[OH:1][C:2]1[C:3]([CH3:11])=[C:4]([CH:8]=[CH:9][CH:10]=1)[C:5]([OH:7])=[O:6].[N+:12]([O-])([OH:14])=[O:13]>OS(O)(=O)=O>[OH:1][C:2]1[C:3]([CH3:11])=[C:4]([CH:8]=[CH:9][C:10]=1[N+:12]([O-:14])=[O:13])[C:5]([OH:7])=[O:6]To a solution of 3-hydroxy-2-methylbenzoic acid (5.00 g, 25.3 mmol) in concentrated H2SO4 (98%, 10 mL) was added concentrated HNO3 (63%, 10 mL) dropwise. The mixture was stirred at −40° C. for 10 min. The mixture was poured into ice-water and extracted by EA (3 times). The organic layers were combined, dried over anhydrous Na2SO4. Then the solvent was evaporated in vacuo to afford 3-hydroxy-2-methyl-4-nitrobenzoic acid as a yellow solid, which was used for next step without further purification.
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