To a solution of 1-(tert-butoxycarbonyl)-4-methanesulfonylpiperazine (1.32 g) in tetrahydrofurane (10 ml) was added dropwise at 0° C. 1.6 M n-butyllithium in hexane (6.25 ml), and the mixture was stirred at room temperature for 30 minutes. The reaction solution was cooled to −78° C., and to the mixture was added a solution of methyl 4-chlorobenzoate (853 mg) in tetrahydrofurane (5 ml). The mixture was stirred for 3 hours at the same temperature and raised to 0° C. To the reaction solution was added water (10 ml), and the mixture was made acidic with citric acid aqueous solution and extracted with ether. The organic layer was washed with brine, dried, concentrated, and purified with silica gel column chromatography (hexane/ethyl acetate=2/1) to give a colorless solid of the title compound (790 mg).
