[Cl:1][C:2]1[CH:3]=[C:4]([C:13]([OH:15])=[O:14])[C:5]2[O:9][C:8]([CH3:11])([CH3:10])[CH2:7][C:6]=2[CH:12]=1.S(OOS([O-])(=O)=O)([O-])(=O)=[O:17].[K+].[K+].C(#N)C.O.C(OCC)(=O)C>O>[Cl:1][C:2]1[CH:3]=[C:4]([C:13]([OH:15])=[O:14])[C:5]2[O:9][C:8]([CH3:11])([CH3:10])[C:7](=[O:17])[C:6]=2[CH:12]=1
A solution of 5-chloro-2,2-dimethyl-2,3-dihydrobenzofuran-7-carboxylic acid (654) (2.39 g, 10.5 mmol), potassium persulfate (8.55 g, 31.6 mmol), cupric sulfate pentahydrate (2.62 g, 10.5 mmol) and acetonitrile/water (1:1) (90 mL) were heated at reflux for 1 hour. Ethyl acetate and water were added and the layers separated. The aqueous phase was extracted with ethyl acetate and the combined organic layers were dried over sodium sulfate, filtered and concentrated in vacuo to provide 5-chloro-2,2-dimethyl-3-oxo-2,3-dihydrobenzofuran-7-carboxylic acid (655) as a yellow solid.
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