A Homodimer of Withaferin A Formed by Base-Promoted Elimination of Acetic Acid from 27--Acetylwithaferin A Followed by a Diels-Alder Reaction.

Treatment of 27--acetylwithaferin A () with the non-nucleophilic base, 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), afforded 5β,6β-epoxy-4β-hydroxy-1-oxo-witha-2(3),23(24),25(27)-trienolide () and , a homodimer of withaferin A resulting from a Diels-Alder [4 + 2] type cycloaddition of the intermediate α,β-dimethylene-δ-lactone (). Structures of and were elucidated using HRMS and 1D and 2D NMR spectroscopic data. The structure of was also confirmed by single crystal X-ray crystallographic analysis of its -4---nitrobenzoate (). Formation of withaferin A homodimer () as the major product suggests regio- and stereoselectivity of the Diels-Alder [4 + 2] cycloaddition reaction of . Acetylation of - afforded their acetyl derivatives -, respectively. Compounds - and - were evaluated for their cytotoxic activities against four prostate cancer (PC) cell lines (LNCaP, 22Rv1, DU-145, and PC-3) and normal human foreskin fibroblast (HFF) cells. Significantly, exhibited improved activity compared to the other compounds for most of the tested cell lines.