In 2 ml of acetic acid was dissolved 100 mg of 2-amino-3,4-diethoxycarbonyl-5-nitro-6-phenylpyridine, and 1.5 ml of 42% hydrofluoroboric acid was added to the solution. The mixture was cooled to -5° C., followed by adding dropwise a solution of 21 mg (1.1 eq) of sodium nitrite dissolved in 0.5 ml of water. Thirty minutes later, the temperature of the mixture was elevated to room temperature and the mixture was neutralized with sodium hydrogencarbonate, followed by extraction with ethyl acetate. After the organic layer was washed with water and a saturated aqueous solution of sodium chloride, and dried. After the solvent was distilled off, the resulting crystals (2-pyridone compound) were collected by filtration with hexane, and washed. The solvent was distilled off from the obtained filtrate, to give an oily substance.
