[Cl:1][C:2]1[CH:3]=[C:4]([CH2:10][C:11]([OH:13])=[O:12])[CH:5]=[CH:6][C:7]=1[S:8][CH3:9].S(=O)(=O)(O)O.[CH3:19]O>>[CH3:19][O:12][C:11](=[O:13])[CH2:10][C:4]1[CH:5]=[CH:6][C:7]([S:8][CH3:9])=[C:2]([Cl:1])[CH:3]=1A solution of (3-chloro-4-methylsulfanyl-phenyl)-acetic acid (7.00 g, 32.30 mmol) in methanol (150 mL) was treated slowly with concentrated sulfuric acid (2.8 mL, 52.65 mmol). The resulting reaction mixture was heated under reflux for 1.5 h. The reaction mixture was allowed to cool to 25° C. and then concentrated in vacuo to remove methanol. The residue was diluted with ethyl acetate (500 mL). The organic phase was successively washed with a saturated aqueous sodium bicarbonate solution (1×200 mL) and a saturated aqueous sodium chloride solution (1×200 mL), dried over sodium sulfate, filtered, and concentrated in vacuo to afford (3-chloro-4-methylsulfanyl-phenyl)-acetic acid methyl ester (7.41 g, 99.4%) as a light yellow oil: EI-HRMS m/e calcd for C10H11ClO2S (M+) 230.0168. found 230.0166.
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