Total Syntheses of Sesterterpenoid Ansellones A and B, and Phorbadione.

Ansellane-type sesterterpenoids including, ansellones A-G and (+)-phorbadione are structurally novel marine secondary metabolites which exhibit anticancer and anti-HIV activity. The first, asymmetric total syntheses of three structurally representative members, (-)-ansellones A and B and (+)-phorbadione, were accomplished in 16-23 steps from (+)-sclareolide. The route features the first regioselective cyclization of vinyl epoxides with internal alcohol nucleophiles in a 1,4-addition manner (SN 2'). Additionally, the allylic C-H oxidation was exploited at a late stage of the synthesis of (-)-ansellone A and (+)-phorbadione. This strategy is expected to be applicable to the synthesis of other ansellane sesterterpenoids.