(R)-ethyl 6-acetamido-2-aminohexanoate 186 (1 equivalent, 120 mg, 0.55 mmol) was dissolved in anhydrous THF (4 mL) and trisphosgene (0.33 equivalent, 55 mg, 0.18 mmol) was added while stirring at −10° C. The mixture was allowed to warm to room temperature, then refluxed for 2 h. tert-butyl-N-[4-(aminomethyl)phenyl]carbamate (1 equivalent, 123 mg, 0.55 mmol) and triethylamine (2 equivalent, 154 μl, 1.1 mmol) were added successively and the resulting solution was stirring at room temperature overnight. The reaction mixture was concentrated, washed by 30 ml of NaHCO3 and extracted by 3×30 ml EtOAc. The organic phase was dried over Na2SO4, filtered and concentrated. The crude product was purified by flash chromatography (CH2Cl2/MeOH) to afford the urea protected (21 mg, 8%) Rf=0.18 (CH2Cl2/MeOH 95/5). 1H NMR (200 MHz, CDCl3) δ 7.30 (d, J=8.6 Hz, 2H), 7.18 (d, J=8.5 Hz, 2H), 6.68 (s, 1H), 6.10-5.90 (m, 1H), 5.43 (d, J=7.7 Hz, 1H), 5.30 (t, J=5.7 Hz, 1H), 4.51-4.35 (m, 1H), 4.35-4.25 (m, 2H), 4.15 (q, J=7.1 Hz, 2H), 3.26-3.08 (m, 2H), 1.91 (s, 3H), 1.85-1.15 (m, 6H), 1.50 (s, 9H), 1.25 (t, J=7.1 Hz, 3H). Finally, the urea was dissolved in 2 ml of DCM and 2 ml of TFA was added then the reaction mixture was let 1 h at room temperature. The reaction mixture was concentrated and purified on reverse phase (H2O/MeCN) to afford the urea deprotected (7 mg, 44%) as a colorless oil Rf=0.42 (CH2Cl2/MeOH 90110). 1H NMR (200 MHz, CDCl3): δ 7.06 (d, J=8.4 Hz, 2H), 6.62 (d, J=8.4 Hz, 2H), 6.15-5.92 (m, 1H), 5.36 (d, J=8.1 Hz, 1H), 5.16 (t, J=5.8, 1H), 4.55-4.35 (m, 1H), 4.22 (d, J=5.6 Hz, 2H), 4.14 (q, J=7.2 Hz, 2H), 3.28-3.10 (m, 2H), 1.92 (s, 3H), 1.90-1.27 (m, 6H), 1.25 (t, J=7.1 Hz, 3H). HPLC method A tr=6.75 nm (91.30%). ESI-MS m/z: 365.2 [M+H]+.
