The 4-[(4-chlorophenyl)sulfonyl]-4-(2,5-difluorophenyl)piperidine hydrochloride (100 mg, 0.245 mmol) obtained in Example 260, triethylamine (0.070 ml, 0.50 mmol) and benzaldehyde (0.050 ml, 0.428 mmol) were added to dichloromethane (5 ml), followed by the addition of sodium triacetoxyborohydride (110 mg, 0.500 mmol) at room temperature. The resulting mixture was stirred at room temperature for 14 hours. A 1N aqueous sodium hydroxide solution (3.0 ml) was then added. The resulting mixture was stirred at room temperature for 30 minutes. After diethyl ether was added, the resulting mixture was washed with a 1N aqueous sodium hydroxide solution. The organic layer was dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated under reduced pressure. The residue thus obtained was subjected to flash silica gel chromatography, and the fraction obtained from the dichloromethane:methanol=50:1 eluate was concentrated under reduced pressure, whereby the title compound (87 mg, 0.19 mmol, 77%) was obtained as a white foam.
