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In 500 ml of methanol were dissolved 14.7 g of isatin and 10.6 g of trimethyl orthoformate, and 100 mg of p-toluenesulfonic acid was added thereto. The mixture was heated under reflux for 7 hours, followed by concentration. To the residue was added chloroform, and the mixture was washed with saturated aqueous sodium hydrogencarbonate. The organic layer was dried over anhydrous sodium sulfate and concentrated. The resulting crude product was purified by silica gel column chromatography (eluent: chloroform/ethyl acetate=10/1) to give 14.7 g (76%) of the title compound as an oil.

1. additionwas added
2. temperatureThe mixture was heated
3. temperatureunder reflux for 7 hours
4. concentrationfollowed by concentration
5. additionTo the residue was added chloroform
6. washthe mixture was washed with saturated aqueous sodium hydrogencarbonate
7. dry with materialThe organic layer was dried over anhydrous sodium sulfate
8. concentrationconcentrated
9. customThe resulting crude product was purified by silica gel column chromatography (eluent: chloroform/ethyl acetate=10/1)