Into 100 ml of dimethylformamide were added 3.1 g of 1-benzyl-6-(4-carboxybutoxy)-3,4-dihydrocarbostyril and 1.7 ml of triethylamine. The outside of the reaction vessel containing the above mentioned mixture was ice-cooled and 1.4 ml of isobutyl chloroformate was added dropwise to the mixture under stirring condition. After the addition operation, stirring was continued for 30 minutes and 1.75 g of N-ethyl-N-(2-tetrahydropyranylmethyl)amine was added to the reaction mixture and further stirred at a room temperature for 3 hours. After the reaction was completed, the solvent was removed by distillation and the residue thus obtained was extracted with about 300 ml of chloroform and washed with a diluted NaHCO3 aqueous solution, water, a diluted hydrochloric acid and water in this order. The residue thus obtained was treated by silica-gel column chromatography (solvent used was a mixture of chloroform:methanol=20:1) and the elue thus obtained was concentrated to obtain 2.3 g of 1-benzyl-6-{4-[N-(2-tetrahydropyranylmethyl)-N-ethylaminocarbonyl]butoxy}-3,4-dihydrocarbostyril in the form of a colorless syrupy substance.
