Lithium aluminum hydride (1.9 g, 49 mmol) was added batchwise to a solution of 2-methoxyisonicotinic acid (5.0 g, 33 mmol) in tetrahydrofuran (40 mL) at 0 °C. The reaction mixture was stirred continuously at 0 °C for 1 h, followed by slow dropwise addition of saturated sodium sulfate solution for quenching. The reaction solution was filtered and the filtrate was extracted with ethyl acetate and the organic layer was washed with saturated brine and subsequently concentrated under reduced pressure. The resulting residue was dissolved in dichloromethane (30 mL) and chromium trioxide pyridine complex (10.6 g, 49 mmol) was added. The mixture was stirred at room temperature for 2 h. After completion of the reaction, the mixture was filtered through a short silica gel column and eluted with ethyl acetate. The eluate was concentrated under reduced pressure to remove the solvent. The residue was purified by column chromatography (petroleum ether: ethyl acetate = 10:1) to afford the target product 2-methoxypyridine-4-aldehyde (646 mg, 14% yield). The product was characterized by 1H NMR (CDCl3): δ 10.01 (1H, s), 8.36 (1H, d, J = 5.2 Hz), 7.29 (1H, dd, J = 1.2 Hz, 5.2 Hz), 7.14 (1H, d, J = 1.2 Hz), 3.99 (3H, s).
