Applications of vinylogous Mannich reactions. Concise enantiospecific total syntheses of (+)-croomine

A novel method for the synthesis of (+)-croomine is devised to determine the factors that control the stereochemical course in the vinylogous Mannich reactions of substituted silyloxyfurans with cyclic iminium salts. Two concise enantiospecific asymmetric syntheses of (+)-croomine were performed using the method. Results show that the first reaction took only 11 chemical steps and the second reaction took 10 steps to give (+)-croomine.