Oxidation of 1-substituted 2-aminoimidazo[1,5-a]pyridinium salts with an excess of concentrated nitric acid gave the corresponding 2-pyridylmethylenecarbamic acid hydro-salts. Oxidation of 3-substituted -2-aminoimidazo[1,5-a]pyridinium bromide with nitric acid in acetic acid gave the corresponding 1-nitro-3-substituted imidazo[1,5-a]-pyridines. The parent 2-aminoimidazo[1,5-a] pyridinium bromide yielded either type of product depending upon the reaction conditions.Oxidation of 2-amino-1-phenylimidazo[1,5-a]pyridinium bromide with lead(IV) acetate in acetic acid solution gave 2-acetamido-3-oxo-1-phenyl-2,3-dihydroimidazo[1,5-a] pyridine.
Imidazo[1,5-a]pyridines in acetic acid solution are readily nitrated by nitric acid–sulphuric acid, although in some instances treatment of the base hydrogensulphate with nitric acid–acetic acid is preferred. Nitration occurs most readily in the 1-position, but 3-nitration occurs if the 1-position is already substituted.
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