Synthetic approaches to some aza-analogues of benzimidazole N-oxides. Part 1. The imidazo[4,5-b]pyridine series

Abstract

The 3-oxide of 2-p-nitrophenylimidazo[4,5-b]pyridine (8) may be obtained in good yield by baseinduced cyclisation of 2-nitro-3-(p-nitrobenzylamino)pyridine (18). It is also obtained in lower yield, along with p-nitrobenzoic acid and other cleavage products, by the corresponding reactions of bases with the N-ethoxycarbonyl, N-methylsulphonyl, and N-p-tolylsulphonyl derivatives of 2-nitro-3-(p-nitrobenzylamino)pyridine [(7), (16), and (15), respectively]. Cleavage is the main reaction when N-(2-nitro-3-pyridyl)-N-phenacylmethanesulphonamide (17) is treated with bases: phenylglyoxal, or its Schiff base with 3-amino-2-nitropyridine, are possible intermediates in the cleavage process. 3-Nitro-2(p-nitrobenzylamino)pyridine (31) is similarly cyclised in basic media to give the 1-oxide of 2-p-nitrophenylimidazo[4,5-b]pyridine (32).