[Cl:1][C:2]1[CH:12]=[CH:11][C:5]2[CH2:6][CH:7]([CH2:9][OH:10])[O:8][C:4]=2[C:3]=1[Cl:13].S(=O)(=O)(O)[OH:15].O>CC(C)=O.[O-2].[O-2].[O-2].[Cr+6]>[Cl:1][C:2]1[CH:12]=[CH:11][C:5]2[CH2:6][CH:7]([C:9]([OH:15])=[O:10])[O:8][C:4]=2[C:3]=1[Cl:13]To a solution of 6,7-dichloro-2,3-dihydro-2-hydroxymethylbenzofuran (10.4 gm.) in acetone (200 ml.) cooled to 20° C. is added an oxidizing solution prepared from chromium trioxide (6.0 g.), concentrated sulfuric acid (5.3 ml.) and water (43 ml.) over a one-half hour period. The reaction mixture is stirred at 25° C. for 18 hours. The acetone layer is separated from the precipitated chromium salts, added to water (600 ml.) and extracted with ether (2 × 150 ml.). The ether extract is washed with water then extracted with aqueous sodium bicarbonate. Acidification of the basic solution gives 3.4 g. of 6,7-dichloro-2,3-dihydrobenzofuran-2-carboxylic acid which is purified by reprecipitation from aqueous sodium hydroxide with aqueous hydrochloric acid.
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