To a solution (5S)—N2,N2-bis(tert-butoxycarbonyl)-5-(2,3-difluorophenyl)-D-lysine from Step D (22.0 g, 48.0 mmol) in CH2Cl2 (700 mL) were added EDC (11.0 g, 57.6 mmol) and HOAT (3.27 g, 24.0 mmol) followed by triethylamine (10.0 mL, 72.0 mmol). After 1 h, aqueous NaHCO3 was added, the layers were separated, and the aqueous phase was extracted with CH2Cl2. The combined organic extracts were dried over MgSO4, filtered and concentrated in vacuo. The residue was purified by silica gel chromatography, eluting with CH2Cl2:MeOH—90:10, to give the title compound. MS: m/z=341 (M+1).
