Compound ((3R,3aS)-1-oxo-7-(3-oxomorpholin-4-yl)-1,3,3a,4-tetrahydrobenzo[b]oxazolo[3,4-d][1,4]oxazin-3-yl)methylmethanesulfonate (3.91 g, 9.81 mmol) prepared as above in (c) was dissolved in 50 ml of DMF. Phthalimide potassium (2.72 g, 14.72 mmol) was added and the mixture was allowed to react at 80° C. for 2 h. TLC (DCM/MeOH=10/1) was employed to monitor the reaction. After the reaction completed, water (80 ml) was added for dilution. There was white solid precipitated, which was filtered and dried. The filtrate was extracted with EA (50 ml×3). The organic phase was combined, and washed sequentially with water (30 ml) and saturated NaCl (30 ml) solution, dried over anhydrous sodium sulfate, filtered, and the solvent was evaporated to afford totally 3.71 g of white solid, yield: 84.2%.
