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General procedure for the synthesis of 5-nitroindol-2-one from 2-indolone: 2-indolone (26 g) was dissolved in 100 mL of concentrated sulfuric acid at -15 °C and fuming nitric acid (8.4 mL) was added slowly and dropwise. The reaction temperature needs to be strictly controlled to ensure that it is maintained at -15 °C. After the dropwise addition was completed, the reaction mixture was continued to be stirred for 30 minutes. Subsequently, the reaction solution was carefully poured into ice water and a yellow precipitate was immediately generated. The precipitate was separated by filtration and 34 g of 5-nitroindol-2-one was finally obtained in 98% yield.

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