To a solution of Z-Glu(OBzl)OH (5.62 g, 15 m mole) (Hayakawa et al. Bull. Chem. Soc. Japan, 39: 391 (1966)) in tetrahydrofuran (20 ml), N-methylmorpholin (1.53 ml) and isobutyl chloroformate (1.98 ml) were added with stirring at -15° C. To this mixture a solution of HOSu (N-hydroxy-succinimide) (3.45 g) in tetrahydrofuran (10 ml) was added with stirring at -15° C. The mixture was stirred at -15° C. for 30 min., and at 0° C. for 30 min. To this mixture a solution of H-Ser-OH (3.15 g, 30 mmole) in H2O (15 ml) containing triethylamine (4.2 ml) was added with stirring at 0° C. The mixture was stirred at 0° C. for 2 hr. and at room temperature for 15 hr. After removal of the organic solvent, the residue was extracted with three portions of ethyl acetate, which were washed successively with 1N citric acid and sat. NaCl solution, and dried over Na2SO4. After evaporating the residue was solidified by addition of ether. Reprecipitation from methanol-ether to yield (I) (5.29 g, 76.9%), RfI 0.74 RfII 0.70, mp 104°-105° C. [α]D27 +5.7 (C=1.12 in methanol).
