Toward an enantioselective total synthesis of sarain A: construction of an advanced intermediate and rearrangement of the sarain A core under mild conditions.

A high-yielding N-sulfonyliminium ion-enoxysilane cyclization and a ring-closing metathesis are key steps in the enantioselective synthesis of late-stage intermediates en route to sarain A. Also revealed is an unprecedented rearrangement of the tetracyclic sarain A core under mildly acidic conditions. [structure: see text]