Synthesis of the tricyclic core of labiatin A and australin A.

A concise synthesis of the tricyclic core of the marine diterpene natural products labiatin A and australin A has been accomplished. The key ring-forming transformation is a cascade reaction comprising generation of a copper carbenoid from a diazo ketone, intramolecular reaction of the carbenoid with a cyclic ether, and rearrangement of the resulting free oxonium ylide or its metal-bound equivalent with ring expansion of the original cyclic ether.