[CH3:1][C:2]1[O:6][C:5]([CH:7]=[O:8])=[CH:4][CH:3]=1.CC(=CC)C.P([O-])(O)(O)=[O:15].[Na+].Cl([O-])=O.[Na+].Cl>CC(O)(C)C.O>[CH3:1][C:2]1[O:6][C:5]([C:7]([OH:15])=[O:8])=[CH:4][CH:3]=15-Methylfuran-2-carbaldehyde (2.20 g, 20.0 mmol) obtained in Step 1 was dissolved in a mixed solvent of 2-methyl-2-propanol (150 mL) and water (40 mL), and 2-methyl-2-butene (10.6 mL, 100 mmol) and sodium dihydrogenphosphate (2.40 g, 20.0 mmol) were added thereto, followed by stirring at room temperature for 15 minutes. 79% sodium chlorite (8.01 g, 70.0 mmol) was added to the reaction mixture little by little, followed by stirring at room temperature for 1 hour. 4 mol/L hydrochloric acid was added to the reaction mixture to adjust the pH to 3, followed by extraction with chloroform. The organic layer was dried over anhydrous magnesium sulfate, and then the solvent was distilled away under reduced pressure. The resulting residue was purified through silica gel column chromatography (chloroform:methanol=17:3) to afford 5-methylfuran-2-carboxylic acid (606 mg, 24%).
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