A solution of (5S)—N2,N2-bis(tert-butoxycarbonyl)-5-(2,3-difluorophenyl)-D-lysine (0.569 g, 1.24 mmol), 1-chloro-2-methyl-2-propanol (0.202 g, 1.86 mmol) and diisopropylethylamine (0.529 g, 4.10 mmol) in EtOH (5 mL) was heated at 75° C. overnight. The reaction was concentrated to dryness, diluted with DCM (20 mL) and EDC (0.358 g, 1.87 mmol), HOAT (0.252 g, 1.87 mmol) were added followed by diisopropylethylamine (0.650 mL, 3.73 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 (10%→35% EtOAc/hexanes) to give the title compound (0.21 g). MS 513.1 (M+1). 1H NMR (500 MHz, CD3OD) δ 7.1 (m, 3H), 5.24 (d, J=10.7 Hz, 1H), 4.02 (m, 1H), 3.69 (d, J=13.9 Hz, 1H), 3.60 (d, J=15.1 Hz, 1H), 3.39 (m, 1H), 3.24 (d, J=14.2 Hz, 1H), 2.4 (m, 1H), 2.1 (m, 3H), 1.5 (s, 18H), 1.20 (s, 3H), 1.16 (s, 3H).
