Crude crystals (1.26 g) of 2-acetyl-1,2,3,4-tetrahydroisoquinoline-7-thiol were dissolved in dimethylformamide (15 ml) and thereto were added 60% sodium hydride (243 mg) and 4-ethoxycarbonyl-4-(2-iodoethyl)piperidine-1-carboxylic acid tert-butyl ester (1 g), and the mixture was stirred at 70° C. for 1 hour. After completion of the reaction, water was added and the mixture was extracted with ethyl acetate and washed with saturated brine. The organic layer was dried over anhydrous magnesium sulfate and the solvent was evaporated. The obtained residue was purified by silica gel column chromatography (hexane:ethyl acetate=3:7) and dried under reduced pressure to give the title compound (535 mg).
