General procedure for the synthesis of 4-fluoro-3-nitrobenzaldehyde from 3-nitro-4-fluorobenzyl alcohol: a dimethyl sulfoxide (12 mL) suspension of the pyridine-sulfur trioxide complex (4.65 g, 29.2 mmol) was added to a solution containing 4-fluoro-3-nitrobenzyl alcohol (1.0 g, 5.84 mmol) and triethylamine (4.07 mL, 29.2 mmol) in a dichloromethane (20 mL) solution. The reaction mixture was stirred at room temperature for 15 minutes. After completion of the reaction, the reaction solution was diluted with ether (150 mL), washed sequentially with water, 5% potassium bisulfate solution and water, and the organic layer was dried with anhydrous magnesium sulfate. The solvent was removed by distillation under reduced pressure. The residue was purified by silica gel column chromatography [eluent: hexane-ethyl acetate (5:1)] to afford colorless crystalline 4-fluoro-3-nitrobenzaldehyde (0.86 g, 87% yield). The melting point of the product was 37-38 °C. 1H-NMR (CDCl3) δ: 7.51 (1H, t, J = 9.4 Hz), 8.10-8.30 (1H, m), 8.60 (1H, dd, J = 7.4, 2.2 Hz), 10.05 (1H, s).
