A solution of 1-[4-amino-1-(4-pyridyl)-4-piperidylmethyl]-4-(6-chloronaphthalene-2-sulfonyl)-2-piperazinone dihydrochloride (235 mg), O-propoxycarbonyl-4-nitrophenol (270 mg) and N-ethyldiisopropylamine (0.348 ml) in DMF (5 ml) was stirred at 80° C. overnight. To the reaction solution was added ethyl acetate, the mixture was washed successively with a 10% aqueous sodium carbonate solution, an aqueous saturated sodium bicarbonate solution and an aqueous saturated sodium chloride solution, dried, and concentrated. The residue was purified by silica gel column chromatography (ethyl acetate:10% aqueous ammonia-containing methanol=90:10→80:20), and crystallized from a mixed solution of acetone and ethyl acetate to obtain the pale brown crystalline title compound (114 mg).
