Synthesis of Pyrrolo[1,2-b]isoquinolines via Gold(I)-Catalyzed Cyclization/Enyne Cycloisomerization/1,2-Migration Cascade

A gold(I)-catalyzed cascade transformation of N-alkynic 2-ynamides for the rapid and efficient synthesis of the indolizidine scaffold is developed. Through a sequential nucleophilic cyclization/enyne cycloisomerization/1,2-migration process, diverse pyrrolo[1,2-b]isoquinolines are obtained under mild conditions in a regiospecific and convergent manner. Various alkyl and aryl migrating groups are tolerated in this process. The electronic effect of the migrating group is comprehensively investigated. The study of the mechanism indicates that the pathway involving a gold carbenoid species is the main pathway and that the 1,2-migration of alkyl and aryl groups to the gold carbenoid occurs in an intramolecular fashion. This cascade reaction is also employed as the key step for the synthesis of a decumbenine B analogue.