4-(Trifluoromethoxy)aniline (1.271 ml) was added to a 1,2-dichloroethane solution (50 ml) of 1-[4-((R)-2-nitro-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazin-7-ylmethoxy)phenyl]piperidin-4-one (2.5 g). Subsequently, sodium triacetoxyborohydride (1.992 g) and acetic acid (0.538 ml) were added thereto and the mixture was stirred at room temperature for 21 hours. A saturated sodium hydrogen carbonate aqueous solution was added to the reaction mixture, followed by extraction with methylene chloride. The organic layer was washed with a saturated sodium chloride aqueous solution and dried over anhydrous sodium sulfate. Thereafter, the solvent was distilled off under reduced pressure. The resulting residue was purified by silica gel column chromatography (ethyl acetate:n-hexane=1:1) and recrystallized from ethyl acetate to afford the title compound as a yellow powder (2.55 g).
