Synthetic studies toward the zoanthamine alkaloids: synthesis of the fully functionalized BC ring motif

A synthetic approach toward lactone 20, representing the fully functionalized BC ring motif of the zoanthamine alkaloids, has been developed and is presented herein. The challenging C9 quaternary center of 20 was installed by a Wilkinson hydrogenation of enone 17 followed by construction of an α-bromo acetal and intramolecular cyclization exclusively from the α face of the BC ring system.