143 mg of methyl 4-[(3R,4S)-3-({(tert-butoxycarbonyl)[(1R)-1-(1-naphthyl)ethyl]amino}methyl)-4-(2-fluorophenyl)piperidin-1-yl]-3,5-difluorobenzoate was dissolved in 4.0 mL of THF-2.0 mL of methanol and added with 2.0 mL of a 1 M aqueous sodium hydroxide solution at room temperature. After stirring at room temperature for 3 days, it was neutralized by addition of 2.1 mL of 1 M hydrochloric acid. The reaction mixture was concentrated under reduced pressure, and then extracted with ethyl acetate, and the organic layer was dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated under reduced pressure, and the residue was purified by silica gel column chromatography (methanol-chloroform) to obtain 138 mg of 4-[3-({(tert-butoxycarbonyl)[(1R)-1-(1-naphthyl)ethyl]amino}methyl)-4-(2-fluorophenyl)piperidin-1-yl]-3,5-difluorobenzoic acid. ESI+: 619
