D-Lac(oAc)-L-Phe-D-GluoBzl (2) (1.0 g) was dissolved in methylene chloride (40 ml) and N-methylmorpholine (200 mg) was added thereto. To this stirred solution cooled at -10° C. in an ice salt-bath, isobutylchlorocarbonate (270 mg) was added and the mixture was allowed to react at the same temperature for 20 minutes. To the reaction mixture was added the silyl ester prepared from Boc-(D)-mesoDAP-(D)-NHNHBoc-(L)-GlyOH (1) (920 mg) in a mixture of methylene chloride (20 ml) and dimethylformamide (10 ml) containing bis(trimethylsilyl)acetamide (2 ml). The reaction mixture was stirred for 2 hours and concentrated under reduced pressure to give an oily residue which was dissolved in ethyl acetate and washed with 2% hydrochloric acid. The organic layer was washed successively with water and saturated sodium chloride and then dried. Evaporation of the solvent gave a residue which was throughly washed with ether and collected by filtration to give D-Lac(oAc)-L-Phe-γ-D-Glu(α-oBzl)-(L)-Boc-(D)-mesoDAP-(D)-NHNHBoc-(L)-GlyOH (3) (1.660 g).
