Compound 1 (172 mg, 0.429 mmol) obtained in Example 1 was dissolved in ethanol (4.0 mL), and hydroxyamine hydrochloride (89.5 mg, 1.29 mmol) and triethylamine (178 μL, 1.29 mmol) were added thereto at room temperature, followed by stirring for 20 minutes. A saturated aqueous ammonium chloride solution was added to the reaction mixture, followed by extraction with ethyl acetate twice. The organic layer was washed with saturated brine, dried over anhydrous magnesium sulfate, and concentrated under reduced pressure to yield a residue. The residue was purified by silica gel column chromatography to obtain the title compound (160 mg, 90%) as a mixture of E-form and Z-form.
