In a mixture of 3 ml of ethanol and 3 ml of tetrahydrofuran is dissolved 0.28 g of isopentyl 2-[5-[(1,3-diisopentyl-2-oxo-2,3-dihydro-1H-benzimidazol-5-yl)carbonyl]-2-(isopentyloxy)phenyl]acetate. After adding 1.4 ml of 1 mol/L sodium hydroxide solution, the mixture is stirred at ambient temperature for 3 hours. Water and chloroform are added to the reaction mixture, pH is adjusted to 2 with 2 mol/L hydrochloric acid, and the organic layer is separated. The organic layer thus obtained is washed with water and saturated aqueous solution of sodium chloride successively and dried over anhydrous magnesium sulfate, and the solvent is distilled off under reduced pressure. Thus, 0.21 g of 2-[5-[(1,3-diisopentyl-2-oxo-2,3-dihydro-1H-benzimidazol-5-yl)carbonyl]-2-(isopentyloxy)phenyl]-acetic acid is obtained as a white foaming product.
