Cytotoxic withanolides from the aerial parts of Tubocapsicum anomalum.

Ten new withanolides (1-10) and three artificial withanolides (11-13) were isolated from the aerial parts of Tubocapsicum anomalum, together with five known analogues (14-18). Their structures were determined on the basis of extensive spectroscopic and chemical methods. They include seven acnistin-type (1-4, 11, 14 and 15), three withajardin-type (5-7), and eight normal-type (8-10, 12, 13 and 16-18) withanolides. Of normal-type withanolides, a chemical conversion from the 16α,17α-epoxywithanolide (16) to Δ13,14-16α-hydroxywithanolide (18) was achieved by Wagner-Meerwein rearrangement. All isolates were evaluated for their cytotoxicity against four human tumor cell lines (HCT-116, HepG2, MCF-7 and A375). Among them, compounds 1-3, 6-8, 14, 16-18 showed cytotoxic activity with IC50 values of 0.24-8.71 μM.