To a solution of the compound obtained in Example 4 (1) (265 mg) in tetrahydrofuran (7 ml), an aqueous solution (3.5 ml) of OXONE (registered trademark) (970 mg) was added dropwise and the mixture was stirred at 0° C. for 4 hours. To the reaction solution was added water, the mixture was extracted with ethyl acetate, dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. The resulting residue was separated using silica gel column chromatography (hexane:ethyl acetate=1:1) to obtain 262 mg of the desired product.
