In a mixed solvent comprising 10 ml of MeOH and 5 ml of acetic acid is dissolved 2.2 g of BOC-Tyr-(D)-Met-Gly-OH, followed by adding 0.6 ml of 30% aqueous hydrogen peroxide solution, and the mixture is stirred at roomtemperature for 5 hours. Following the addition of a small amount of palladium black, the mixture is stirred. The catalyst is filtered out, andthe solvent is distilled off, whereby the precipitated crystals are collected by filtration. Thus obtained is 2.1 g of the desired product, m.p. 185° C. (decomposed), Rf1 =0.20, [α]D21 +1.8° (c=0.5, DMF)
