Rearrangement of o-nitrobenzaldehyde in the Hantzsch reaction

Abstract

The reaction of 5-hydroxy-2-nitrobenzaldehyde with ethyl acetoacetate in ammonia gave the two expected isomeric 1,4- and 1,2-dihydropyridines resulting from the normal Hantzsch reaction. However, the combination of 2-nitrobenzaldehyde with ethyl acetoacetate under the same conditions yielded four products: the two normal isomeric dihydropyridines and two tricyclic compounds. When we attempted to independently synthesize the two tricyclic compounds by reductive cyclization of 4-(2-nitrophenyl)-2,6-dimethyl-3,5- dicarbetoxy-1,4-dihydropyridine and 2-(2-nitrophenyl)-4,6-dimethyl-3,5- dicarbetoxy-1,2-dihydropyridine with tin (II) chloride in hydrochloric acid media, we obtained instead an indole and a quinoline derivative, respectively.