Regioselective Synthesis of Polysubstituted Carbazoles from Indoles and Simple 1,4-Dicarbonyl Compounds

Polysubstituted carbazoles were efficiently synthesized through direct benzannulation reaction between 1,4-dicarbonyl compounds and indoles with catalytic amount of inexpensive zirconium(IV) chloride. This transformation proved to be regioselective and furnishes 1,4-disubstituted and 1,2,4-trisubstituted carbazoles with yields ranging from 26% to 91% and broad substrate scope. Moreover, this protocol benefits from using readily accessible starting materials without the need of their pre-functionalization. The synthetic utility of the products was exemplified by functionalization of an iodocarbazole by means of Suzuki-Miyaura reactions and by the synthesis of a 3-deaza-derivative of the natural product canthin-6-one.