To a suspension of 107 mg of ethyl 4-[[2-oxo-4-(piperidin-4-yl)piperazin-1-yl]acetyl]phenoxyacetate dihydrochloride synthesized according to the method described in WO962503 in 2 ml of N,N-dimethylformamide were added 110 mg of proton sponge and 85.7 mg of 1-acetoxyethyl 4-nitrophenyl carbonate with stirring at 0° C., and the mixture was stirred at room temperature for 18 hours. The reaction mixture was poured into 2 ml of water and extracted three times with 4 ml of ethyl acetate. The organic layer was dried over magnesium sulfate, and the solvent was removed by distillation. The residue thus obtained was purified by chromatography on an LH-20 column (chloroform:methanol=1:1) to give 119 mg of the title compound (100%).
