Br[CH:2](Br)[C:3]1[CH:11]=[CH:10][C:6]([C:7]([OH:9])=[O:8])=[CH:5][C:4]=1[F:12].Cl.[OH2:15]>C(O)C.[N+]([O-])([O-])=O.[Ag+]>[F:12][C:4]1[CH:5]=[C:6]([CH:10]=[CH:11][C:3]=1[CH:2]=[O:15])[C:7]([OH:9])=[O:8]AgNO3 (0.39 mol) in hot H2O (90 mL) is added dropwise to a solution of crude 4-dibromomethyl-3-fluorobenzoic acid (2, 0.19 mol) in ethanol (EtOH) (480 mL) at 50° C. over 10 min and then the mixture is stirred at the same temperature for 45 min. After cooling to room temperature, the mixture is poured into 1 N HCl (200 mL) and filtered off. The residue is washed with EtOH and the filtrate is concentrated to ca. 300 mL. The mixture is extracted twice with ethylacetate (EtOAc) and the combined organic layers are washed with brine, dried over MgSO4 and evaporated in vacuo. The crystals are collected by filtration and washed with ether/hexane (1:1) to give 3-fluoro-4-formyl benzoic acid (3).
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