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  • Asymmetric synthesis  (18)
  • Wiley-Blackwell  (18)
  • Wiley
  • 2000-2004  (18)
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  • Wiley-Blackwell  (18)
  • Wiley
Years
Year
  • 1
    Electronic Resource
    Electronic Resource
    Preparation of Kingiside from Aucubin (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1623-1626 
    Details
    ISSN: 1434-193X
    Keywords: Iridoid glucoside ; (8S)-Kingiside ; (8S)-Loganin ; (8S)-7-Ketologanin ; Asymmetric synthesis ; Natural products ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The tetraacetyl derivative 8 of the naturally occurring kingiside (8a) was prepared from aucubin (1). Intermediates in the synthesis were (8S)-tetraacetyl loganin (6) and (8S)-tetraacetyl-7-ketologanin (7), whose free (8R)-epimers occur in many different plants (Caprifoliaceae, Loganiaceae). The 13C NMR spectrum allows the structure to be unequivocally identified.
    Additional Material: 1 Tab.
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  • 2
    Electronic Resource
    Electronic Resource
    Ring Closure Reactions of Disubstituted 4-Penten-1-oxyl Radicals - Towards a Stereoselective Synthesis of allo-Muscarine (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1677-1683 
    Details
    ISSN: 1434-193X
    Keywords: Radicals ; Cyclizations ; Pyridinethione ; Tetrahydrofurans ; Asymmetric synthesis ; Thiazolethione ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The trisubstituted functionalized tetrahydrofurans 10, 11, 16, 18, and 19 were photochemically prepared from 2,3-syn- and 2,3-anti-configuredN-(3-benzoyloxy-5-hexen-2-oxy)thiazole-2(3H)-thione anti-6, pyridinethiones 7, anti-8, and BrCCl3. The formation of tetrahydrofurans was achieved by an efficient and highly regioselective alkoxyl radical cyclization (5-exo-trig). The 2,3-anti substituted intermediates 9 and 12 cyclize stereoselectively whereas a 2,3-syn-configured O-radical affords both possible diastereomeric addition products in equal amounts. The cyclized tetrahydrofuryl methyl radicals were trapped with the bromine atom donor BrCCl3 to afford the bromomethyl-substituted cyclic ethers 10, 11, 18, and 19 in excellent yields. The utility of this reaction was stressed by conversion of one of the newly prepared tetrahydrofurans in a two-step synthesis into (+)-allo-muscarine (+)-20.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/2000/99590_s.pdf or from the author.
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  • 3
    Electronic Resource
    Electronic Resource
    Increasing Enantioselectivities and Reactivities by Stereochemical Tuning: Fenchone-Based Catalysts in Dialkylzinc Additions to Benzaldehyde (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1785-1792 
    Details
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Zinc ; Transition structures ; QM/MM computations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Trimethylsilyl substitutions of the fenchyl alcohols [(1R,2R,4S)-exo-(2-Ar)-1,3,3-trimethylbicyclo[2.2.1]heptan-2-ol, Ar = 2-methoxyphenyl (1) and Ar = 2-(dimethylaminomethyl)phenyl (2)] yield the chiral ligands 3 [Ar = 2-methoxy-3-(trimethylsilyl)phenyl] and 4 [Ar = 2-(dimethylaminomethyl)3-(trimethylsilyl)phenyl]. Increased reactivities and enantioselectivities in diethylzinc additions to benzaldehyde are obtained from 3 (63% ee R) and 4 (93% ee S), relative to 1 (26% ee S) and 2 (73% ee S). X-ray crystal structures of 3 and of its methylzinc complex 3-Zn reveal out-of-plane bending of the methoxy groups as major geometrical consequences of the trimethylsilyl substitutions. Analyses of QM/MM ONIOM μ-O transition-structure models for 1, 2, 3, and 4 show that trimethylsilyl-induced distortions of methoxy and of dimethylaminomethyl groups explain the observed increased enantioselectivities.
    Additional Material: 10 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    Enantioselective Preparation of 2,4-Disubstituted Azetidines (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1815-1820 
    Details
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Palladium-catalysed couplings ; Palladacycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Chiral C2-symmetric N-benzylazetidines have been conveniently prepared from optically pure anti-1,3-diols without loss of enantiomeric purity. N-Debenzylation led to the corresponding N-unsubstituted azetidines, which were then subjected to palladium-catalysed coupling reactions with aryl bromides to afford chiral N-arylazetidines. (R,R)-N-Benzyl-2,4-dimethylazetidine has been employed in the synthesis of a new cyclopalladated complex, which can be used, for instance, as a chiral recognition agent for phosphorus ligands.
    Additional Material: 1 Tab.
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  • 5
    Electronic Resource
    Electronic Resource
    Synthesis of Silaproline, a New Proline Surrogate (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 807-811 
    Details
    ISSN: 1434-193X
    Keywords: Proline analogues ; Asymmetric synthesis ; Schöllkopf's method ; Silicon ; Amino acids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The asymmetric synthesis of a new proline surrogate, incorporating the dimethylsilyl group at position 4 of proline using Schöllkopf's bis-lactim ether method, is described.
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  • 6
    Electronic Resource
    Electronic Resource
    Asymmetric Synthesis of α-Substituted β-Amino Sulfones by Aza-Michael Addition to Alkenyl Sulfones and Subsequent α-Alkylation (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 879-892 
    Details
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Addition reactions ; Sulfones ; Alkylations ; Amines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The aza-Michael addition of enantiopure 1-aminopyrrolidines to (E)-alkenyl sulfones in the presence of a catalytic amount of ytterbium trifluoromethanesulfonate [Yb(OTf)3] yields β-hydrazino sulfones in moderate to good yields and with diastereoselectivities of up to ≥ 98%. The latter undergo reductive N-N bond cleavage with BH3 · THF and, after N-protection with Boc2O or benzyl bromide, afford N-protected β-amino sulfones with moderate to high enantiomeric excesses (ee = 42 to ≥96%) without racemization. Subsequent α-alkylation of the N,N-dibenzyl protected β-amino sulfones with various electrophiles yields α-alkyl-β-amino sulfones in excellent yields (88-97%) with high diastereomeric (de ≥96 to ≥98%) and enantiomeric purity (ee = 94 to ≥96%). The absolute configuration of the new stereogenic centre was determined by X-ray structural analysis and confirmed by NMR spectroscopy (NOE experiments). Possible reaction mechanisms for the conjugate addition and α-alkylation are presented.
    Additional Material: 5 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Addition of Grignard Reagents to Chiral 1,2-Bisimines: A Diasteroselective Preparation of Unsymmetrical 1,2-Diamines (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 611-616 
    Details
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Grignard reactions ; Dynamic kinetic resolution ; Asymmetric synthesis ; Unsymmetrical 1,2-diamines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A diastereoselective synthesis of tert-butyl-1,2-diamines has been developed from the addition of tert-butylmagnesium chloride to the 1,2-bisimines derived from glyoxal and chiral amines such as 1-(S)-ethylphenylamine, 1-(S)-phenylpropylamine or 1-(S)-(p-chlorophenyl)ethylamine. The influence of solvent, temperature and chiral auxiliaries on the chemical reactivity and stereoselectivity has been fully studied. Evidence of a dynamic kinetic resolution during the bis-addition process of the organometallic, leading to the 1,2-di-tert-butylethanediamine,[1] as a single diastereomer, has been demonstrated. This resolution has been applied with high diastereoselectivities to the synthesis of unsymmetrical disubstituted 1,2-diamines, by addition of one equivalent of tert-butylmagnesium chloride, followed by one equivalent of a second Grignard reagent. Several chiral 1-tert-butyl-1,2-diamines have also been synthesized by monoaddition of tert-butylmagnesium chloride to the bisimines, followed by hydride reduction of the chiral intermediate imines.
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  • 8
    Electronic Resource
    Electronic Resource
    Synthesis of 2-Fluoro Analogues of Frontalin (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1387-1389 
    Details
    ISSN: 1434-193X
    Keywords: Natural products ; Epoxidations ; Asymmetric synthesis ; Fluorine ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The synthesis of enantiomerically and diastereomerically pure (-)-(1R,2R,5R)- and (-)-(1R,2S,5R)-2-fluoro frontalin (7) starting from (+)-(1S)-menthyl-(R)-toluene-4-sulfinate, methylmagnesium bromide, methyl fluoroacetate, 4-pentenyl bromide and diazomethane is described. The absolute stereochemistry was unambiguously determined by X-ray analysis of (+)-(1S,2R,5S,RS)-5, an intermediate in the synthesis of the enantiomeric (+)-(1S,2R,5S)-2-fluoro frontalin (7).
    Additional Material: 2 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Stereocontrolled Alkylation of Chiral Pyridinium Salt Toward a Short Enantioselective Access to 2-Alkyl- and 2,6-Dialkyl-1,2,5,6-Tetrahydropyridines (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1391-1399 
    Details
    ISSN: 1434-193X
    Keywords: Alkaloids ; Alkylations ; Asymmetric synthesis ; Grignard reactions ; Nitrogen heterocycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Treatment of salts 1a-b with Grignard reagents gives, after reduction of the resulting unstable dihydropyridines 7, the tetrahydropyridines 8a-c, with modest selectivities but in very few steps and under practical conditions. Higher stereo- and regioselectivities are obtained with salt 1c which gives the tetrahydropyridines 15a-e. In addition, the dihydropyridine intermediates 11b cyclize to give the new oxazolidine derivatives 12a-e, which turn out to be good precursors of the 2,6-trans-disubstituted tetrahydropyridines 21a-e. Selective syntheses of (-)-lupetidin, (+)-solenopsin, and indolizidines (-)-5 and (-)-6 are presented as representative examples of applications.
    Additional Material: 3 Tab.
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  • 10
    Electronic Resource
    Electronic Resource
    N-Monoalkylation of α-Amino Acid Esters under Solid-Liquid PTC Conditions (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1443-1449 
    Details
    ISSN: 1434-193X
    Keywords: Amino acids ; Alkylations ; Phase-transfer catalysis ; Asymmetric synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---N-(2-Nitrophenylsulfonyl)- (o-NBS-AA-OMe, 4) and N-(4-Nitrophenylsulfonyl)-α-amino acid methyl esters (p-NBS-AA-OMe, 5) were N-alkylated with a variety of alkyl halides 6 under solid-liquid phase-transfer catalysis (SL-PTC) conditions, affording the alkylated products o-NBS-N-R2-AA-OMe 7 and p-NBS-N-R2-AA-OMe 8 in excellent yields without any detectable racemization.
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