To 1 l of 1,2-dichloroethane were added 56.8 g of titanium tetraisopropoxide, 168 g of D-(-)-diethyl tartarate, 0.91 ml of water and 80 g of 6-{3-[1-(2-methylphenyl)-2-imidazolylthio]propoxy}-3,4-dihydrocarbostyril. The mixture was stirred and then cooled to -5° C. Thereto was added 124 g of cumene hydroperoxide slowly. After the completion of the addition, the mixture was stirred at 5° C. for 18 hours. The reaction was repeated. The reaction mixtures from the two reactions were combined and purified by a silica gel column chromatography (eluant: dichloromethane/ethyl acetate/methanol=3/1/0→30/10/1), followed by recrystallization from ethanol-ethyl acetate. The mother liquor was concentrated to obtain crystals. This procedure was conducted 14 times. The resulting crystals were purified by a silica gel column chromatography (eluant: dichloromethane/ethyl acetate/methanol=30/10/1), followed by recrystallization from ethanol to obtain 2.7 g of (S)-(+)-6-{3-[1-(2-methylphenyl)-2-imidazolylsulfinyl]propoxy}-3,4-dihydrocarbostyril. White powder.
