrogenation (Pd/C) afforded the expected 7aα-hydro epimer 1b with cis-orieted H-atoms at C(3), C(5), and C(7a). NaBH3CN/NH4OAc reduction of the nitrodione 11 yielded all four diastereoisomers with the 7aβ-hydro epimer 1a as the predominant component; 1a is a pheromone of the cryptic thief ant Solenopsis sp. The N-atom of the pyrrolizidine ring stems from NH4OAc exclusively as shown by reduction of 11 with NaBH3CN/(15N)H4OAc.
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    URL: http://dx.doi.org/10.1002/hlca.19910740624
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  • 43
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    Glycosylidene Carbenes. Part 4. Synthesis of Spirocyclopropanes from Acetamidoglycosylidene-Derived Diazirines (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1362-1372 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the first glycosylidene-derived 2-acetamido-2-deoxydiazirine 4 from N-acetylglucosamine 6 is described. Thus, 6 was transformed into the 3-O-mesylglucopyranoside 9 by glycosidation with allyl alcohol, benzylidenation, and mesylation (Scheme 2). Solvolysis of 9 gave the allopyranoside 10 which, upon benzylation and glycoside cleavage, yielded the hemiacetals 12. Using our established method (via the lactone oxime 14 and the diaziridines 16), 12 gave the diazirine 4. Thermolysis of this diazirine in the presence of i-PrOH gave the dihydro-1,3-oxazole 5 (Scheme 1); in the presence of acrylonitrile, the four diastereoisomeric spirocyclopropanes 17-20 and the acetamidoallal 21 were obtained and separated by prep. HPLC (Scheme 3). Assignment of the configuration of 17-20 is based on NOE measurements and on the effect of diamagnetic anisotropy of the CN group. The ratio of the four cyclopropanes, which is in keeping with earlier results, is rationalized.
    Additional Material: 4 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740625
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  • 44
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    Mammalian alkaloids: Synthesis and O-methylation of (S)- and (R)-3′-hydroxycoclaurine and their N-methylated analogues with S-adenosyl-L-[methyl-14C]methionine in presence of mammalian catechol O-methyltransferase (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1399-1411 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: O-Methylation of the optically active 3′-hydroxycoclaurines 3a and 3b and of the N-methylated analogs 5a,b with S-adenosyl-L-[methyl-14C]methionine in presence of mammalian COMT was investigated in vitro. The N-unsubstituted (1S)- and (1R )-isomers 3a and 3b, respectively, afforded almost equal amounts of the corresponding N-norreticuline 4 and N-nororientaline 19, besides two unknown by-products (see Fig. and Table 1). The N-methylated (1S)-isoquinoline 5a, on the other hand, afforded largely (S)-orientaline ((S)-19), while an almost equal mixture of (R)-reticuline (6b) and (R)-orientaline ((R)-19) was obtained from the (1R)-enantiomer 5b. The isoquinolines 3a,b and 5a,b were prepared by a Bischler-Napieralski cyclization yielding O-benzyl-protected isoquinoline 10 (Scheme 1). The optical resolution of 10 was accomplished with 2′-bromotartranilic acid. The N-methylated isoquinolines were prepared by N-formylation of 10a,b and reduction of the formamides 13a,b with diborane (→ 14a,b). Deblocking of the benzyl-ether moieties of 10a,b and 14a,b was accomplished by catalytic hydrogenation in presence of HCl, affording directly 3a,b·HCl and 5a,b·HCl, respectively.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740704
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  • 45
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    A Novel, Degraded Polyketidic Lactone, Leptosphaerolide, and Its Likely Diketone Precursor, Leptosphaerodione. Isolation from Cultures of the Marine Ascomycete Leptosphaeria oraemaris (LINDER)Presented in part by A. G. at the ‘Giornate di chimica delle Sostanze Naturali’ meeting, Maratea (Potenza), on June 3, 1991. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1445-1450 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acetone extraction of cultures of the marine ascomycete Leptosphaeria oraemaris (LINDER) on cornmeal disk gave the novel polyketide derivative leptosphaerolide ( = (+)-7-[(1E)-l,3-dimethylpent-1-enyl]-10-hydroxy-3-methoxybenzo[1,2-b:5,4-c′]dipyran-2(9H)-one; (4+)-8) besides the o-dihydroquinone 3-[(1E)-1,3-dimethylpent-1-euyl]-8,10-dihydroxy-7-methoxy-8-(2-oxopropyl)-1H-naphtho[2,3-c]pyran-9(8H)-one (1) as a 10:9 mixture of epimers. retro-Aldol reaction of 1 gave leptosphaerodione ( = (-)-3-[(1E)-1,3-dimethylpent-1-enyl]-10-hydroxy-7-methoxy-1H-naphtho[2,3-c]pyran-8,9(8H)-dione; (-)-6) which was also present in small amounts in the extracts and which gave 1 on reaction with acetone. It is thus likely that 1 is an artefact of the extraction by acetone. Biogenetically (+)-8 might derive from (-)-6 via an unusual oxidation with loss of CO2.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740707
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  • 46
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    1,2-Stereoinduction in Radical Reactions: Stereoselective Synthesis of 2-Alkyl-3-hydroxybutanoates (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1600-1607 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Radical addition to 2-cyclohexyl-5-methylidene-6-methyl-1,3-dioxan-4-one (2) affords stereoselectively 5,6-trans-products trans-3. The size of the radical has no influence on the selectivity. These trans-acetals are converted into threo-3-hydroxy-butanoates 4. Methyl 2-methylidene-3-[(tert-butyl)diphenylsilyloxy]butanoate (5), treated under the same conditions, leads mainly to the erythro-isomer of 4 after deprotection. An influence of the steric bulkyness of the radical is observed. The stereochemical course of the reactions is discussed.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740722
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  • 47
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    Gas-Phase Chemistry of Ti+ to Zn+ with Butyl Isothiocyanate (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1609-1621 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using a complete set of 2H-labeled isotopomers, the gas-phase reactions of Ti+ to Zn+ with butyl isothio-cyanate are studied. The main product for most of the metal ions is MHNCS+ formed by an ion/dipole mechanism. Exceptions are Cr+, which yields significant amounts of H2 and H2S loss, Mn+, which is the only ion that does not form MHNCS+ at all but produces mainly C2H4 and C3H6, and Zn+, which predominantly reacts by charge transfer.
    Additional Material: 6 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740802
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  • 48
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    1,3-Oxathiolan-Synthese: Spirocyclische 1,3-Oxathiolane aus der Lewis-Säure-katalysierten Umsetzung von cyclischen Trithiocarbonaten und OxiranenHerrn Professor Dr. C. H. Eugster zum 70. Geburtstag gewidmet (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1500-1510 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1,3-Oxathiolanc Synthesis: Spirocyclic 1,3-Oxathiolanes from the Lewis-Acid-Catalyzed Reaction of Cyclic Trithiocarbonates and OxiranesThe cyclic trithiocarbonates 1.3-dithiolane-2-thione (4) and 1,3-dithiole-2-thione (9) in 1,2-dichloroethane and MeCN, respectively, react with alkyl- and phenyl-substituted oxiranes 2 in the presence of Lewis acids to give 1-oxa-4,6,9-trithiaspiro[4.4]nonanes 5 and 6 (Scheme 2) and 1-oxa-4,6,9-trithiaspiro[4.4]non-7-enes 10 and 11 (Scheme 3), respectively. The reactions proceed regioselectively yielding 2-alkyl (5, 10) and 3-phenyl derivatives (6, 11) as the main products. From the reaction of 4 and 2-phenyloxirane (2e) with TiCl4, 2-phenyl-1,4,6,9-tetrathia-spiro[4.4]nonane (7) is isolated as a minor product. The molecular structures of 5a, 6e, and 7 are established by X-ray crystallography.
    Additional Material: 2 Ill.
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  • 49
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    2D-NMR ConformationaL Studies of ChiraL Functionalized Ferrocenylphosphine Ligands for the Gold(I)-Catalyzed Aldol Reaction (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1533-1543 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformations in solution of six selected chiral 1,1′-bis (diphenylphosphino)ferrocenyl ligands bearing functionalized side chains, the diastercoisomers 4 and 5 and 6-9, have been elucidated by 2D-NMR methods (COSY, TOCSY, NOESY, and 1H,31P and 1H,13C correlations). The possible relationship between the preferred conformation and the stereoselectivity observed in the gold(I)-catalyzed asymmetric aldol condensation in the presence of these ligands is discussed.
    Additional Material: 5 Ill.
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  • 50
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    Crystal Structure and ESR Study of a Bimetallic Copper/Iron Crown Ether Inclusion Complex (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1591-1599 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bimetallic inclusion complexes have been synthesized by a secondary coordination interaction between the guest complex [Fe(η5-C5H5)(CO)2(NH3)][PF6] and copper(II) complex 1a or nickel(II) complex lb containing crown-ether hosts. The X-ray crystal-structure analysis established that the Cu,Fe inclusion complex 2 crystallize as a centrosymmetric dimer with a Cu—Cu separation of 3.73 Å and a novel out-of-plane Cu—N interaction. The magnetic parameters for 2 were obtained by ESR and ENDOR spectroscopy. ESR susceptibility measurements down to 6 K exclude the presence of any antiferromagnetic coupling interaction between the CuIIcenters of the dimer.
    Additional Material: 5 Ill.
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    Synthesis of Aristotelia-type alkaloids. Part VIIIPart VII: [1].. Synthesis of (±)-aristolasicone (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1636-1642 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The revised structure of the indole alkaloid aristolasicone (2) was confirmed through a convergent total synthesis of the racemic form of this metabolite. The key step involves a one-pot condensation/cyclization reaction between 1-(4-methoxyphenylsulfonyl)-1H-indole-2-acetaldehyde (9) and (±)-trans-5-(2,6-difluorobenzyloxy)-p-menth-l-en-8-amine ((±)-7). The resulting allohobartine derivative (±)-13, obtained in 84% yield, was deprotected and oxidized to (±)-alloscrratenone ((±)-15) which cyclized smoothly to the target molecule (±)-2 upon exposure to BF3 · Et2O.
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    From 1-(Silyloxy)Butadiene to 4-Amino-4-Deoxy-DL-Erythrose and to 1-Amino-1-Deoxy-DL-Erythritol Derivatives via Hetero-Diels-Alder Reactions with Acylnitroso Dienophiles (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1653-1670 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acylnitroso dienophiles 4 reacted instantly with 1-(silyloxy)butadiene 5α and led in good yield to the regioisomeric cycloadducts 6 (major) and 7 (minor; Scheme 2, Table 1). cis-Hydroxylation of these primary cycloadducts with OsO4 (catalyst) occurred stereospecifically and in high yield (→8 and 9, resp.; Scheme 2). It was followed by reductive ring cleavage to give either 1-amino-1-deoxy-DL-erythritol or 4-amino-4-deoxy-DL-erythrose derivatives 10 and 14, respectively, depending on the nature of the reducing agent (Schemes 3 and 4).
    Additional Material: 7 Tab.
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    Cuprous complexes and dioxygen. Part 11Part 10: [1].. Concomitant one- and two-electron reduction of O2 by the Cuaq+ ion (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1679-1688 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The autoxidation of CuI in aqueous MeCN has been studied using a Clark oxygen electrode in the presence and absence of Cu11. The reaction is inhibited by Cu11 in the pH range of 0.5 to 5.0, reaching a lower limiting value at the highest concentrations. The reaction order changes from 1 to 2 with respect to CuI under the influence of Cu2+ ion. Detailed kinetics analysis of a total of 275 measurements has shown that an unstable primary adduct CuO+2 decomoses to give .O2- or HO2., depnding on pH, and also reacts directly with a second Cu+ ion, avoiding one-electrton reduction of O2 by this path. Reaction of HO2. is faster with CuI than with Cu11 by a factor of 20, and single-electron transfer within CuO+2 to Cu2+ and .O2- predominates over reaction with a second copper ion for [CuItot] 〈 2. 10-3M in the absence of Cu2+. The most likely value for the reaction of .O2- with CuI is 5.3 · 108 M-1S-1, but even this high rate constant is at the limit of significance. All secondary reactions followinfg the initial formation of CuO+2 are shown to be very fast, a fact that should be properly considered in the discussion of mechanisms of copper-catalyzed oxidations and oxygenations.
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    A Molecular-Mechanics Analysis of Complexes of the Sexidentate Macrocycles cis- and trans-6,13-Dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1834-1842 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular-mechanics calculations of the sexidentate coordinated complexes of the pendant arm macrocyclic hexaamines trans- (L1) and cis-6,13-dimethyl-3,4,8,11-tetraazacyclotetradecane-6,13-diamine (L2) reveal that the trans-isomer shows a preference toward small metal ions, whereas the L2 may coordinate a wider range of metal ions without significant intramolecular strain. Conformational disorder in previously reported crystal structures of complexes of L1 is interpreted, and predictions for some, as yet, unknown complexes of L2 are made.
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    URL: http://dx.doi.org/10.1002/hlca.19910740826
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    3-Deazaguanine N7- and N9-(2′-Deoxy-β-D-ribofuranosides): Building Blocks for Solid-Phase Synthesis and Incorporation into Oligodeoxyribonucleotides (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1790-1800 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligonucleotides continuing 3-deaza-2′-deoxyguanosine (I) or its N7-regioisomer 2 were prepared by solid-phase synthesis using P111 chemistry. Protection of 1 or 2 with N,N V-dimethylformamide diethyl acetal followed by 4,4′-dimethoxytritylation afforded imidazo[4,5-c]pyridines 10b and 11b, respectively. The latter were converted into the 3′-phosphonates 10c or lie, respectively; the cyanoethyl N,N-diisopropylphosphoramidite 10d was also prepared. The oligonucleotide building blocks were employed in automated solid-phase synthesis. 1 he self-complementary oligomers 13, 15, and 17 were prepared and characterized by enzymatic hydrolysis with snake-venom phosphodiesterase followed by alkaline phosphatase. There CD spectra exhibited the general structure of a B-DNA.
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    Steric Effects on Reaction Rates. Part XII. Force-Field Calculations for the Solvolysis of Cyclobutyl and Tricyclyl Derivatives (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1808-1816 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Force-field parameters have been developed for the molecular-mechanics calculation of tertiary carbenium ions with tricyclane structure, for tertiary cyclobutyl and cubyl cations. The cyclobutyl parameters are also applicable to tertiary 7-norbornyl cations. Satisfactory plots are obtained for correlation of the rates of solvolysis with the differences in steric energies between carbenium ions and the corresponding bromides.
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    URL: http://dx.doi.org/10.1002/hlca.19910740823
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    Imidazole-Analogues of 6-Epicastanospermine and of 3,7a-Diepialexine (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 934-940 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chiral bicyclic imidazol derivatives 7 and 8 were obtained from D-glucose derivative 9 by a sequence of selective protection/deprotection and intramolecular SN2 reactions. Triols 7 and 8 are analogues of 6-epica-stanospermine (4) and of 3,7a-diepialexine (6), respectively, and are potential glycosidase inhibitors. However, their anti-HIV activity proved to be only marginal.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740503
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    Ionization Behavior and Ionization-Dependent Conformation of Raclopride, a Dopamine D2 Receptor Antagonist (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 956-968 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Raclopride, an antipsychotic 6-methoxysalicylamide (= 2-hydroxy-6-methoxybenzamide) derivative, was shown by titrimetry and UV-pholometry to exist in zwitterionic form at physiological pH. Calculations revealed that the neutral and zwitterionic forms differ considerably in their conformational behavior, the latter form being energetically favored by an intramolecular phenolate-ammonium ionic bond. These findings indicate that raciopride and other halogenated 6-melhoxysalicylamides with a highly acidic phenolic group may not resemble other ortho-methoxybenzamides in their stereoelectronic structure and mode of binding to the dopamine D2 receptor.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740506
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  • 59
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    Synthesis and Structure of the Copper(II) Complex of a Chiral Bis(dihydrooxazole) Ligand (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1-6 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bridged bis(dihydrooxazole) compound (-)-2 and its silylated derivative (-)-3 have been synthesised as building blocks for chiral ligands. The complex of (-)-3 with CuII has been obtained, and its crystal structure has been determined. The CuII ion is bound to two deprotonated ligands in a twisted tetracoordinated geometry.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740103
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  • 60
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    Generation and Trapping of Cyclopropenes from 2-Alkoxy-1,1-dichlorocyclopropanes (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 55-64 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In presence of crown ether, 2-alkoxy-1,1-dichlorocyclopropanes react with t-BuOK/THF preferentially via ring opening to 2-chloroalk-2-en-1-ones and alkynones or to chlorocyclopropenes. The latter may be intercepted with 1,3-diphenylisobenzofuran, but in the absence of trapping agent, the rearrangement to vinylcarbenes does not occur.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740108
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  • 61
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    1H-Phenalen-1-one: Photophysical Properties and Singlet-Oxygen Production (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 79-90 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The efficiency of aromatic ketones as singlet-oxygen (1O2(1Δg)) sensitizers can vary considerably with the electronic configuration of their lowest triplet state and the solvent used. Near-infrared measurements of tie luminescence of singlet oxygen have shown that the quantum yield of singlet-oxygen production (ΦΔ) by 1H-phenalen-1-one (1) is close to unity in both polar (ΦΔ = 0.97±0.03 in methanol) and non-polar solvents (ΦΔ = 0.93±0.04 in benzene). Analysis of the absorption spectra of the ground state and phosphorescence measurements show that the lowest singlet and triplet states have dominant π, π* electronic configurations. The quantum yield of intersystem crossing (ΦISC) of 1, determined by laser flash photolysis (partial-saturation method), is equal to unity. In comparison with other aromatic ketones, these parameters are important for the discussion of the surprisingly high ΦISC of 1 and the efficient energy transfer from its triplet state to molecular oxygen. The 1H-phenalen-1-one (1), being one of the most efficient singlet-oxygen sensitizers in both polar and non-polar media, could be used as a reference sensitizer, in particular in the area of relatively high energies of excitation.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740110
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  • 62
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    Neurotoxic Sesquiterpenoids from the Yellow Star Thistle Centaurea solstitialis L. (Asteraceae) (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 117-123 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ingestion of yellow star thistle (Centaurea solstitialis L.) by horses produces parkinsonism due to nigro-pallidal degeneration. The toxin responsible has not been identified so far. A CH2Cl2 extract from the aerial parts of C. solstitialis exhibited significant neurotoxicity against primary neuronal cultures of foetal rat brain. Activity-guided fractionation yielded the known sesquiterpene lactones solstitialin A (1), 13-0-acetylsolstitialin A (3), cynaropicrin (4), and the hitherto unknown 3-O-acetylsolstitialin A (2). In the bioassay with rat foetal full cell culture, 3 and 4 were toxic in a concentration-dependent manner and may be responsible for the ability of the plant to cause neurodegenerative changes in the brain of horses.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740114
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  • 63
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    1-Methoxycarbonyl-substituiertes 2,3-Dihydropyridin-4(1H)-on (= Methyl-1,2,3,4-tetrahydro-4-oxopyridin-1-carboxylat) als Chromophor für die photochemische [2 + 2]-Cycloaddition (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 163-178 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1-Methoxycarbonyl-Substituted 2,3-Dihydropyridin-4(1H)-one(= Methyl 1,2,3,4-Tetrahydro-4-oxopyridine-1-carboxylate) as Chromophore for Photochemical [2 + 2]-CycloadditionsWith olefins having an electron-acceptor as well as with olefins having an electron-donor substituent, 1-methoxycarbonyl-substituted dihydropyridinone 12 undergoes [2 + 2] cycloaddition in good preparative yields. The photochemical cycloaddition is highly regioselective. For preparative purposes, the ring junction can be equilibrated to the thermodynamically more stable cis-junction. Only the ‘endo’/‘exo’ selectivity at the C-atom bearing the olefin substituent cannot be controlled. The photodimerization of 12 is the only side reaction. Using a slight excess of the olefin, the photodimerization can be suppressed. The protecting group at the N-atom of the dihydropyridinone can be varied in order to introduce an internal sensitizer, as shown with 1-acyl-substituted compound 29, which underwent the cycloaddition process even with sunlight.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740118
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    C2-Symmetric 4,4′,5,5′-Tetrahydrobi(oxazoles) and 4,4′,5,5′-Tetrahydro-2,2′-methylenebis[oxazoles] as Chiral Ligands for Enantioselective Catalysis Preliminary Communication (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 232-240 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of a series of enantiomerically pure, C2-symmetric 4,4′,5,5′-tetrahydro-2,2′-methylenebis[oxazoles] and 4,4′,5,5′-tetrahydro-2,2′-bi(oxazoles) is reported. Copper complexes with anionic tetrahydromethylenebis[oxazole] ligands are efficient catalysts for the enantioselective cyclopropane formation from olefins and diazo compounds (up to 96% ee in the reaction of styrene with menthyl diazoacetate). Tetrahydrobi(oxazole)iridium(I) complexes were found to catalyze transfer hydrogenations of aryl alkyl ketones with i-PrOH (up to 91% ee). Tetrahydrobi(oxazole)palladium complexes can be used as enantioselective catalysts for allylic nucleophilic substitution (up to 77% ee in the reaction of PhCH=CHCH(OAc)Ph with NaHC(COOMe)2).
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740123
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    Masthead (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991) 
    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.19910740201
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  • 66
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    Electronic and Conformational Effects on the Lipophilicity of Isomers and Analogs of the Neurotoxin 1-Methyl-4-phenylpyridinium (MPP+) (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 290-296 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of isomers and analogs of the neurotoxin 1-methyl-4-phenylpyridinium (MPP+) were examined for their lipophilic behavior. Their partition coefficients in the 1-octanol/H2O system were measured by centrifugal partition chromatography (CPC), which, being much faster and markedly more precise than the classical shake-flask method, proved a very promising alternative for assessing lipophilicity. A smaller-than-expected lipophilicity was shown by the ortho-isomer of MPP+ (M2PP+) and is explained in terms of electronic effects and rigidity, as revealed by UV and NMR spectroscopy, and conformational analysis. Implications of lipophilicity in modulating the in vivo dopaminergic neurotoxicity of the examined compounds are also discussed.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740205
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    On the Regioselectivity Control in the Palladium-Catalyzed Hydro-alkoxycarbonylation of α,β-Unsaturated Esters (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 323-325 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The regioselectivity of the hydro-alkoxycarbonylation of methyl acrylate, methacrylate, and crotonate catalyzed by [PdCl2L2] complexes (L = phosphine ligands) can be largely controlled by variation of the ligands. PPh3, promotes preferential carbonylation at the α-position, whereas with [(2,2-dimethyl-1,3-dioxolane-4,5-diyl)bis-(methylene)]bis[diphenylphosphine] as ligand, the β-position is overwhelmingly carbonylated.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740210
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    The Synthesis of Boronolide (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 336-342 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic boronolide (1) is prepared in six steps in 4.4% overall yield from acrolein dimer 6 and 1-(trimethyl-silyl)hex-1-yne (8). The latter, by hydromagnesiation, is condensed with 6 to give the corresponding threo-allylic alcohol 13 (Scheme 2). Conversion of 13 to the erythro-allylic alcohol 5 (Scheme 3), bis-hydroxylation, and acetylation afford 1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740213
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  • 69
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    C-Alkylation of Peptides through Polylithiated and LiCl-Solvated Derivatives Containing Sarcosine Li-Enolate Units (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 197-224 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The tripeptide and hexapeptide derivatives Boc-Gly-Sar-MeLeu-OH (5b), Boc-Ala-Sar-Sar-OH (6b), Boc-Ala-Sar-MeLeu-OH (7b), and Boc-Abu-Sar-MeLeu-Val-MeLeu-Ala-OH (12b) can be poly-deprotonated (tri- and pentalithio derivatives K and P, respectively), and thus C-alkylated on sarcosine (Sar) moieties with MeI and allyl or PhCH2Br. The polylithiated species are solubilized in THF, and their reactivity modified by excess base (lithium diisopropylamide (LDA)), by added LiCl, and/or the cosolvent N,N′-dimethylpropyleneurea (DMPU). Optimization of the reaction conditions for methylation in the cases of 7b (Table 3) and 12b (Scheme 8) gave products in which the Sar residue of the educt has been transformed into a Me-D-Ala unit in yields of 80 (9c/8c) and 67% (14c/13c), respectively, and with a diastereoselectivity of ca. 4:1. Less selective methylations and benzylations were observed with the tripeptides 5b and 6b containing only one stereogenic center; also, excess base and alkyl halide may lead to double alkylations in those latter two cases (Tables 1 and 2). No epimerization of stereogenic centers was detected under the strong-base conditions. The analysis of the products was accomplished by a combination of NMR and FAB-MS spectroscopy, as well as by hydrolysis to the parent amino acids, subsequent formation of derivatives with isopropyl isocyanate, and GC analysis on the chiral column Chirasil-Val®.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740121
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    Quantitative Structure-Affinity Relationships of Dopamine D2 Receptor Antagonists: A Comparison between Orthopramides and 6-Methoxysalicylamides (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 241-254 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A large series of orthopramides (= 2-methoxybenzamides), 6-methoxysalicylamides, and 2,6-dimethoxy-benzamides were examined for their affinity to the dopamine D2 receptor. The binding data were correlated with physicochemical parameters and 13C-NMR chemical shifts using the cross-validated partial least-squares method and multiple linear regression analysis. The results quantitate the influence of electronic factors and lipophilicity to D2 receptor binding. They also show that the N-[(1-ethylpyrroIidin-2-yl)methyl] and N-(1-benzylpiperidin-4-yl) side-chains affect the mode of binding of these compounds.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740202
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    The Synthesis of 3-Acyl-2,3-dihydro-2,2-dimethyl-1,3-thiazoles (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 297-303 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acylation of 2,5-dihydro-2,2-dimethyl-1,3-thiazoles leads to 3-acyl-2,3-dihydro-2,2-dimethyl-1,3-thiazoles as potential starting materials for the total synthesis of racemic cephalosporins.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740206
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    Stereoselective Synthesis of Indazole 2′-Deoxy-β-D-ribonucleosides: Glycosylation of the Nucleobase Anion (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 315-322 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The glycosylation of indazolyl anions derived from 4a, b with 2-deoxy-3,5-bis-O-(4-methylbenzoyl)-α-D-erythro-pentofuranosyl chloride (5) is described. The reaction was Stereoselective - exclusive β-D-anomer formation - but regioisomeric N1- and N2-(2′-deoxy-β-D-ribofuranosides) (i.e. 6a and 7a, resp., and 6b and 7b, resp.) were formed in about equal amounts. They were deprotected to yield 8a, b and 9a, b. Compound 1, related to 2′-deoxyadenosine (3), and its regioisomer 2 were obtained from 8b and 9b, respectively, by catalytic hydrogenation. The anomeric configuration as well as the position of glycosylation were determined by 1D NOE-difference spectroscopy. The first protonation site of 1 and 2 was found to be the NH2 group. The N-glycosylic bond of 1H-indazole N1-(2′-deoxyribofuranosides) is more stable than that of the parent purine nucleosides. Compound 1 is no substrate for adenosine deaminase.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740209
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    Enantioselective Synthesis of Both Enantiomers of a Isoproterenol Analogue and of Di-O-pivaloylepinephrine (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 390-396 
    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 analogue 4a of isoproterenol and of di-o-pyvaloylepinephrine 3 which provides both enantiomers in optically pure state is reported. The key step of the method is the highly diastereo-selective reduction (using DIBAL or DIBAL/ZnCl2) of a β-ketosulfoxide 7 which leads, as desired, to the (S)- or the (R)-configuration at C(1) (Schemes 4 and 5).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740216
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  • 74
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    Über die Reaktion einiger 2H-Pyran-2-on-Derivate mit primären Aminen (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 407-416 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the Reaction of Some 2H-Pyran-2-one Derivatives with Primary AminesThe versatile reactivity of 6-unsubstituted 2H-pyran-2-ones towards aliphatic and aromatic amines has been studied. It was found that the result of the reaction depends not only on the substitution of 2H-pyran-2-ones and on the structure of amines, but also on the stoichiometric ratio of reacting compounds.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740218
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  • 75
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    A New Synthesis of (2S,3R,4R)-2-(Hydroxymethyl)pyrrolidine-3,4-diol (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 445-450 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compound 6 was synthesized from 2,3,5-tri-O-benzyl-D-arabinofuranose (7) in three steps and 48% overall yield. Moreover, it was shown, in the case of γ-hydroxy amide 9, that the Mitsunobu reaction is not suitable for the preparation of γ-lactams, because O-alkylation is predominant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740222
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    Synthesis of a Phosphonic Acid Analogue of N-Acetyl-2,3-didehydro-2-deoxyneuraminic Acid, an Inhibitor of Vibrio cholerae Sialidase (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 451-463 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the phospha analogue 10 of DANA (2) is described. Bromo-hydroxylation of the known 11 (→ 12 and 13) followed by treatment of the major bromohydrin 13 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gave the oxirane 14 (Scheme 1). Depending on the solvent, TiBr4 transformed 14 into 16 or into a 15/16 mixture. Reductive debromination of 16 (→ 17), followed by benzylation provided 18. Oxidattve decarboxylation (Pb(OAc)4) of the acid, obtained by saponification of 18, yielded the anomeric acetates 19 and 20. While 19 was inert under the conditions of phosphonoylation, the more reactive imidate 22, obtained together with 23 from 19/20 via 21 (Scheme 2), gave a mixture of the phosphonates 24/25 and the bicyclic acetal 26. Debenzylation of 24/25 and acetylation led to the acetoxyphosphonates 27/28. Since β-elimination of AcOH from 27/28 proved difficult, the bromide 34 was prepared from 27/28 by photobromination and subjected to reductive elimination with Zn/Cu (→ 35; Scheme 3). This two-step sequence was first investigated using the model compounds 30 and 31. Transesterification of 35, followed by deacetylation gave 10, which is a strong inhibitor of the Vibrio Cholerae sialidase.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740223
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  • 77
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    Palladium-Catalyzed Ene-Type Cyclizations of Terpenoid 1,6-Enynes (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 517-530 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Palladium-catalyzed ene-type cyclizations of 1,6-enynes 2 yield methylidene-cyclopentane derivatives 3. Removal of protecting groups from 3 furnishes the diols 4 and 5, whose configurations were established by a combination of X-ray and 1H-NMR techniques. The predominant formation of isomers 4 is discussed in the context of two alternative hypothetical mechanisms previously proposed for this type of cyclization.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740308
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    Katalytische Carbonylierung von Aceton-oxim an Bisoximato-diruthenium-Komplexen: Ein einfacher Zugang zu wasserfreiem Acetonin. Isolierung und Molekülstruktur von [Ru2(CO)5(Me2CNO)2(Me2CNOH)] (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 549-553 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Catalytic Carbonylation of Acetone Oxime at Bisoximatodiruthenium Complexes: A Simple Access to Anhydrous Acetonine. Isolation and Molecular Structure of [Ru2(CO)5(Me2CNO)2(Me2CNOH)]In the presence of catalytic amounts of [Ru2(CO)4(Me2CNO)2(Me2CNOH)2] (1), acetone oxime reacts with carbon monoxide to give 2,3,4,5-tetrahydro-2,2,4,4,6-pentamethylpyrimidine (‘acetonine’). CO2, and NH3. The reaction proceeds presumably via carbonylation of Me2C=NOH to the unstable intermediate Me2C=N—OCHO which undergoes decarboxylation to give the corresponding imine Me2C=NH. The final product is assumed to result from the cyclotrimerisation of Me2C=NH with elimination of NH3. Evidence for the intermediacy of the imine comes from the analogous reaction of acetophenone oxime, PhMeC=NOH, which gives the corresponding imine PhMeC=NH as a stable product. The isolation of the complex [Ru2(CO)5(Me2CNO)2(Me2CNOH)] (2) suggests the carbonylation to take place at the bridging oximato ligand of 1.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740311
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    Synthesis of 1,2,3,4-Tetrahydro-4-oxothieno[3,2-d]pyrimidine and Perhydropyrimidine Derivates from Alkyl DicyanoacetatesDedicated to Michael Hanack, Tübingen, on the occasion of his 60th birthday (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 579-584 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nucleophilic addition of methyl 3-aminothiophene-2-carboxylate (2) as well as of β-alanine ethyl ester (11) to one CN group of the salts 1 of alkyl dicyanoacetates, followed by cyclization, yielded 1,2,3,4-tetrahydro-4-oxothieno[3,2-d]pyridimine derivatives 6-10 (Scheme 1) and perhydropyrimidine derivatives 17-19 (Scheme 2), respectively.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740315
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  • 80
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    Lithium-Salt Effects in Peptide Synthesis. Part I. Conditions for the Use of Lithium-Salts in Coupling Reactions (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 617-627 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The influence of Li-salts on the course of peptide-coupling reactions was investigated. As a model for segment couplings, Ac-Phe-OH was coupled to HCl·H-Ala-OMe using the mixed anhydride, DCC1, DCCl/HOBt, BOP-Castro and TBTU-Knorr methods. As a model for stepwise synthesis Z-Phe-OH was coupled with HCl·Ala-O(t-Bu), using symmetrical anhydrides and active esters. The effects of salt additives such as LiCl, LiBr, LiC104, and ZnCl2 on yields, side-product formation, racemisation, and reaction rates are reported.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740319
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  • 81
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    Mitteilungen (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 670-671 
    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.19910740324
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  • 82
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    α-D-Glycosyl-Substituted α,α-D-Trehaloses with (1 → 4)-Linkage: Syntheses and NMR Investigations (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 682-696 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two symmetrical trehalose glycosyl ‘acceptors’ 4 and 6 were prepared and three of the unsymmetrical type, 8, 10, and 11. Glucosylation of symmetrical ‘acceptor’ 4 gave a higher yield of trisaccharide (44%) than protect ve-group manipulation, namely via selective debenzylidenation 2 → 9 or monoacetylation 2 → 5 which proceeded in moderate yields (33-34%). A comparison of catalysts in the cis-glucosylation of trehalose ‘acceptor’ 10 with tetra-O-benzyl-β-D-glucopyranosyl fluoride 13 profiled triflic anhydride ((Tf)2O) as a new reactive promoter yielding 92% of trisaccharide 14, deblocking gave the target saccharide α-D-glucopyranosyI-(1 → 4)-α,α-D-trehalose. 1H-NMR spectra of most compounds were analyzed extensively. The use of the ID TOCSY technique is advocated for its time efficiency, if needed supplemented by ROESY experiments.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740403
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  • 83
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    Sequence-Specific Spin Labeling of Oligothymidylates by Phosphotriester Chemistry (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 739-747 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligothymidylates (oligo(dT)'s) with the sequence-specifically-incorporated one-atom-tethered C(5)-nitio-xide-labeled nucleoside 1 were synthesized by the phosphotriester method. Some modifications of the protocol were required to account for the chemical reactivity of the nitroxide, the stability of which was monitored during the synthesis by electron paramagnetic resonance (EPR) spectroscopy. The EPR specific activity (AEPR) of the FPLC-purified nitroxide-labeled oligomers was determined and found to be in agreement with enzymatically prepared spin-labeled nucleic acids. Annealing the nitroxide-labeled oligo(dT)'s to (dA)n or oligo(dA) resulted in different EPR-lineshape changes suggesting a strong coupling of the short-tethered nitroxide to global macro-molecular motion.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740407
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  • 84
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    Synthèse des (-)-(6R)-et(+)-(6S)-tétrahydro-6-[(Z)-pent-2-ényl]-2H-pyran-2-one, lactones de Jasminum grandiflorum L. et de Polianthes tuberosa L. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 787-790 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of (-)-(6R)- and (+)-(6S)-Tetrahydro-6-[(Z)-pent-2-enyl]-2H-Pyran-2-one, lactones from Jasminum grandiflorum L. and from Polianthes tuberosa L.(-)-(2S)-Ethyl 2-hydroxyhexanedioate ((2S)-2) was obtained by kinetic resolution of racemic ethyl 2-hydroxy-hexanedioate with baker's yeast. The key intermediates (+)-(5R)- and (-)-(5S)-ethyl 5,6-epoxyhexanoate ((5R)- and (5S)-6, resp.) are proved to be useful synthons for the total synthesis of chiral 6-alkyl-δ-lactones, as exemplified by the preparation of both enantiomers of jasmine lactone ((6R)- and (6S)-10, resp.).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740412
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  • 85
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    Synthesis of 1H,2,3-Triazoles from 2-Substituted Cyclododecanones and Phenyl AzideDedicated to Prof. Dr. Burchard Franck on the occasion of his 65th birthday (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 899-903 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: By reaction with phenyl azide, 2-cyano- and 2-(ethoxycarbonyl)-substituted cyclododecanones are converted into 5-amino- and 5-hydroxy-1H-1,2,3-triazoles, respectively. The possible reaction mechanism is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740421
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  • 86
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    Chiral Cooperativity: The Effect of Distant Chiral Centers in Ferrocenylamine Ligands upon Enantioselectivity in the Gold(I)-Catalyzed Aldol Reaction (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 905-933 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Long-range chiral cooperativity in enantiomerically pure ferrocenylamine ligands containing both planar and multiple centers of chirality (multiple stereogenic C-atoms) was demonstrated in the AuI-catalyzed reaction of aldehydes and isocyanoesters. Synthetic methodology was developed for the synthesis of ferrocenylamine ligands with two and three chiral centers of known absolute configuration in the C-side chain in addition to the planar chirality of the molecule. The diastereo- and enantioselectivity of the AuI-catalyzed formation of the trans- and cis-dihydrooxazoles 5 and 6, respectively, from benzaldehyde (1) and methyl isocyanoacetate (2) depend upon the sequence of chirality (absolute configuration of the chiral centers) in the side chain of the ferrocenylamine ligands. Particularly significant effects were observed upon the enantioselectivity for the minor cis-dihydrooxazole 6, for which, in certain cases, resulted in a change in the enantiomeric dihydrooxazole 6 produced in excess with a change in the absolute configuration of a distant chiral center. Significant effects upon diastereo- and enantioselectivity were observed when chiral ferrocenylamine ligands containing free OH groups were utilized. Using ligands containing a free OH group gave 6 with an absolute configuration opposite to that produced by the corresponding ester and carbamate derivatives. The possible mechanisms for the transmission of chiral information in the proposed stereoselective transition state (TS) was discussed, including both the formation of a stereogenic N-atom and steric effects based upon Newman's rule of six.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740502
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  • 87
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    Synthesis and X-Ray Crystal Structure of an Optically Pure Tripodal C3-Symmetric Tritertiary Phosphine Bearing Chirality on Phosphorus (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 983-988 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preparation of the optically pure tritertiary phosphine (RRR)-MeSi(CH2P(t-Bu)Ph)3 (2) is reported. The route followed involves deprotonation of optically pure (R)-P(BH3)Me(t-Bu)PH (2) the reaction of the resulting carbanion with MeSiCl3, followed by removal with morpholine of the BH3-protecting groups from the triertiary phosphine-borane 3. The latter's X-ray crystal structure and that of [Rh(NBD)((RRR)-1]TOf)(4), are also rported. Furthermore, it is shown that the separation of the racemic phosphine-borane 2 can be conveniently carried out using medium-pressure liquid chromatgrapy with cellulose-riacetate as a chiral stationary phase.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740508
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  • 88
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    Conformational Analysis of the cis- and trans-Isomers of FK506 by NMR and Molecular Dynamics (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1027-1047 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The potent immunosuppressant drug FK506 (2) has been examined by 1H- and 13C-NMR spectroscopy and NOE-restrained molecular dynamics to elucidate the conformation in solution. A combination of two- and three-dimensional NMR techniques was used to completely assign the 1H- and 13C-NMR chemical shifts of the two configurational isomers resulting from the cis-trans isomerization about the single amide bond. Hetero- and homonuclear coupling constants were measured to assign the diastereotopic methylene protons at C(16), C(18), and C(23). Intramolecular H—H distances were defined from NOESY spectra recorded at -30° in CDCl3 and used as constraints in molecular-dynamics simulations. The conformational preferences of 2 in solution are discussed in light of the constitutional features recently proposed to be necessary for binding and activity.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740513
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  • 89
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    3′-Substituted and 2′,3′-Unsaturated 7-Deazaguanine 2′,3′-Dideoxynucleosides: Syntheses and Inhibition of HIV-1 Reverse Transcriptase (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1081-1090 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of 2′,3′-dideoxy-2′,3′-didehydro-β-D-ribofuranoside 1 and 2′,3′-dideoxy-3′-fluoro-β-D-ribo-furanoside 5 of 7-deazaguanine as well as 7-deaza-2′-deoxyxyloguanosine (3) are described. The corresponding 2,4-diamino compounds 2 and 4 were also prepared. Thus, silytation of 2-amino-4-chloro-7-(2-deoxy-β-D-erythro-pentofuranosyl)-7H-pyrrolo[2,3-d]pyrimidine (6) afforded 7, which gave the oxo-nucleoside 13 after oxidation with CrO3. NaBH4 reduction yielded 14 which, upon deprotection (Bu4NF) and nucieophific displacement, afforded 3 and 4. On the other hand, the N2-formyl derivative of 7 was mesylated (→ 10), treated with Bu4NF, and deprotected with NH3 yielding the 2′,3′-dideoxy-2′,3′-didehydro-nucleoside 12, Nucleophilic displacement reactions on 12 yielded 1 and 2. The fluoro-nucleoside 5 was obtained from 14 after methoxytritylation of the NH2 group (→ 16), fluorination with DAST (→ 17), and treatment with 2M NaOH. The 5′-triphosphates of 5 and other pyrrolo[2,3-d]pyrimidine 2′,3′-dideoxy-3′-fluoro-nucleosides were found to be highly active inhibitors of HIV-1. reverse transcriptase, similar to those of 1 and 2.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740517
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  • 90
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    Transesterifications with 1,8-Diazabicyclo[5.4.0]undec-7-ene/Lithium Bromide (DBU/LiBr) - Also Applicable to Cleavage of Peptides from Resins in Merrifleld Syntheses (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1102-1118 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A mixture of the amidine base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and LiBr (preferably 0,5 and 5 equiv., resp.) turns out to be a highly efficient catalyst (at 0-25°) for saponifications (in THF/H2O) and transesterifications (in ROH). The scope and limitations of the method are determined using ca. two dozens of different ester/alcohol combinations (Schemes 2 and 3). The investigation is focused on peptides as substrates. Under carefully controlled conditions, no epirnerization occurs with N-Boc- and N-Z-protected peptide esters, when methyl, ethyl, isopropyl, or allyl esters are the products, as shown for peptides containing up to six amino acids, with Ala, Len MeLeu, Asp(OEt), or Tyr at the C-terminus (Scheme 3 and Tables 1 and 2). Hydrolytic and transesterifying detachments of Boc-Leu-Ala-Gly-Val-OR and Boc-Leu-Ala-Gly-Phe-OR (R = H, Me) from PAM and Wang resins (1-8 h at 0-25°, 2 equiv. of DBU, 5 equiv. of LiBr) can be achieved by this method without epimerization of the C-terrninal stereogenic center; a comparison with other methods (HF, Ti(OR)4) is given (Schemes 4 and 5). Possible protecting-group strategies involving the DBU/LiBr method are discussed (Table 3), Extensive experimental details are given.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740520
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  • 91
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    C—C Bond Formation via Carbon-Centered Radicals Generated from Dicarbonyl(η5-cyclopentadienyl) organoiron Complexes (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1143-1155 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Irradiation of benzyldicarbonyl(η5-cyclopentadienyl)iron complex (2) leads to hemolytic cleavage of the Fe—C bond. In the presence of activated alkenes, radical addition occurs and both saturated and unsaturated addition products 7-9 are formed. Photolysis of alkyliron complexes 2,3 and 20 in the presence of acrylonitrile leads to the same products as the irradiation of the respective acyliron complexes 28-30. This indicates that, under photolytical conditions, alkyl and acyl complexes are in equilibrium with each other.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740524
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    Site-Selectivity in the Fe+-Mediated C—H/C—C Bond Activation of Aldimines Short CommunicationDedicated to Prof. Gerhard Spiteller on the occasion of his 60th birthday (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1171-1174 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is demonstrated for the first time that the site-selectivity for the Fe+-mediated C—H bond activation of aldimines R1N=CHR2 (R1, R2 = alkyl) involves the alkyl chain R1 by a factor ≥ 12 in comparison to the alkenyl part R2. This finding explains previous observations that dehydrogenation of intermediates formed by alkene loss from either R1 or R2 of R1N=CHR2/Fe+ preferentially involves the alkyl part.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740604
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  • 93
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    Lagotoside: A New Phenylpropanoid Glycoside from Lagotis stolonifera (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1273-1277 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: From the aerial parts of Lagotis stolonifera (Scrophulariaccae), a new phenylpropanoid glycoside, lagotoside (8), and the three known glycosides ehrenoside (5), verbascoside (= acteoside; 6), and plantamajoside (7) were isolated, together with the four known iridoid glucosides aucubin (1), catalpol(2), globularin (4), and lythantosalin (3). The structure of the new compound 8 was elucidated on the basis of chemical and spectral data as 2-(3-hy-droxy-4-methoxyphenyl)ethyl O-[α-L-arabinopyranosyl-(1 → 2)]-O-[α-L-rhamnopyranosyl-(1 → 3)]-4-O-feruloyl-β-D-glucopyranoside.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740615
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  • 94
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    High-Resolution Solid-State MAS13C- and 1H-NMR Spectra of Benzenoid Aromatics Adsorbed on Alumina and Silica: Successful Applications of 1D and 2D Pulse Experiments from Liquid-State NMRDedicated to Prof. Edgar Heilbronner on the occasion of his 70th birthday (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1296-1304 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Strong line-narrowing effects in solid-state, magic-angle-spinning (MAS) 13C- as well as 1H-NMR spectra of benzenoid aromatics adsorbed at alumina or silica surfAccs indicate high mobility of the organic adsorbates. Even under modest spinning rales (1 kHz), dipolar couplings are sufficiently reduced to allow scalar 13C,1H couplings to be measured. Hetero- and homonuclear pulse sequences known from high-resolution NMR in liquids, like SEFT, J-RESOLVED, DEPT, COSY, and 13C,1H shift-correlation experiments are successfully applicable. 13C spin-lattice relaxation limes are as short as 0.5 s (CH) and 1.1 s (Cq), and T1(1H) values are in the order of 0.3 s.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740619
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  • 95
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    Madeiranes, a New Class of Pentacyclic Triterpenes: D-Friedo-madeir-14-en-3β-ol and -3-one, D:C-Friedomadeir-7-en-3β-ol and -3-oneDedicated to Prof. Dr.Albert Eschenmoser on the occasion of his 66th birthday (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1329-1338 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The pentacyclic triterpenes 1a, b and 2a, b are isolated from extracts of Euphorbia mellifera Arr. Their structures, determined by X-ray analysis, are supported in part by MS, NMR, and IR data. The skeleton differs from that of representatives of the lupane or hopane class by (i) an unusual arrangement of the substituents in the cyclopentane ring E, the Me group being on C(19) and the i-Pr group in an angular position on C(17), and (ii) a cis D/E ring junction. We propose to name Die unknown pentacyclic parent ‘madeirane’, thus 1a, b and 2a, b are D-friedomadeir-14-ene and D:C-friedomadeir-7-ene derivatives. The biosynthesis of these compounds may be rationalized via ‘spirodammaranol’-cation intermediates 5 and 5′ and madeiranol cation 6, as outlined in Scheme 2.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740622
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  • 96
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    Masthead (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991) 
    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.19910740701
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  • 97
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    Regioselektive 1,3-dipolare Cycloadditionen von Thiocarbonyl-yliden mit 1,3-Thiazol-5(4H)-thionen (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1386-1398 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regioselective 1,3-Dipolar Cycloadditions of Thiocarbonyl Ylides with 1,3-Thiazole-5(4H)-thionesThe thiocarbonyl ylides 13 and 1,3-thiazol-5(4H)-thiones 1 undergo a smooth reaction to yield spirocyclic 1,3-dithiolanes 14-16 (Schemes 4-6). The 1,3-dipolar cycloadditions occur in a regioselective manner, but the orientation of the thiobcnzophenone-S-methylide (13b) differs from that of the cycloalkane thione-S-methylides 13a and 13c. Whereas the 1,3-cycloadduct with 13b is formed in accordance with frontier-orbital considerations, the inverse orientation in the reactions with 13a and 13c most likely is the result of steric hindrance in the transition state. The thiocarbonyl ylides have been prepared in situ from the corresponding 2,5-dihydro-1,3,4-thiadiazoles 12. The more stable aliphatic precursors 12a and 12c undergo decomposition at 50°, the unstable 12b at -30°.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740703
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  • 98
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    Asymmetric Catalysis by Vitamin B12. The Mechanism of the Cob(I)alamin-Catalyzed Isomerization of 1,2-Epoxycyclopentane to (R)-Cyclopent-2-enol (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1425-1444 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The isomerization of 1,2-epcxycyclopentane (1) to enantiomerically enriched (R)-cyclopent-2-enol (2) in protic solvents is catalyzed by cob(I)alamin. The enantiomeric excess (e.e.) of (R)-2 is usually ca. 60%; it is only slightly dependent on the temperature, but increases with decreasing dielectric constant ε of the solvent. Standard kinetic methods show the reaction to be first order in vitamin B12 and zero order in 1. The rate constant increases exponentially with increasing ε of the solvent. An Arrhenius plot at ε = 40 gives activation parameters ΔH≠ = 78 ± 4 kJ·mol-1 and ΔS≠ = -49 ± 1 J·mol-1·K-1. The isomerization 1 → 2 proceeds in two steps (Schemes 2 and 7): (i) The epoxide ring is first opened by the proton-assisted fast and irreversible nucleophilic attack of the chiral CoI catalyst to form diastereoisomeric (1R,2R)- and (1S,2S)-(2-hydroxycyclo-pentyl)cob(III)alamins 6 in a ratio of ca. 4:1 which are the dominant species in the steady state; (ii) The intermediates 6 then decompose in the rate-limiting step to form 2 and recycled catalyst. Experiments with specifically 2H-labeled 1 showed the hydro-cobalt elimination 6 → 2 to be non-stereoselective. It proceeds via reversible Co—C bond homolysis to a free 2-hydroxycyclopentyl radical from which stereoelectronically controlled H-abstraction by Co11 takes place.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740706
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    Electronic Resource
    Fluxional Behaviour of Isocyanide Derivatives of Dodecacarbonyltetrairidium (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1464-1470 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The fluxional behaviour of [Ir4(CO)11(t-BuNC)] (1) and [Ir4(CO)11(ArNC)] (Ar = 4-methoxyphenyl, 4-tolyl, phenyl, 4-chlorophenyl, 4-nitrophenyl; 2-5) has been investigated by 2D and variable-temperature 13C-NMR. These techniques give new evidence of the processes responsible for CO exchange, namely two successive merry-go-rounds causing Cotton-like scrambling and a CO rotation about one metal center.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740710
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  • 100
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    Chiral Acylsilanes in Organic Synthesis. Diastereoselective 1,2-Additions to Racemic Alkoxymethyl-Substituted Acylsilanes (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1477-1488 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the three alkoxymethyl-substituted acyisilanes 1-3 is described (Schemes 1 and 2). Their reactions with NaBH4 as well as PhLi gave the corresponding alcohols with moderate to good diastereoisomeric induction (up to 78% de; see Table), depending upon the solvent used (Scheme 3). The results indicate that in Et2O, the reactions with PhLi proceed via 6-membered chelates (see C in Scheme 4) leading to the products with high de's (74-78%). In THF, these chelates are not formed, and as a consequence, the additions take place with reversed and lower stereoselectivities (34-50% de).
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19910740712
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