[Br:1][C:2]1[CH:3]=[C:4]([CH2:10][CH2:11][C:12]([O:14][CH3:15])=[O:13])[CH:5]=[CH:6][C:7]=1[O:8][CH3:9].[N+:16]([O-])([O-:18])=[O:17].[K+].S(=O)(=O)(O)O.[OH-].[Na+]>C(OC(=O)C)(=O)C>[Br:1][C:2]1[CH:3]=[C:4]([CH2:10][CH2:11][C:12]([O:14][CH3:15])=[O:13])[CH:5]=[C:6]([N+:16]([O-:18])=[O:17])[C:7]=1[O:8][CH3:9]A solution of Intermediate 20 (3.20 g) in acetic anhydride (25 ml) was added with potassium nitrate (1.30 g) under ice cooling and stirred for 10 minutes, and the solution was added dropwise with concentrated sulfuric acid (730 μl) over 10 minutes. The reaction mixture was stirred for 10 minutes for 10 minutes at the same temperature, then warmed to room temperature, and further stirred for 30 minutes. The reaction mixture was poured into 1 N aqueous sodium hydroxide (250 ml) containing ice, and extracted with isopropyl ether (200 ml×2). The organic layer was successively washed with saturated aqueous sodium hydrogencarbonate, saturated aqueous ammonium chloride, and saturated brine and dried, and then the solvent was evaporated under reduced pressure. The residue was purified by column chromatography (Quad, hexane:ethyl acetate=10:1) to obtain the title compound (Intermediate 21, 2.73 g).
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