20780-72-7

65971-74-6

6326-79-0

20780-72-7

The general procedure for the synthesis of 6-bromodihydroindole-2,3-diones and 4-bromoindoline-2,3-diones using 2-hydroxyimino-N-(3-bromophenyl)-acetamide as starting material is as follows: 1. to a solution of hydrogen chloride (50.0 g, 0.247 mol) in water (237 mL) was sequentially added a mixture of anhydrous sodium sulfate (69.0 g, 0.486 mol) and 3-bromoaniline (40.0 g, 0.233 mol) in 37% hydrochloric acid (25 mL). 2. Water (632 mL) was added slowly with vigorous stirring. After addition, the reaction mixture was heated to reflux for 10 minutes and then cooled to room temperature. 3. The precipitate formed was collected by filtration, washed with water (3 x 100 mL) and dried under vacuum to give crude isonitrosoacetanilide. 4. The crude product was added in batches to rapidly stirred concentrated sulfuric acid (790 mL), with the rate of addition controlled to maintain the reaction temperature between 50 and 70 °C. The reaction was carried out in a controlled manner. 5. The reaction mixture was heated to 80°C, held for 20 minutes, and then cooled to room temperature. 6. The cooled mixture was poured into crushed ice (about 320 g) and allowed to stand for 1 hour. 7. The orange precipitate was collected by filtration, washed with water and dried to give a mixture of 4-bromoindigo (K-98) and 6-bromoindigo (K-99) (40 g, 83% yield).