To a solution of 5-[[(4-fluorophenyl)sulfonyl](2,3,6-trifluorophenyl)methyl]-4-methylpyridine-2-carboxamide (150 mg, 0.34 mmol) obtained in Example 39 in ethylene glycol dimethyl ether (3 ml), an aqueous solution of formaldehyde (37%, 0.1 ml) and 5% aqueous sodium hydroxide (0.4 ml) were added at 0° C., and the mixture was stirred overnight at room temperature. Sodium carbonate (20 mg) was added to the reaction mixture, and the mixture was stirred for 20 minutes at room temperature. The mixture was concentrated under reduced pressure, and then the residue was dissolved in chloroform, dried over magnesium sulfate, and filtered. Subsequently, the filtrate was concentrated under reduced pressure, and the resulting residue was subjected to silica gel chromatography. The fraction obtained from an elution with hexane:ethyl acetate=1:1 was concentrated under reduced pressure, and the resulting residue was washed with diethyl ether, and then was collected by filtration, to obtain the title compound (93 mg, 0.20 mmol, 58%) as a white solid.
