4-Fluoro-2-nitrobenzoic acid (4 g, 21.6 mmol) was dissolved in 40 mL of methanol at 0 °C and 8.0 mL of concentrated sulfuric acid was slowly added. The reaction mixture was warmed to 60 °C and stirred continuously for 16 hours. Upon completion of the reaction, it was cooled to room temperature and the pH was adjusted with aqueous potassium carbonate to 8. Subsequently, the organic solvent was evaporated under reduced pressure. The remaining aqueous phase was extracted with ethyl acetate, the organic phases were combined and dried over anhydrous sodium sulfate. Finally, the organic phase was concentrated under reduced pressure to afford the red oily product methyl 2-nitro-5-fluorobenzoate (3.6 g, 82% yield). Mass spectrometry (ESI positive ion mode): m/z = 200 [M + H]+. LC-MS analytical conditions [Method 4, gradient elution: starting solvent water (containing 0.05% trifluoroacetic acid)-acetonitrile, initial acetonitrile ratio 10%]: retention time Rt = 1.01 min.
