Unified Asymmetric Total Syntheses of (-)-AlotaketalsA-D and (-)-PhorbaketalA

The novel tricyclic spiroketal alotane-type sesterterpenoids showed strikingly different biological activities and potency with subtle structural alterations. Asymmetric total syntheses of the tricyclic sesterterpenoids (-)-alotaketalsA-D and (-)-phorbaketalA were accomplished [29-31 steps from (-)-malic acid] in a collective way for the first time. The key features of the strategy included 1)a new cascade cyclization of vinyl epoxy [delta]-keto-alcohols to forge the common tricyclic spiroketal intermediate, 2)a late-stage allylic C-H oxidation, and 3)olefin cross-metathesis to install the different side chains.