In 9 ml of chloroform, 300 mg of the optically active 5-[3-(3-methoxyphenyl]indolin-1-ylcarbonyl!-4,5,6,7-tetrahydrobenzimidazole obtained in Example 24 was solved. This solution and 3.1 ml of a 1.0M dichloromethane solution of boron tribromide added thereto in a stream of argon at -78° C. were together stirred at -78° C. for three hours and further at normal room temperature for three hours. The resultant reaction solution was poured into ice water, neutralized with an aqueous sodium hydrogen carbonate solution, and then extracted with a mixed solution of chloroform and ethanol. The organic layer was dried over magnesium sulfate and then distilled. The residue of the distillation was refined by silica gel column chromatography to obtain 220 mg of optically active 5-[3-(3-hydroxyphenyl)indolin-1-ylcarbonyl]-4,5,6,7-tetrahydrobenzimidazole (76.1%).
