C[O:2][C:3]1[C:16]2[O:15][C:14]3[C:9](=[CH:10][CH:11]=[CH:12][CH:13]=3)[C:8](=[O:17])[C:7]=2[CH:6]=[CH:5][CH:4]=1.B(Br)(Br)Br.N>C(Cl)Cl.CO>[OH:2][C:3]1[C:16]2[O:15][C:14]3[C:9](=[CH:10][CH:11]=[CH:12][CH:13]=3)[C:8](=[O:17])[C:7]=2[CH:6]=[CH:5][CH:4]=1To a solution of 4-methoxy-xanthen-9-one (12 g, 53 mmol) in methylene chloride (100 mL) at 0° C. was added a 1M solution of boron tribromide in methylene chloride (160 mL, 160 mmol) and the mixture was stirred at rt for 2 h. The reaction was slowly poured into a solution of ammonia in methanol (2M). The reaction was concentrated under reduced pressure and the resulting residue was partitioned between 1N HCl and methylene chloride. The organic phase was separated, dried over MgSO4, filtered, and evaporated to afford 7.5 g of Compound 3a, without further purification. MS m/z (M-H)− 243.1.
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