Dibenzylphosphoryl-L-alanyl-L-proline (1.2 g, 2.5 m mole), HOBt (338 mg, 2.5 m mole) and L-tyrosine ethylester hydrochloride (737 mg, 3 m mole) was suspended in DMF (10 ml), and WSC (0.5 ml) was added gradually thereto while cooling to -15° C. and stirring. The reaction was carried out for 3 hours under cooling and then overnight at room temperature. To the reaction solution ethyl acetate (100 ml) was added, and the mixture was washed with 1N hydrochloric acid and water in order, and dried with anhydrous sodium sulfate. The solvent was removed by distillation under reduced pressure to obtain an oily dibenzylphosphoryl-L-alanyl-L-prolyl-L-tyrosine ethylester (1.4 g). This oily material gave a single spot with Rf 0.50 by thin layer chromatography (developing solvent; chloroform:methanol:acetic acid=95:5:3, colour forming method: spraying of 25% hydrobromic acid and 0.1% ninhydrine, and heating).
