In dichloromethane (10 ml), 1-[(6-chloronaphthalen-2-yl)sulfonyl]-4-[4-(pyridin-4-yl)benzoyl]piperazine (300 mg) obtained in Example A-1 was dissolved, followed by the addition of 3-chloroperbenzoic acid (382 g) at −20° C. The resulting mixture was stirred at −20° C. for 21 hours. An aqueous solution of sodium sulfite was added to decompose an excess peroxide. Dichloromethane and a saturated aqueous solution of sodium bicarbonate were added to separate an organic layer. After drying the organic layer over anhydrous magnesium sulfate, the residue obtained by distilling off the solvent was purified by chromatography on a silica gel column (2-5% methanol—dichloromethane). After the solvent was distilled off, ether was added to the residue to solidify it, followed by collection through filtration, whereby the title compound (200 mg, 63%) was obtained as a colorless solid.
