[CH3:1][O:2][C:3]1[CH:4]=[C:5]([CH2:11][C:12]([OH:14])=[O:13])[CH:6]=[C:7]([O:9][CH3:10])[CH:8]=1.S(=O)(=O)(O)O.[CH3:20]O>>[CH3:20][O:13][C:12](=[O:14])[CH2:11][C:5]1[CH:6]=[C:7]([O:9][CH3:10])[CH:8]=[C:3]([O:2][CH3:1])[CH:4]=1To a solution of 3,5-dimethoxyphenylacetic acid (1.99 g, 10.0 mmol) (Transworld) in methanol (20 mL) was added concentrated sulfuric acid (1.0 mL) and the reaction mixture was heated at reflux for 24 hours. The reaction mixture was concentrated in vacuo. The residue was then diluted with ethyl acetate (100 mL) and successively washed with water (50 mL) and brine (50 mL), dried over anhydrous sodium sulfate, filtered, and concentrated in vacuo to give the crude 3,5-dimethoxyphenylacetic acid methyl ester as a black oil which was used in the next step without further purification. (Yield 2.05 g, 97.6%).
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