Michael reaction of indoles with 3-(2′-nitrovinyl)indole under solvent-free conditions and in solution.: An efficient synthesis of 2,2-bis(indolyl)nitroethanes and studies on their reduction

Michael reaction of 3-(2'-nitrovinyl)indole with eight 3-unsubstituted indoles on TLC-grade silica gel furnished unsymmetrical bis(indolyl)nitroethanes in 7-12 min under microwave irradiation and in 8-14 h at rt. In contrast, the p- TsOH-catalysed reaction of the nitrovinylindole with the 3-unsubstituted and two 3-substituted indoles in solution under reflux furnished both unsymmetrical and symmetrical bis(indolyl)nitroethanes, the latter resulting from novel tandem Michael addition-elimination-Michael addition reactions. The synthesis of a 2',3"-bis(indolyl)nitroethane, the precursor core structure of two bioactive marine metabolites, and the reduction of 2,2-bis(3'-indolyl)nitroethane to the corresponding ethylamine, isolated as its N-acetyl derivative, have been achieved. Significantly, attempted hydrolysis of three nitronates, derived from the corresponding bis(indolyl)nitroethanes, with buffered aqueous TiCl3 has led to the first isolation of oximes (syn/anti-mixture) as the only products. (C) 2003 Elsevier Ltd. All rights reserved.