Conjugate addition-dipolar cycloaddition cascade for the synthesis of benzo[a]quinolizine and indolo[a]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.