To a solution of 1-(tert-butoxycarbonyl)-4-(4-chlorobenzoylmethanesulfonyl)piperazine (640 mg) and 2-chloro-1-methylpyridinium iodide (650 mg) in dichloromethane (8 ml) was added dropwise triethylamine (4 ml) at 0° C., and the mixture was stirred at room temperature for 24 hours. The reaction solution was concentrated and purified with silica gel column chromatography (hexane/ethyl acetate=2/1) to give a colorless solid of 1-(tert-butoxycarbonyl)-4-[2-(4-chlorophenyl)ethynesulfonyl]piperazine (521 mg).
