4-(((Tert-butoxy)carbonyl)amino)butanoic acid → {3-[3-(3-Ethylcarbamoyl-propylcarbamoyl)-propylcarbamoyl]-propyl}-carbamic acid tert-butyl ester

Boc-Protected derivative 4 (22.3 g, 70.7 mmol) was stirred in a mixture of MeOH (200 ml) and HCl (12%, 81 ml) for 48 h, concentrated to dryness in vacuo, dissolved in H2O, and washed 3 times with CHCl3. Water was removed on a rotary evaporator at 45° C. The resulting hydrochloride was dissolved in a mixture of CH2Cl2 (600 ml) and aq. Na2CO3 (20%, 100 ml), cooled on an ice bath, and Bt-derivative 2 (22.47 g, 70.15 mmol) was added. The cooling bath was removed and the mixture was stirred for 18 h with a mechanical stirrer. The main part of the CH2Cl2 was removed on a rotary evaporator, the reaction mixture was suspended in water and filtered. The product was triturated in EtOAc, filtered, washed with petr. ether and dried in vacuo. Yield 25.46 g (90%). mp: 170-171° C. 1H NMR (CDCl3/CD3OD): 1.14 (t, J=7.30, 3H), 1.44 (s, 9H), 1.72-1.85 (m, 6H), 2.10-2.23 (m, 6H), 3.06-3.12 (m, 2H), 3.18-3.26 (m, 6H), 5.57 (bs, 1H), 7.32 (bs, 1H), 7.46 (bs, 1H), 7.60 (bs, 1H). 13C NMR (CDCl3/CD3OD): 14.22, 24.73, 25.32, 25.39, 25.96, 28.15, 33.33, 33.59, 34.12, 34.25, 38.60, 39.64, 79.29, 156.68, 173.39, 173.88. ESI-MS: 423.4 (MNa+), 401.4 (MH+).