The compound obtained in Example 2 (252 mg, 0.60 mmol), triethylamine (0.201 ml) were dissolved in THF (5 ml), and under ice cooling, to the solution was added acetyl chloride (0.085 ml). The mixture was stirred at the same temperature for 30 minutes. To the mixture was added a saturated aqueous solution of sodium hydrogencarbonate (15 ml) under ice cooling, and the mixture was extracted with ethyl acetate (15 ml×3). The organic layer was dried over magnesium sulfate and concentrated under reduced pressure. The concentrate was subjected to column chromatography (silica gel log, ethyl acetate/methanol=1/0 to 9/1 to 4/1), and the desired fraction was concentrated under reduced pressure. To the concentrate was added ethyl acetate and the insolubles were filtered off. The filtrate was concentrated under reduced pressure to give the titled compound (237 mg, 0.51 mmol, Yield 85%) as colorless oily substance.
