General procedure for the synthesis of 5-fluoro-2-methoxy-3-formylpyridine from 5-fluoro-3-hydroxymethyl-2-methoxypyridine: manganese(IV) dioxide (39.8 g, 458 mmol) was added to a 300 mL ethyl acetate solution of 5-fluoro-2-methoxy-pyridin-3-yl-methanol (36.8 g, 50.9 mmol), and the reaction mixture was at room temperature The reaction mixture was stirred overnight at room temperature. Upon completion of the reaction, the mixture was filtered through a diatomaceous earth pad and the diatomaceous earth pad was washed thoroughly with ethyl acetate. The filtrate and washings were combined and concentrated under reduced pressure to remove the solvent. The crude product was purified by silica gel column chromatography using 50% ethyl acetate in heptane solution as eluent to afford the target compound 5-fluoro-2-methoxy-3-formylpyridine as a light yellow solid (4.5 g, 29.0 mmol, 57.0% yield).
