Recyclization of 2-indolyl cyclopropyl ketones into carbazoles

Abstract

An acid-triggered cascade transformation of donor acceptor cyclopropanes, containing 2-indolyl fragment as a donor and acyl group as an acceptor, into carbazole derivatives was developed. The starting cyclopropanes, which were easily synthesized via aldol condensation / Corey-Chaykovsky reaction sequence in a gram scale, were readily converted into 4-substituted carbazoles by the action of both equimolar hydrochloric acid in methanol and catalytic HCl-dioxane in methylene chloride. The reaction mechanism involves small ring opening and common ring closure via Friedel-Crafts reaction as key steps, however, the current results do not allow for conclusion about the order of the proceeding steps in each case.