The 4-[(4-chlorophenyl)sulfonyl]-4-(2,5-difluorophenyl)piperidine hydrochloride (90 mg, 0.22 mmol) obtained in Example 260 and triethylamine (0.092 ml, 0.66 mmol) were dissolved in dichloromethane (4 ml), followed by the addition of acetyl chloride (0.024 ml, 0.34 mmol) at room temperature. After stirring at room temperature for 14 hours, a saturated aqueous solution of sodium bicarbonate (0.5 ml) was added. The residue obtained by concentrating the resulting mixture under reduced pressure was subjected to flash silica gel chromatography, and the fraction obtained from the dichloromethane:methanol=30:1 eluate was concentrated under reduced pressure to give a white solid. The solid thus obtained was washed with hexane, whereby the title compound (48 mg, 0.12 mmol, 53%) was obtained as a white powder.
