Generation of Fused Seven-Membered Polycyclic Systems via Ring Expansion and Application to the Total Synthesis of Sesquiterpenoids.

Seven-membered polycyclic architectures, widely present in natural products and molecular drugs, are challenging synthetic targets. However, methods for synthesizing fused medium-sized bicyclo[ m . n .0] ring systems, including the benzo-cycloheptane systems, are still urgent. Herein we describe a base-induced ring expansion as a general strategy to construct a wide range of fused seven-membered ring systems. The application of this method was demonstrated by the efficient total syntheses of two sesquiterpenoids, plecarpenene and plecarpenone, both bearing a fused bicyclo[5.3.0]decane skeleton.