General procedure for the synthesis of 7-chloroindolin-2-one from 7-chloroindigo: Indigo derivatives (1a-j, 3.0 g), 80% hydrazine hydrate (13 mL) and water (13 mL) were added to a three-necked flask equipped with a thermometer under vigorous stirring. The reaction mixture was placed in an oil bath at 140 °C for 6 h and subsequently cooled to room temperature. The pH of the reaction mixture was adjusted to 2 with 2.0 mol/L hydrochloric acid and stirring was continued for 12 hours at room temperature. Upon completion of the reaction, the products were collected by vacuum filtration and recrystallized with anhydrous ethanol to finally obtain the target compounds 2a-j.
