1-Acetyl-N-{3-[4-(4-aminobenzyl)-1-piperidinyl]propyl}-N-(3,4-dichlorophenyl}-4-piperidinecarboxamide (Example 303) (18 mg, 6.033 mmol) was dissolved in tetrahydrofuran (0.3 mL). To the solution were added triethylamine (5.0 mg, 0.050 mmol) and acetyl chloride (3.4 mg, 0.043 mmol), and the mixture was stirred at room temperature for 30 minutes. To the reaction mixture was added aqueous solution of 1N-sodium hydroxide (5 mL), and the mixture was extracted with ethyl acetate (5 mL×2). The organic layer was dried over magnesium sulfate and concentrated under reduced pressure. The concentrate was subjected to alumina column chromatography (alumina 2 g, ethyl acetate), and the desired fraction was concentrated under reduced pressure to give the titled compound (18 mg, 0.031 mmol, Yield 93%) as a colorless oily substance.
