To a solution of 5-[(2,5-difluorophenyl)[(4-fluorophenyl)thio]methyl]-4-meth ylpyridine-2-carboxylic acid (307 mg, 0.79 mmol) obtained in Example 20 in methylene chloride (8 ml), dimethylamine hydrochloride (130 mg, 1.58 mmol), 1-hydroxybenzotriazole (213 mg, 1.58 mmol), 4-methylmorpholine (0.347 ml, 3.15 mmol), and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (302 mg, 1.58 mmol) were added at room temperature. After stirring for 17 hours at room temperature, the reaction mixture was washed with saturated aqueous sodium hydrogencarbonate. The organic layer was dried over anhydrous sodium sulfate and filtered, then the filtrate was concentrated under reduced pressure, and the resulting residue was subjected to flash silica gel chromatography. The fraction obtained from an elution with hexane:ethyl acetate=1:1 was concentrated under reduced pressure. To a solution the resulting residue in methylene chloride (5 ml), 3-chloroperbenzoic acid (112 mg, 0.42 mmol) was added at 0° C. After stirring for 1 hour at room temperature, the reaction mixture was washed with 1 N aqueous sodium hydroxide. The organic layer was dried over anhydrous sodium sulfate and filtered, then the filtrate was concentrated under reduced pressure, and the resulting residue was subjected to preparative thin-layer chromatography (developed with hexane:ethyl acetate=1:2, eluted with methylene chloride:methanol=4:1). The obtained fraction was concentrated under reduced pressure, and the resulting residue was washed with a mixed solvent of ethanol and hexane, and was collected by filtration, to obtain the title compound (78 mg, 0.18 mmol, 23%) as a white solid.
