Cascade radical cyclization on 3-propargyl-2-alkenyl indole gives stereoselective access to cyclohepta[ b ]indole over carbazole.

A protecting group-dependent diastereoselective synthesis of cyclohepta[ b ]indole over carbazole derivatives is developed. This strategy involves a regioselective 6- exo-trig radical cyclization-cyclopropanation-ring expansion cascade of 3-propargyl-2-alkenyl indole. The cascade radical cyclization was also performed on indole derivatives possessing alkyne, acrylate and vinylogous carbamate moieties, which delivered pyridocarbazole giving credence to the mechanistic hypothesis. Furthermore, cyclohepta[ b ]indole could be selectively converted to sulfone and sulfoxide as well as benzoazulenoindole via intramolecular Friedel-Crafts acylation.