O[C:2]([C@H:5]1[CH2:9][O:8][C:7]([CH3:11])([CH3:10])[N:6]1[C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])([CH3:4])[CH3:3].C(N(S(F)(F)[F:25])CC)C>C(Cl)Cl>[F:25][C:2]([C@H:5]1[CH2:9][O:8][C:7]([CH3:11])([CH3:10])[N:6]1[C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])([CH3:4])[CH3:3]tert-Butyl (4R)-4-(1-hydroxy-1-methylethyl)-2,2-dimethyl-1,3-oxazolidine-3-carboxylate (3.80 g, 14.7 mmol), prepared by the method of Joullie, Tetrahedron, 52, pp. 11673-11694, (1996) was dissolved in 90 mL of anhydrous CH2Cl2. The solution was cooled to −78° C. under an atmosphere of N2. (Diethylamino)sulfur trifluoride (DAST) (2.25 mL, 17.0 mmol) was added and the reaction was allowed to warm to ambient temperature overnight. The reaction was quenched with saturated NaHCO3 solution and the layers were separated. The organic portion was washed successively with H2O and brine, dried over Na2SO4 and concentrated under reduced pressure. Chromatography (SiO2, 7% EtOAc/hexanes) gave tert-butyl (4R)-4-(1-fluoro-1-methylethyl)-2,2-dimethyl-1,3-oxazolidine-3-carboxylate (1.98 g) as a nearly colorless liquid.
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