Compound (1f) (0.450 g) was dissolved in dimethyl sulfoxide (hereinafter referred to as DMSO, 2 mL) and ethanol (4 mL). An aqueous sodium hydroxide solution (4 M, 0.851 mL) and a 30% hydrogen peroxide solution (0.193 mL) were added to the resulting solution in this order at room temperature, followed by stirring for 30 min. Water was added to the reaction solution, the reaction solution was distributed between ethyl acetate and water, and the organic layer was washed with saturated saline. The organic layer after the washing was dried over anhydrous sodium sulfate, and then the solvent was distilled away. The residue was purified by neutral silica gel column chromatography (chloroform/methanol) to obtain compound (1) (0.300 g, 65%) as a white solid.
