| 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(DOTTED BOND)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 1H-NMR data of both diasterisomers were elucidated in light of the theoretical results. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1853–1861, 1998 |
