[CH2:1]([C:3]1[CH:11]=[C:10]([CH3:12])[C:9](I)=[CH:8][C:4]=1[C:5]([OH:7])=[O:6])[CH3:2].[Li]CCCC.CN([CH:22]=[O:23])C>O1CCCC1>[CH2:1]([C:3]1[CH:11]=[C:10]([CH3:12])[C:9]([CH:22]=[O:23])=[CH:8][C:4]=1[C:5]([OH:7])=[O:6])[CH3:2]To a stirred solution of 2-ethyl-5-iodo-4-methylbenzoic acid (compound 211.3, 5.00 g, 17.2 mmol, 1.00 equiv) in tetrahydrofuran (100 mL) at −78° C. under nitrogen was added dropwise n-BuLi (17 mL, 2 M in THF, 2.00 equiv). After 1 h at −78° C., DMF (7.00 g, 95.8 mmol, 3.00 equiv) was added dropwise at −78° C. The resulting solution was slowly warmed up to 25° C., then carefully quenched with water followed by the addition of 10 mL of HCl (aq., 6 M). The aqueous phase was extracted with 3×100 mL of ethyl acetate. The combined organic layers were dried over anhydrous sodium sulfate and concentrated in vacuo. The residue was purified using silica gel column chromatography with ethyl acetate/petroleum ether (2:1) as eluent to furnish 450 mg (14%) of 2-ethyl-5-formyl-4-methylbenzoic acid as a yellow solid.
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