Longipinane derivatives from Stevia connata.
The hexane extracts of the roots of Stevia connata afforded three new longipinene derivatives, longipinane-7beta,8alpha, 9alpha-triol-1-one 7-angelate-8-methylbutyrate (1), longipin-2-ene-7beta,8alpha,9alpha-triol-1-one 8,9-diangelate (6), and longipin-2-ene-7beta,8alpha,9alpha-triol-1-one 8-angelate-9-methylbutyrate (8), together with the known longipinane-7beta,8alpha,9alpha-triol-1-one 8,9-diangelate (2), longipinane-7beta,8alpha,9alpha-triol-1-one 7,9-diangelate (3), longipinane-7beta,8alpha,9alpha-triol-1-one 7,8-diangelate (4), longipin-2-ene-7beta,8alpha,9alpha-triol-1-one 7,8-diangelate (5), longipin-2-ene-7beta,8alpha,9alpha-triol-1-one 7-angelate-8-methylbutyrate (12), and stigmasterol. The structures of the new compounds were determined by chemical transformations and spectral methods including 2D NMR measurements. Spontaneous intramolecular transesterifications starting from the 8-angelate-9-methylbutyrate 8 provided an equilibrated mixture of the 7-angelate-9-methylbutyrate 10, the 7-angelate-8-methylbutyrate 12 and the starting material when stored in MeOH-H(2)O solution, while the 8,9-diangelate 6 only provided a binary mixture of the 7, 9-diangelate 7 and the starting material under the same conditions. The structures of 6-8, 10, and 12 and those of the nonisolable reaction intermediates 9, 11, and 14 were further evaluated by AM1 semiempirical calculations.