[CH2:1]([C:3]1[CH:11]=[CH:10][CH:9]=[C:8]2[C:4]=1[C:5](=[O:13])[C:6](=[O:12])[NH:7]2)[CH3:2].ClC1C=CC=C(C(OO)=[O:22])C=1>C(O)(=O)C.C(OCC)(=O)C>[CH2:1]([C:3]1[CH:11]=[CH:10][CH:9]=[C:8]2[NH:7][C:6](=[O:22])[O:12][C:5](=[O:13])[C:4]=12)[CH3:2]20 g (0.114 mol) of 4-ethylisatin are suspended in 75 ml of 100 percent acetic acid, treated portionwise with 26 g (0.137 mol) of m-chloroperbenzoic acid, the temperature not rising above 50°, and then the mixture is stirred at this temperature for a further 15 minutes. The mixture is poured into ice-water and filtered. The crude product obtained is taken up in ethyl acetate and extracted cautiously with a mixture of 2 N sodium hydroxide and ice. The ethyl acetate phase is dried over magnesium sulphate and concentrated to give crude 6-ethylisatoic acid anhydride which melts at 202°-204° after recrystallization from ethyl acetate.
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