Tert-butyl N-methyl-{1-(2-methyl-6-nitro-2,3-dihydroimidazo[2,1-b]oxazol-2-ylmethyl)piperidin-4-yl}carbamate prepared in Example 236 (207 mg, 0.52 mmol) was dissolved in methylene chloride (5 ml). To the solution, trifluoroacetic acid (5 ml) was added followed by stirring at room temperature for 5 hours. The reaction mixture was concentrated under reduced pressure. To the residue, methylene chloride (5 ml), triethylamine (1 ml) and benzyl chlorocarbonate (0.15 ml, 1.05 mmol) were added followed by stirring at room temperature for 30 minutes. The reaction mixture was poured into water, extracted with methylene chloride. The organic phase was dried over sodium sulfate and then filtered. The resulting filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (methylene chloride/methanol=200/1) and crystallized from ethyl acetate-isopropyl ether to afford benzyl N-methyl-[1-(2-methyl-6-nitro-2,3-dihydroimidazo[2,1-b]oxazol-2-ylmethyl)piperidin-4-yl]carbamate (184 mg, yield 82%) as a white powder.
