In 12 ml of ethanol is dissolved 1.20 g of isopentyl 2-[5-(3,4-diisopentyloxybenzoyl)-2-isopentyloxyphenyl]acetate. After adding 0.6 ml of 5 mol/L aqueous solution of sodium hydroxide, the mixture is stirred at 50° C. for one hour. Chloroform and water are added to the reaction mixture, pH is adjusted to 2 with 6 mol/L hydrochloric acid, and the organic layer is separated. The organic layer is washed with water and saturated aqueous solution of sodium chloride successively, and the solvent is distilled off from the reaction mixture under reduced pressure. Thus, 0.88 g of 2-[5-(3,4-diisopentyloxybenzoyl)-2-isopentyloxyphenyl]acetic acid is obtained as a white solid product.
