Synthesis of imidazo[4,5-b]- and [4,5-c]pyridines

2-Arylimidazo[4,5-b]- and [4,5-c]pyridines have been prepared by treatment of the appropriate 2,3- or 3,4-diaminopyridine with an aromatic carboxylic acid in the presence of polyphosphoric acid. Other derivatives have been prepared by similar cyclisation of diaminopyridines using triethyl orthoformate, urea, thiophosgene and thiourea and the properties of some N-oxides have been investigated. A number of the arylimidazopyridines have been screened for mutagenicity.