[F:1][C:2]1[C:10]([F:11])=[CH:9][C:5]([C:6]([OH:8])=[O:7])=[C:4]([OH:12])[CH:3]=1.OS(O)(=O)=O.[CH3:18]O>>[F:1][C:2]1[C:10]([F:11])=[CH:9][C:5]([C:6]([O:8][CH3:18])=[O:7])=[C:4]([OH:12])[CH:3]=1A mixture of 4,5-difluoro-2-hydroxybenzoic acid (20 mmol), methanol (100 ml) and concentrated H2SO4 (5 ml) was refluxed for 48 h. Then, the reaction mixture was concentrated. The residue was dissolved in ethyl acetate, washed with brine and a saturated NaHCO3 solution and neutralized with citric acid. The organic layer was dried over MgSO4 and concentrated in vacuo to give the product. Yield: 2.8 g (97% purity, 74% yield).
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