A chemo-enzymatic approach to some indole and quinolizidine alkaloids from Cs-symmetric precursors

In this review we describe the asymmetrization of readily available C-S-symmetric compounds by enzyme-catalyzed reactions to provide chiral building blocks for the effective enantioselective synthesis of certain indole and quinolizidine alkaloids. By the asymmetrization of 1,2-disubstituted cycle cyclohex-4-enes a series of class I alkaloids, such as (-)-antirhine, (-)-akagerine, and (+)-meroquinene, have been synthesized from the relevant chiral precursors. By employing the same chemo-enzymatic approach, asymmetrization of C-S-symmetric 3,5-disubstituted piperidines provides access to 15,20-dihydrocleavamine and its analogues, as well as to (+)-tacamonine. The synthetic design and details of the various syntheses are presented. In addition, the scope and prospects of the symmetrization-asymmetrization strategy are discussed with special reference to the quinolizidine alkaloids.