The crude product obtained as the residue was purified by silica gel chromatography on silica gel (developer solvent: a 3:1 mixture of chloroform and ethyl acetate), whereby the compound of the formula (IV) shown in the above reaction scheme B where R' was an ethyl group, namely, (2S)-3-ethoxycarbonyl-2-pentylpropionyl-L-valyl-L-prolinol (73 mg) was obtained.