Ti(III)-promoted radical cyclization of epoxy enones. Total synthesis of (+)-paeonisuffrone.

The total synthesis of (+)-paeonisuffrone starting from (+)-10-hydroxycarvone is described. The key step of our synthetic strategy is a titanium-catalyzed stereoselective cyclization initiated by epoxide opening through electron transfer. This reaction stereoselectively affords the highly oxygenated pinane skeleton present in the target molecule and opens a new and effective approach to the synthesis of the complex, biologically active terpenoids isolated from the roots of the Chinese paeony (Paeonia albiflora Pallas and Paeonia suffruticosa Andrews).