[Bis(2-methoxyethyl)amino]sulfur trifluoride (285 μl) was added dropwise to a solution of tert-butyl [(4aR,6R,8aS)-8a-(5-nitro-2-fluorophenyl)-6-hydroxymethyl-4,4a,5,6,8,8a-hexahydro-7-oxa-3-thia-1-azanaphthalen-2-yl]carbamate obtained in Preparation Example 13 (575 mg) in dichloromethane (55 ml) at −78° C. The mixture was stirred overnight with gradual heating to room temperature. A sodium bicarbonate solution was added to the reaction solution, and the aqueous layer was extracted with chloroform. The organic layer was dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure and the residue was purified by NH-silica gel column chromatography to obtain the title compound (240 mg).
