Slowly add 0.6 mmol of dicyclohexylcarbodiimide to a solution, at 0° C., of 0.5 mmol of the compound of Example 3 and 0.5 mmol of 2-[(tert-butoxycarbonyl)amino]acetic acid in 10 ml of dimethylformamide. The reaction mixture is maintained at 0° C. for 5 hours and then at ambient temperature for 16 hours. After filtration and evaporation under reduced pressure, the residue is dissolved in 2 ml of anhydrous pyridine, 2 ml of acetic anhydride are added, and stirring is carried out for 48 hours at ambient temperature in darkness. After concentration of the reaction mixture under reduced pressure, chromatography of the residue on silica gel (dichloromethane) allows the expected product to be isolated.
