Copper(II)-Catalyzed Indolizines Formation Followed by Dehydrogenative Functionalization Cascade to Synthesize 1-Bromoindolizines

A one-pot, three-component cascade reaction between pyridine, α-acylmethylbromide, and maleic anhydride leading to direct access of 1-bromoindolizines in high yields has been developed. This protocol is accomplished via a reaction sequence of 1,3-dipolar cycloaddition of the pyridinium ylide with maleic anhydride, oxidative decarboxylation of the primary cycloadduct, and dehydrogenative bromination of the resulting 1-unsubstituted indolizine. Copper chloride was used as a catalyst and oxygen as the terminal oxidant. This reaction represents the first example of transition-metal-catalyzed direct dehydrogenative bromination of indolizine at the C-1 position. Moreover, the obtained 1-bromoindolizines can be transformed to other 1-substituted indolizines such as 1-arylindolizines via a simple reaction process.