An improved synthesis of 6H-pyrido[4,3-b]carbazole derivatives

Abstract

During the acid-catalysed cyclization of 5-bromo- and 5-amino-2-{1-[3-(1-methoxyethyl)-4-pyridyl]ethylidene}-indolin-3-ones to the corresponding 6H-pyrido[4,3-b]carbazoles (ellipticines), loss of the substituent on the benzenoid ring severely reduces the product yield. The mechanism of this reaction is discussed, and the problem has been solved by modification of the side chain at position 3 of the pyridyl group. An oxidative mode of ring closure is employed and ellipticine and 9-acetamidoellipticine have thus been prepared. In general, product yields compare favourably with other routes to the 6H-pyrido[4,3-b]carbazole system. 9-Phenylellipticine has also been prepared and some circumstantial evidence for the mode of action of ellipticines in neoplastic systems is presented.