Development of Organocatalytic Reactions for the Synthesis of Natural Products and Pharmaceuticals

This review describes novel organocatalytic methods for the enantioselective construction of spiroindans and spirochromans and the application of the methods to the total synthesis of natural products. We developed an intramolecular Friedel- Craftstype 1,4-addition in which the substrates were a resorcinol derivative and 2-cyclohexenone linked by an alkyl chain. The reaction proceeded smoothly in the presence of a cinchonidine-based primary amine (30 mol%) with water and p-bromophenol as additives. A variety of spiroindanes were obtained with high enantioselectivity under these conditions. The reaction was applied in the first total synthesis of the unusual proaporphine alkaloid (-)-misramine, which included the key steps of enantioselective spirocyclization and double reductive amination of the keto-aldehyde to form a piperidine ring toward the end of the synthesis. The total synthesis of misrametine was achieved by selective demethylation of the methoxy group from the precursor to misramine. Next, a method for highly enantioselective organocatalytic construction of spirochromans containing a tetrasubstituted stereocenter was developed. An intramolecular oxy-Michael addition was catalyzed by a bifunctional cinchona alkaloid thiourea catalyst. A variety of spirochroman compounds containing a tetrasubstituted stereocenter were obtained with excellent enantioselectivity of up to 99% enantiomeric excess. The reaction was applied to the asymmetric formal synthesis of (-)-(R)-cordiachromene.