Stereocontrolled Construction of ABCD Tetracyclic Ring System with Vicinal All-Carbon Quaternary Stereogenic Centers of Calyciphylline A Type Alkaloids.

A unique approach to the synthesis of an ABCD tetracyclic core bearing the vicinal all-carbon quaternary stereogenic centers of the calyciphylline A type alkaloids is reported. The synthesis features two C-C bond formations at a sterically congested position: one is an intramolecular pinacol coupling to construct the central A ring; the other is a semipinacol rearrangement to construct a quaternary stereogenic center adjacent to another quaternary center.