Step D (5): tert-Butyl 3-((4S,5S)-4-(3,5-difluorobenzyl)-2-oxooxazolidin-5-yl)morpholine-4-carboxylate. To a solution of (2S,3S)-2-(3,5-difluorobenzyl)-3-(4-(tert-butoxycarbonyl)morpholin-3-yl)-3-hydroxypropanoic acid (step D (4)), 1.6 g, 4 mmol) in toluene (100 mL) was added diphenylphosphoryl azide (1.65 g, 6 mmol) and triethyl amine (1.01 g, 10 mmol.). This reaction mixture was stirred at 80° C. for 4 h. Ethyl acetate (500 mL) was added. The mixture was washed with sodium carbonate solution, H2O, dried over Na2SO4 and concentrated. The crude mixture was purified by silica gel Flash Chromatography (0% to 20% to 30% to 50% to 70% EtOAc/Hexane step gradient) to give 800 mg of the title compound: MS (ESI) (M−H)− 397.12.
