A solution, cooled to -20° C., of 32.3 g of Boc-Asp-(OBzl)-OH in 150 ml of DMF is treated with 11 ml of N-methylmorpholine and 13.07 ml of isobutyl chloroformate. The suspension obtained is stirred at -15° C. and treated with a suspension, cooled to -20° C., of 37.95 g of H-Val-OBzl.tosylate and 11 ml of N-methylmorpholine in 150 ml of DMF. The reaction mixture is stirred at below -10° C. for 10 minutes and at room temperature for 2 hours, filtered and the filtrate is concentrated. The residue is dissolved in ethyl acetate and washed with 5% KHSO4 /10% K2SO4 solution, water, saturated NaHCO3 solution, water and saturated NaCl solution and dried. The organic phase is concentrated. 52.9 g of Boc-Asp(OBzl)-Val-OBzl are obtained. 10.24 g thereof are dissolved in 30 ml of TFA and then concentrated. The residue, is crystallized from ethyl acetate/hexane. There are obtained 8.3 g of H-Asp- (OBzl)-Val-OBzl.TFA (1:1),.m.p. 147°-148° C.
