Brønsted acid catalyzed cascade reactions of 2-[(2-aminophenyl)ethynyl]phenylamine derivatives with aldehydes: a new approach to the synthesis of 2,2'-disubstituted 1H,1'H-3,3'-biindoles.

An unusual Brønsted acid catalyzed cascade reaction of 2-[(2-aminophenyl)ethynyl]phenylamine derivatives with aryl(heteroaryl)aldehydes to afford an efficient alternative entry into 2,2'-disubstituted-1H,1'H-3,3'-biindoles under metal-free conditions is reported.