A solution of ethyl-(S)-3-[4-aminophenyl]-2-[t-butoxycarbonylamino]propanoate (638 mg, 2.07 mmol) and Intermediate 11 (310 mg, 1.88 mmol) in ethoxyethanol (2 ml) was stirred at 120° for 15 min and at 100° for 1 h under nitrogen. The volatiles were removed in vacuo and the dark residue partitioned between EtOAc (70 ml) and saturated aqueous NaHCO3 (10 ml). The phases were separated and the aqueous layer re-extracted with EtOAc (2×30 ml). The combined organic extracts were washed with brine (10 ml)T dried (Na2SO4) and evaporated in vacuo to afford a dark foam. Chromatography (SiO2; 5 to 10% MeOH/DCM) afforded a mixture of ethyl-(S)-3-[4-(2,7-naphthyridin-1-ylamino)phenyl]-2-[(t-butoxycarbonyl)amino]propanoate and some of the title compound (730 mg). This mixture was treated with a solution of trifluoroacetic acid (5 ml) and DCM (5 ml) at room temperature for 1 h. The volatiles were removed in vacuo and the residue partitioned between EtOAc (75 ml) and saturated aqueous NaHCO3 (20 ml). The phases were separated and the aqueous layer re-extracted with EtOAc (3×30 ml). The combined organic extracts were washed with brine (10 ml), dried (Na2SO4) and evaporated in vacuo to afford an orange solid. Chromatography (SiO2; 10% MeOH/DCM) afforded the title compound as a straw-coloured solid (420 mg, 60% over two steps). δH (CDCl3) 10.70 (1H, s), 10.31 (1H, s), 9.44 (1H, d, J 5.6 Hz), 8.94 (1H, d, J 5.6 Hz), 8.55 (1H, d, J 7.3 Hz), 8.54 (2H, d, J 8.5 Hz), 8.46 (1H, d, J 5.6 Hz), 7.94 (2H, d, J 8.5 Hz), 4.84 (2H, q, J 7.1 Hz), 4.35 (1H, t, J 6.6 Hz), 4.10 (2H, br s), 3.64 (1H, dd, J 13.5, 6.4 Hz), 3.56 (1H, dd, J 13.5, 7.0 Hz) and 1.95 (3H, t, J 7.1 Hz); m/z (ES+, 70V) 337 (MH+).
