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  • 1
    Electronic Resource
    Electronic Resource
    Anisochrony of the O-methylene protons of the C-2-ethyl ester function in triethyl meso, cis-1,3-dimethylcyclohexane-1,2,3-tricarboxylate - Atropisomerism of an ester group (1981)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 16 (1981), S. 224-229 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The methylene protons of the ethyl ester function at C-2 in triethyl meso, cis-1,3-dimethylcyclohexane-1,2,3-tricarboxylate (A) are anisochronous, despite the apparent symmetry of structure A. The necessary diastereotopy is thought to be engendered by the central ester being stabilized in a rotamerization in which the O:C·O plane is held parallel to the general plane of the 6-membered ring while fast exchanges take place among rotamers about the alkyl-oxygen bond; other rotamers about the ester axis are thought to encounter high order repulsive steric/polar interactions with the flanking ester functions and C-methyl groups. Evidence for the axial orientation of the C-1 and C-2 ester functions in A, based on comparisons of its 1H NMR spectral characteristics with those of its RS-cis, trans and meso, trans analogues, is presented. The role of ion-pairing in the stereoselectivity of methylation of precursor enolates, leading to the formation of these three systems, is briefly discussed. A comment on the appropriateness, or otherwise, of drawing conformational conclusions from the magnitudes of the anisochrony in comparable systems is included. Triester A is thought to be the first instance where atropisomerism about an sp3—sp2 bond involving an ordinary ester function has been detected employing a prochiral sensor group.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1270160312
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  • 2
    Electronic Resource
    Electronic Resource
    1H and 13C nuclear magnetic resonance study of 1,3-dipyridylthioureas for chemical shift assignments and conformational analysis (1987)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 25 (1987), S. 474-479 
    Details
    ISSN: 0749-1581
    Keywords: 1H NMR ; 13C NMR ; 1,3-Dipyridylthioureas ; Conformation ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A study of molecular conformation by 1H and 13C NMR methods of three 1,3-dipyridyl thioureas namely, 1,3-di(2-pyridyl)thiourea (1), 1,3-di(3-pyridyl)thiourea (2), 1-(2-pyridyl)-3-(3-pyridyl)thiourea (3), and also of 1-phenyl-3-(2-pyridyl)thiourea (4) and 1,3-diphenylthiourea (5), included for the sake of comparison, was carried out. Evidence was obtained that 3 and 4 exist in solution solely in one form, in an internally hydrogen bonded E,Z conformation, whereas 1, 2 and 5 exist in two (or more) rotamer forms. The data reveal an interesting dynamic exchange phenomenon occurring in 1 between two intramolecularly hydrogen bonded conformers. The 1H and 13C chemical shifts, 1H,1H and 13C,1H coupling constants are reported. The 13C and 1H chemical shifts are correlated with the electron densities calculated by the CNDO method.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260250603
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