1.9 Grams of 6-carboxy-3,4-dihydrocarbostyril was suspended in 200 ml of methylene chloride, then 2 ml of pyridine was added to the suspension and under stirring 1.4 g of thionyl chloride was added dropwise in keeping the inside temperature at 0°-20° C. After the addition of thionyl chloride, the reaction mixture was kept at the same temperature and stirred for 1 hour, then 1.74 g of benzylpiperazine in 10 ml of methylene chloride solution was added to the mixture. Then the reaction mixture was further stirred at a room temperature for 4 hours. The reaction mixture was washed thoroughly with an aqueous solution of potassium carbonate, then washed with water and a diluted hydrochloric acid, dried with anhydrous sodium sulfate and the solvent was removed by distillation. The residue thus obtained was treated by a silica gel column chromatography (Silica gel: Wako C-200, eluate:chloroform:methanol (volume/volume)=20:1). The objective product was recrystallized from ethanol to obtain 325 mg of 6-(4-benzyl-1-piperazinylcarbonyl)-3,4-dihydrocarbostyril. Colorless needle-like crystals. Melting point: 198°-200° C.
