[Cl:1][C:2]1[CH:10]=[C:9]([N+:11]([O-:13])=[O:12])[C:8]([O:14][CH3:15])=[CH:7][C:3]=1[C:4]([OH:6])=[O:5].S(=O)(=O)(O)O.[CH3:21]O>>[Cl:1][C:2]1[CH:10]=[C:9]([N+:11]([O-:13])=[O:12])[C:8]([O:14][CH3:15])=[CH:7][C:3]=1[C:4]([O:6][CH3:21])=[O:5]A solution of 2-chloro-5-methoxy-4-nitrobenzoic acid (1 mmol) and concentrated sulfuric acid (0.02 mmol) in methanol (200 mL) was heated to reflux for 42 hours. The solvents were evaporated under reduced pressure then the residue partitioned between EtOAc (25 mL) and water (25 mL). The separated aqueous phase was extracted with EtOAc (3×25 mL), then the combined organic extracts were dried (Na2SO4) and concentrated under reduced pressure to afford the title compound (71%) as a solid.
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