[NH2:1][C:2]1[N:7]=[N:6][C:5]([C:8]2[CH:17]=[CH:16][C:11]([C:12]([O:14][CH3:15])=[O:13])=[CH:10][CH:9]=2)=[CH:4][C:3]=1[Br:18].Cl[CH2:20][CH:21](OCC)OCC.CC1C=CC(S(O)(=O)=O)=CC=1>C(O)(C)C>[Br:18][C:3]1[C:2]2[N:7]([CH:20]=[CH:21][N:1]=2)[N:6]=[C:5]([C:8]2[CH:17]=[CH:16][C:11]([C:12]([O:14][CH3:15])=[O:13])=[CH:10][CH:9]=2)[CH:4]=1A solution of methyl 4-(6-amino-5-bromopyridazin-3-yl)benzoate (1.64 g, 5.32 mmol), 2-chloro-1,1-diethoxyethane (0.984 g, 6.39 mmol) and PTSA (1.215 g, 6.39 mmol) in isopropanol (50 mL) was heated to 80° C. for 40 h. After cooling to room temperature, the solution was concentrated in vacuo. The resulting mixture was treated with a saturated NaHCO3 solution (50 mL), extracted with dichloromethane (50 mL×3), dried over Na2SO4, filtered and concentrated. The residue was purified by chromatography (silica gel, 200-300 mesh, petroleum ether:ethyl acetate=3:1) to give methyl 4-(8-bromoimidazo[1,2-b]pyridazin-6-yl)benzoate (640 mg, 36%) as a white solid. LC-MS: [M+H]+, 333.9, tR=1.637 min.
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