[Cl:1][C:2]1[CH:3]=[CH:4][C:5]2[N:6]([C:8]([CH:11]=[O:12])=[CH:9][N:10]=2)[N:7]=1.CC(=CC)C.Cl([O-])=[O:19].[Na+]>CC(O)(C)C.C1COCC1.O>[Cl:1][C:2]1[CH:3]=[CH:4][C:5]2[N:6]([C:8]([C:11]([OH:19])=[O:12])=[CH:9][N:10]=2)[N:7]=1To a solution of 6-chloroimidazo[1,2-b]pyridazine-3-carbaldehyde (1.82 g, 10 mmol), in t-BuOH (90 mL) and 2-methyl-2-butene in THF (30 mL, 2.0 M) was added a solution of sodium chlorite (80%, 9.0 g, 100 mmol) and NaH2PO4 (1 g, 8 mmol) in water (45 mL). The mixture was stirred at rt for overnight, concentrated, diluted with water, acidified to PH=4, precipitated white solid, filtered to give 6-chloroimidazo[1,2-b]pyridazine-3-carboxylic acid as white solid (1.2 g, 67% yield)). 1H NMR (400 MHz, DMSO-d6) δ ppm 8.29 (d, J=9.60 Hz, 1H), 8.22 (s, 1H), 7.50 (d, J=9.60 Hz, 1H); LRMS (ESI) m/e 198.1 [(M+H)+, calcd for C7H6ClN3O2 198.6].
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