Asymmetric Total Synthesis of Shagenes A and B.

We report the first total synthesis of shagenes A and B, which are tricyclic terpenoids containing a cis-substituted cyclopropane, via ring-closing metathesis of an enamide and Ir-catalyzed double-bond isomerization of an alkylidenecyclopropane. Chemo- and diastereoselectivity in the distorted cis-substituted structures were controlled by the alkylidenecyclopropane reactivity and using the ketone functionality as a remote directing group for the Ir catalyst, respectively. The total synthesis suggested the absolute configuration of shagenes.