A solution of (5S)-N2,N2-bis(tert-butoxycarbonyl)-5-(2,3-difluorophenyl)-D-lysine (0.871 g, 1.90 mmol), 2-formylthiazole (0.183 g, 1.62 mmol) and acetic acid (0.137 g, 2.28 mmol) in 1,2-dichloroethane (20 mL) was stirred at room temperature for 30 min. Sodium triacetoxyborohydride (0.604 g, 2.85 mmol) was added and the reaction stirred overnight. The reaction was diluted with saturated aqueous sodium bicarbonate, the layers separated, and the aqueous phase extracted with DCM (2×). The organic washes were combined, dried over magnesium sulfate, and concentrated. This residue was diluted with dichloromethane (20 mL) and EDC (0.569 g, 2.97 mmol), HOAT (0.401 g, 2.97 mmol) were added followed by diisopropylethylamine (1.04 mL, 5.94 mmol). After stirring overnight, NaHCO3 was added, the layers separated and the aqueous phase backwashed with DCM. The combined organics were dried over magnesium sulfate, filtered, concentrated and the residue purified by silica gel chromatography (0%→40% EtOAc/hexanes) to give the title compound (0.49 g). MS 538.0 (M+1)). 1H NMR (500 MHz, CDCl3) δ 7.73 (d, J=3.5 Hz, 1H), 7.37 (d, J=3.4 Hz, 1H), 7.0 (m, 2H), 6.8 (m, 1H), 5.20 (d, J=10.3 Hz, 1H), 5.03 (d, J=15.5 Hz, 1H), 4.96 (d, J=15.5 Hz, 1H), 3.84 (dd, J=10.3, 15.2 Hz, 1H), 3.48 (d, J=15.5 Hz, 1H), 2.94 (dd, J=11.0, 11.0 Hz, 1H), 2.50 (dd, 1H), 2.0 (m, 4H), 1.5 (s, 18H).
