(Diethylamino)sulfur trifluoride → (3R,4S)-3-azidomethyl-1-benzyloxycarbonyl-4-fluoropyrrolidine

Process A: (3R,4R)-3-Azidomethyl-1-benzyloxycarbonyl-4-hydroxypyrrolidine (1.20 g) was dissolved in dichloromethane (40 mL). While this solution was chilled in a salt/ice bath, diethylaminosulfur trifluoride (1.20 mL) was added dropwise and the mixture was stirred at room temperature for 3 hours. The reaction vessel was again chilled in a salt/ice bath, followed by dropwise addition of a second portion of diethylaminosulfur trifluoride (0.57 mL) and stirring at room temperature for 2 hours. While the reaction mixture was kept chilled in the ice bath, a saturated aqueous solution of sodium bicarbonate (40 mL) was added dropwise and the dichloromethane layer was separated. The dichloromethane layer was then washed with a saturated aqueous solution of sodium bicarbonate (2×20 mL) and then water (20 mL), dried over anhydrous sodium sulfate, and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (eluant: hexane: ethyl acetate=2:1) to give (3R,4S)-3-azidomethyl-1-benzyloxycarbonyl-4-fluoropyrrolidine as a pale brown oil (726 mg).