1 g (2.89·10-3 mol) of [3-[[(1,1-dimethyl-ethoxy)carbonyl]amino]propyl](4-hydroxybutyl)carbamic acid, 1,1-dimethylethyl ester is dissolved in 20 ml of tetrahydrofuran (THF), and 0.45 g (4.5·10-3 mol) of triethylamine is added followed by a solution of 0.58 g (2.89·10-3 mol) of 4-nitrophenyl chloroformate in 5 ml of THF. The reaction mixture is stirred for 15 hours at room temperature, a solution of 1.2 g (2.89·10-3 mol) of 13-amino-3-[[(1,1-dimethylethoxy)carbonyl]-amino]-12-oxo-2,4,11-triazatridec-2-enoic acid, 1,1-dimethylethyl ester in 6 ml of THF is then added and the reaction mixture is heated to 40° C. and stirred for 5 hours. After concentration of the reaction medium under reduced pressure, the residue is purified by medium pressure chromatography on silica gel using a methylcyclohexane/ethyl acetate mixture (7/3; v/v) and then pure ethyl acetate as the eluent to give 1 g of the expected product in the form of a transparent oil (yield=44%).
