Dimethylformamide → 5-Formyl-2,4-dimethylbenzoic acid

To a stirred solution of 5-iodo-2,4-dimethylbenzoic acid (compound 2.1, 5 g, 18.11 mmol, 1.00 equiv) in tetrahydrofuran (150 mL) under nitrogen at −78° C. was added n-BuLi (2.5 M in THF, 18 mL, 2.50 equiv) dropwise. After stirring at −78° C. for 1 h, DMF (5 g, 68.4 mmol, 3.80 equiv) was added dropwise. The resulting mixture was stirred at −78° C. for an additional 0.5 h and then carefully quenched by slow addition of 50 mL of water. The pH was then adjusted to ˜3-4 using aqueous HCl (aq., 6 M). The mixture was extracted with 3×200 mL of ethyl acetate. The combined organic layers were dried (Na2SO4) and concentrated under reduced pressure. The residue thus obtained was purified using silica gel column chromatography with ethyl acetate/petroleum ether (1:10-1:5) as gradient to yield 2.4 g (74%) of the title compound as a white solid.