The epimerization of sesamin and asarinin.

Sesamin (1) and asarinin (2) are two C-7' epimeric lignans. Molecular modeling by semiempirical methods indicated that 1 is more stable than 2 by about 2.5 kcal/mol. However, epimerization under acidic conditions led to a 44.8/55.2 equilibrium ratio of 1 and 2. Single-crystal X-ray diffraction analyses indicated that 1 was monoclinic with a = 10.0435(19) A, b = 6.9151(8) A, c = 11.8460(13) A, and 2 was triclinic with a = 5.595(5) A, b = 9.5910(18) A, c = 15.620(4) A. The unexpected equilibrium ratio of 1 and 2 indicated that structural changes are dependent on the conditions of the extraction processes.