[CH2:1]([O:3][C:4]([C:6]1[C:10]([CH3:11])=[C:9]([CH:12]=[O:13])[O:8][C:7]=1[CH3:14])=[O:5])[CH3:2].S(N)(=O)(=O)[OH:16].Cl([O-])=O.[Na+]>C(O)(=O)C.O>[CH3:11][C:10]1[C:6]([C:4]([O:3][CH2:1][CH3:2])=[O:5])=[C:7]([CH3:14])[O:8][C:9]=1[C:12]([OH:16])=[O:13]2,4-Dimethyl-5-formylfuran-3-carboxylic acid ethyl ester (1.78 g, 9.1 mmol) reported in the references was dissolved in a mixture of acetic acid (32 ml) and water (8 ml). Amidosulfuric acid (1.19 g, 12.2 mmol) was added to the solution, and cooled to 0° C. in an ice bath. Sodium chlorite was added and stirred for 2 hours, and after addition of water to reaction mixture, precipitate was collected by filtration, thereby giving 3,5-dimethyl-4-ethoxycarbonylfuran-2-carboxylic acid (1.01 g, 52%) as a white solid.
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