Metal ions and complexes in organic reactions. Part XVIII. Structural variations in the production of polycyclic heterocyclic systems by iron(II)-promoted cyclisations of nitro-substituted precursors

Abstract

Syntheses are described involving intermediates with a nitro-substituent ortho to the internuclear linkage in structures X–Y or X–NH–Y, where X = phenyl and Y = thienyl, pyrazolyl, imidazolyl, or pyrimidinyl. Heating these compounds with iron(II) oxalate at ca. 280° gave chiefly the corresponding primary amines (up to ca. 65%), sometimes with the corresponding azo-compound. Competitive cyclisation, through the nitro-group, gave compounds (5–20%) in the following polycyclic series: thieno[2,3-b]quinoxaline (3), imidazo[1,2-a]quinoxaline (7), benzimidazo[1,2-a]quinoxaline (9), and benzo[g]pteridine (11); a 2-methyl substituent on an imidazole nucleus contributed to the central pyrazine ring in structures (7) and (9). o-Nitrodiphenylmethane gave o-aminobenzophenone and acridone. Nitro(pyridylamino)pyridines were synthesised as potential sources of dipyridopyrazines but they readily underwent fission to the corresponding aminonitropyridines.