In 3 ml of methylene chloride is dissolved 0.12 g of 2-(ethoxycarbonyl)-1-isobutyl-1H-indole-5-carboxylic acid, to which are successively added 50 μl of oxalyl chloride and 20 μl of N,N-dimethylformamide at ambient temperature. The mixture is stirred at ambient temperature for one hour. Then, at 5-10° C., 0.12 g of aluminum chloride and 0.13 g of 1,3-diisobutoxybenzene are successively added and stirred at the same temperature as above for 30 minutes. The reaction mixture is added to a mixture of chloroform and ice water, 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. The residue thus obtained is purified by silica gel column chromatography (eluent; hexane:ethyl acetate=1:1) to obtain 0.18 g of ethyl 5-(2,4-diisobutoxybenzoyl)-1-isobutyl-1H-indole-2-carboxylate as a colorless oily product.
