Oxygen → Qing Hau Sau

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A solution of dihydroartemisinic acid (4) (1.18 g, 5.0 mmol) and tetraphenylporphyrin (12 mg, 20 μmol) in dichloromethane (total volume of the solution: 10.0 mL, volumetric flask) was prepared. The lamp was turned on 30 min prior to the beginning of the experiment. The reactor was flushed with pure dichloromethane (2.5 mL/min) and oxygen (5 mL/min, 11.5 bar) for 5 min. The reagents were then injected at a flow rate of 2.5 mL/min and the flow of oxygen was readjusted to 5 mL/min (11.5 bar). After the injection of the entire solution of (4), the reactor was flushed with pure dichloromethane (2.5 mL/min) to recover all material. The reaction mixture was collected in a round bottom flask and was cooled down to 0° C. Oxygen was bubbled into the reaction mixture at atmospheric pressure. After 2 min of bubbling, TFA (0.19 mL, 2.5 mmol, 0.5 eq.) was added drop wise. The resulting mixture was stirred at 0° C. for 2 h, while maintaining the oxygen bubbling. Then, the reaction was quenched with a saturated aqueous solution of NaHCO3. The resulting biphasic mixture was stirred at room temperature until disappearance of the green colour. The phases were separated and the aqueous phase was extracted with dichloromethane (3 times). The combined organic layers were dried over anhydrous Na2SO4, filtered and concentrated under reduced pressure. Purification over silica gel (5%-20% EtOAc in cyclohexane) afforded artemisinin (0.707 g, 50%) as a yellow solid.

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