Total synthesis of novel D-ring-modified triptolide analogues: structure–cytotoxic activity relationship studies on the D-ring of triptolide

The total synthesis of a trans-position butenolide analogue of triptolide 3 and the semi-synthesis of analogue 4, with a furan ring, and compound 5, without a planar D-ring, are described. Studies into the antitumor activity of these compounds suggest that the five-membered unsaturated lactone ring (D-ring) of triptolide is essential to its potent anticancer activity and the C18 carbonyl group may exert an important influence on the interaction between triptolide and the target molecule(s) responsible for initiating their cytotoxic effects.