Conjugate Addition−Dipolar Cycloaddition Cascade for the Synthesis of Benzo[<i>a</i>]quinolizine and Indolo[<i>a</i>]quinolizine Scaffolds: Application to the Total Synthesis of (±)-Yohimbenone

A highly efficient total synthesis of (±)-yohimbenone and a formal synthesis of (±)-emetine is described. The key element of the synthesis consists of a conjugate addition−dipolar cycloaddition of 2,3-bis(phenylsulfonyl)-1,3-butadiene with an appropriate oxime. The resulting cycloadducts are cleaved reductively to provide azapolycyclic scaffolds with strategically placed functionality for further manipulation to the target compounds.