Pd-catalyzed enantioselective access to hydrocarbazolones containing contiguous quaternary and tertiary stereocenters

Abstract

The hydrocarbazole scaffold represents the core structure of numerous monoterpenoid indole alkaloids. The development of catalytic methods that provide efficient access to enantioenriched hydrocarbazole derivatives is central for the synthesis of these bioactive alkaloids. We report here a palladium-catalyzed enantioselective formal 5-endo arylative cyclization of enaminones, facilitating the construction of hexahydrocarbazol-4-ones containing contiguous C4a-quaternary and C9a-tertiary stereocenters with high enantioselectivities (86.5 : 13.5–99 : 1 er) and diastereoselectivities (>20 : 1 dr). Notably, enaminone substrates bearing an α-allyl group undertake an arylation/Cope rearrangement cascade, offering a unique route to C1-substituted tetrahydrocarbazol-4-ones. A stereodivergent approach to all four stereoisomers of the quaternary/tertiary chiral center set is achieved by combining the catalyst with Z/E allyl substituents, yielding excellent enantioselectivity. The N-methyl group of the hydrocarbazolone products is readily removed under oxidation conditions. The utility of the method is demonstrated by the access to a variety of hydrocarbazole derivatives and the efficient syntheses of four Aspidosperma alkaloids/analogs, (+)-N-methyl aspidospermidine, (+)-C20-epi-N-methyl aspidospermidine, (+)-N-methyl fendleridine, and (+)-N-methyl limaspermidine from a hexahydrocarbazol-4-one in 3–5 steps.