The compound (130 mg, 0.27 mmol) of Example 1 and triethylamine (69 mg, 0.30 mmol) were dissolved in chloroform (2 ml), acetyl chloride (99 mg, 0.33 mmol) was added under ice-cooling, and the mixture was stirred at room temperature for 1 hr. and at 60° C. for 2 hrs. Water was added to the reaction mixture, and the mixture was extracted with ethyl acetate. The extract was washed with saturated brine, dried (over anhydrous MgSO4), and the solvent was evaporated. The residue was subjected to column chromatography using silica gel (150 g), and eluted with hexane-ethyl acetate (3:1-0:1, v/v) to give the title compound (65 mg, 46%) as an amorphous form.
