Anionic Polycyclization Entry to Tricycles Related to Quassinoids and Terpenoids: A Stereocontrolled Total Synthesis of (+)-Cassaine

A full account of our anionic polycyclization approach to access highly functionalized tricycles related to quassinoids and terpenoids from several optically active bicyclic enone systems and Nazarov reagents is presented. (+)-Carvone is the only chiral source used to fix the entire stereochemistry of all of the tricycles, and the stereochemical outcome of this process was unambiguously determined by X-ray crystallographic analysis. The utility of this strategy was demonstrated by the stereocontrolled construction of advanced tricycles related to the highly potent anticancer natural product bruceantin, a member of quassinoid family, and the total synthesis of the cardioactive terpenoid (+)-cassaine, a nonsteroidal inhibitor of Na⁺-K⁺-ATPase.