Step 3: To a reaction flask containing methyl 4-cyano-2-fluorobenzoate (1.61 g, 9.0 mmol), fuming nitric acid (15 mL) was slowly added, followed by dropwise addition of concentrated sulfuric acid (4 mL). The reaction mixture was stirred at 45 °C for 15 h and cooled to room temperature upon completion. The reaction solution was slowly poured into ice water and the aqueous phase was extracted with ethyl acetate (2×). The organic phases were combined, washed with saturated sodium bicarbonate solution, dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to afford the target product methyl 4-cyano-2-fluoro-5-nitrobenzoate (859 mg, 3.83 mmol, 43% yield). The product was confirmed by 1H NMR (400 MHz, CDCl3): δ 8.95 (d, 1H), 7.72 (d, 1H), 4.04 (s, 3H); the result of mass spectrometry (EI) analysis was consistent with the molecular weight of C9H5FN2O4: 224 (M+).
