1.8 Grams of 6-{3-[4-(3-methylphenyl)-1-piperazinyl]-2-propenyl}-3,4-dihydrocarbostyril and 0.25 g of sodium hydride (50% in mineral oil) were mixed into 60 ml of dimethylformamide and the mixture was stirred at a room temperature for 2 hours then 0.7 g of benzyl chloride was added to the reaction mixture and stirred at a room temperature for 8 hours. The reaction mixture was poured into 150 ml of saturated sodium chloride aqueous solution and was extracted with chloroform, the chloroform layer was washed with water, dried and chloroform was removed by distillation. The residue thus obtained was purified by a silica-gel column chromatography and an oily substance thus obtained was dissolved in 20 ml of acetone, then under stirring condition, the pH of the solution was adjusted to pH 3 to 4 by adding 5%-oxalic acid acetone solution to obtain precipitates. The precipitates were collected by filtration and washed with acetone then recrystallized from ethanol to obtain 2.2 g of 1-benzyl-6-{3-[4-(3-methylphenyl)-1-piperazinyl]-2-propenyl}-3,4-dihydrocarbostyril monooxalate in colorless needle-like crystals, having the melting point of 176°-178° C.
