Pyrrolizidine alkaloids necine bases: II. Conformational analysis of free bases

✍ Scribed by Giordan, Marcelo

• Book ID: 101220941
• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 335 KB
• Volume: 19
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/(sici)1096-987x(199812)19:16<1853::aid-jcc6>3.0.co;2-k

✦ Synopsis

Molecular mechanics calculations were applied to the Ž . conformational analysis of two diasteroisomers, the pyrrolizidine alkaloids PAs retronecine and heliotridine. The application of reoptimized parameters for H Ž . bonding corrected the tendency of MM3 92 calculations to give unrealistic H иии 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 1 H-NMR data of both diasterisomers were elucidated in light of the theoretical results.