A mixture of formic acid (45.0 mL, 1193 mmol) and acetic anhydride (90.0 mL, 952 mmol) is stirred at 50° C. for one hour under nitrogen. A second flask is charged with 4-benzyl-3-[2-cyclopentylmethyl-3-(tetrahydro-pyran-2-yloxyamino)-propionyl]-oxazolidin-2-one (21.62 g, 50.2 mmol), Et3N (170.0 mL, 1220 mmol), and anhydrous DCM (450 mL). This second mixture is cooled to 4° C. under nitrogen, and the mixed acid solution is slowly added to the second flask, while maintaining 10° C. The combined mixture is stirred for 30 minutes at 10° C., quenched with saturated, washed with aqueous NaHCO3 solution, and extracted with DCM. The DCM layers are combined, dried over Na2SO4, and concentrated. The crude product is purified by flash chromatography (SiO2, 50% EtOAc in hexane), which gives N-[3-(4-benzyl-2-oxo-oxazolidin-3-yl)-2-cyclopentylmethyl-3-oxo-propyl]-N-(tetrahydro-pyran-2-yloxy)-formamide (20.10 g, 87% yield) as an oil.
