0.55 ml (3.96 mmol) of triethylamine and 1.07 ml of tert-butanol, followed by 0.31 ml (1.40 mmol) of diphenylphosphoryl azide, are added to 430 mg (1.16 mmol) of 3-[3-(methoxycarbonyl)-4-nitrobenzoyl]imidazo[1,5-a]pyridine-6-carboxylic acid in 20 ml of toluene. The reaction medium is heated at 110° C. for 3 hours and then cooled to ambient temperature. Water and a saturated aqueous sodium bicarbonate solution are added and then the extraction is carried out with ethyl acetate. The organic phase is separated by settling, washed with a saturated aqueous sodium chloride solution, then dried over sodium sulfate and concentrated under reduced pressure. The product is purified by column chromatography on silica gel, elution being carried out with a mixture of dichloromethane and methanol (98/2). 480 mg of an orange-colored solid are obtained. Melting point: 182° C.; 1H NMR (d6-DMSO): 1.55 (9H, s), 3.92 (3H, s), 7.45 (1H, d), 7.90 (1H, s), 7.99 (1H, d), 8.24 (1H, d), 8.67 (1H, d), 8.76 (1H, s), 9.89 (1H, s)
