Trifluoromethanesulfonic anhydride (0.72 ml) was added dropwise at −78° C. into a solution of 1-[1-(4-fluorophenethyl)piperidin-4-yl]-6-hydroxyiminomethylindoline (1.5 g) and triethylamine (1.2 ml) in methylene chloride (1 l) and the resultant mixture was warmed to room temperature. Next, a saturated aqueous solution of sodium bicarbonate and chloroform were added thereto and the layers were separated. The organic layer was dried over anhydrous magnesium sulfate and the residue was purified by NH-silica gel column chromatography (hexane/ethyl acetate system) to give the title compound (1.0 g) as a white powder (yield: 67%).
