To a stirring solution of (S)-2-(benzoyloxy)-4-(benzyloxycarbonylamino)butanoic acid (2, 196 g, 0.548 mol) and pentafluorophenol (111 g, 0.603 mol) in 1.3 L DMF at 0° C. was added EDC (126 g, 0.658 mol) and the reaction was stirred overnight. The reaction mixture was diluted with hexanes:EtOAc (1.5 L, 1:1) and with brine/water (1 L, 1:1), and the organic layer was separated and washed with 1 M citric acid (3×600 mL), followed by sat. NaHCO3:1M NaOH (3×500 mL, 1:1) and brine (500 mL). The organic layer was dried over MgSO4, filtered and concentrated under vacuum to yield (S)-4-(benzyloxycarbonylamino)-1-oxo-1-(perfluorophenoxy)butan-2-yl benzoate (3, 255 g, 88% yield) as a white waxy solid that was used in the next step without further purification.
