2.1 g (5.5 mmol) of O-(benzotriazol-1l-yl)-N,N,N′, N′-tetramethyluronium hexafluorophosphate (HBTU) are added portionwise with stirring, at 0° C. under nitrogen, to a mixture of 2.0 g of (S)-6-amino-α-[[(1,1-dimethylethoxy)carbonyl]amino]pyridine-3-pentanoic acid (5.5 mmol), 1.14 g (7.5 mmol) of 4-ethylpiperidine hydrochloride, N,N-diisopropylethylamine (DIEA) (25 ml; 14 mmol) in dichloromethane (30 ml) and 3 ml of anhydrous dimethylformamide (DMF). The mixture is allowed to warm to room temperature and stirring is continued for 18 hours. The reaction mixture is taken up in 250 ml of ethyl acetate and is washed with 50 ml of 0.1 N hydrochloric acid solution and with 50 ml of saturated sodium hydrogen carbonate solution and with 50 ml of saturated sodium chloride solution. The product obtained is then dried over sodium sulphate and then filtered and concentrated under reduced pressure. The residue obtained is purified by chromatography on a column of silica gel, eluting under pressure with ethyl acetate. 1.48 g of 1,1-dimethylethyl (S)-[4-(6-aminopyrid-3-yl)-1-([4-ethylpiperid-3-yl)carbonyl]butyl]carbamate are obtained in the form of a viscous oil.
