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|Type:||Artigo de periódico|
|Title:||Pyrrolizidine alkaloids necine bases: II. Conformational analysis of free bases|
|Abstract:||Molecular mechanics calculations were applied to the conformational analysis of two diasteroisomers, the pyrrolizidine alkaloids (PAs) retronecine and heliotridine. The application of reoptimized parameters for H bonding corrected the tendency of MM3(92) calculations to give unrealistic H ... O distances for intramolecular OH interactions occurring in both diasterisomers. Inversions in the H-bond direction of exo-retronecine and in the relative stability of heliotridine endo-exo conformers were also observed with the application of the new parameters. A set of probable conformers was obtained for each diasterisomer, based on conformational and Boltzmann population analysis. Only exo-puckered conformers were found in the retronecine set, whereas both exo- and endo-puckered conformers were obtained for heliotridine. Transition state conformations supplied arguments supporting the design of models for H-bond interconversion in the case of exo-retronecine and for the exo-endo interconversion of heliotridine. Reactivity behaviors and H-1-NMR data of both diasterisomers were elucidated in light of the theoretical results. (C) 1998 John Wiley & Sons, Inc.|
|Editor:||John Wiley & Sons Inc|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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