To a DMF solution (80 ml) containing 3-(2-aminopropyl)-7-methoxyindole (12.24 g) and 3-chloromandelic acid (7.46 g) is added benzotriazole-N-oxytris(dimethylamino)phosphonium hexafluorophosphate (17.68 g), and then thereto is added dropwise triethylamine (9.8 ml). The mixture is stirred at room temperature for five hours, and to the reaction mixture is added ethyl acetate. The mixture is washed successively with water, 10% aqueous citric acid solution, water, a saturated aqueous sodium hydrogen carbonate solution, water and a saturated aqueous sodium chloride solution, and dried over anhydrous magnesium sulfate. The mixture is evaporated to remove the solvent under reduced pressure, and the residue is purified by silica gel column chromatography (eluent; ethyl acetate/n-hexane=1:1→2:1) to give N-[3-(7-methoxyindol-3-yl)-2-propyl]-3-chloromandelamide (14.02 g) as an oily product.
