Recyclization of 2-indolyl cyclopropyl ketones into carbazoles.

An acid-triggered cascade transformation of donor-acceptor cyclopropanes, containing a 2-indolyl fragment as a donor and an acyl group as an acceptor, into carbazole derivatives was developed. The starting cyclopropanes, which were easily synthesized via an aldol condensation/Corey-Chaykovsky reaction sequence on 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 the Friedel-Crafts reaction as key steps; however, the current results do not allow conclusions to be drawn about the order of the proceeding steps in each case.