2.13 Grams of Z-Ser-NHNH2 was dissolved in 20 ml of DMF and 4.20 ml of 6N-hydrochloric acid/dioxane was further added, then the mixture was cooled to -15° C., 1.13 ml of isoamyl nitrite was added under stirring. After the reaction mixture shows negative reaction in hydrazid test, then a cold solution of 3.53 ml of triethylamine with 1.20 ml of DMF was added dropwise to neutralize the reaction mixture. This mixture containing the azide was added to a cold DMF solution of H-Lys(Tos)-Glu-OH obtained in above-mentioned step 2(a) with 1.96 ml of triethylamine, then this mixture was stirred at -10° to -15° C. for 2 hours and at 4° C. for 20 hours. Dimethylformamide was removed by distillation, the residue thus obtained was extracted with ethyl acetate, then the ethyl acetate layer was washed with 1N-citric acid and a saturated aqueous solution of sodium chloride, then dried with anhydrous sodium sulfate, and ethyl acetate was removed by distillation under a reduced pressure. The residue thus obtained was solidified by adding ethyl ether and reprecipitated from ethyl acetate-ether to obtain 4.14 g of the desired product.
