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  • 1
    Electronic Resource
    Electronic Resource
    Substituent effects in the 1,3-dipolar cycloaddition reactions of 2-methylenecephalosporins with diazoalkanes (1991)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1991 (1991), S. 699-701 
    Details
    ISSN: 0170-2041
    Keywords: Cephems, 2-methylene-, substituent effects ; Methylenecephems ; 1,3-Dipolar cycloaddition ; Diazoalkanes ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 2-methylenecephalosporins 1 undergo stereoselective 1,3-dipolar cycloaddition reactions with diazoalkanes to give exclusively the spirocyclopropylcephalosporins 3. Substituens at C-3, C-4 and C-7 of the cephalosporin nucleus do not display a significant effect on the reactivity of the 2-exo double bond. However, reduction of the sulfoxides to sulfides has an inactivating influence. Reactions with substituted diazoalkanes proceed more slowly in every case. The results are explained by theoretical (PM3, AM1 and MNDO) calculations.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.1991199101123
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  • 2
    Electronic Resource
    Electronic Resource
    Conformation energy around the N(sp3)—O single bond (1994)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 15 (1994), S. 1091-1104 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A detailed conformational analysis was performed on simple substituted hydroxylamines using either ab initio (from HF/6-31G* to RQCISD/6-311G**) or popular semiempirical (MNDO, AM1, PM3) methods to ascertain the allowed conformations and to establish the influence of the level of theory on the results. All the ab initio results (provision being made for their expected divergences) are similar and show a simple twofold character for the 〉 N—O— rotational energy, without any appreciable populations of the cis conformer. On the other hand, the predictive value of the semiempirical methods for structural and energetical parameters of molecules bearing 〉 N—O— moieties is limited, a situation like that prevailing for peptide bonds. The inversional barriers for the methyl-substituted hydroxylamines were also calculated and compared to the corresponding rotational energy barriers. Rotation is generally favored over inversion for hydroxylamine and its methylated derivatives. © 1994 by John Wiley & Sons, Inc.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540151005
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    Paper (German National Licenses)
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