Sulfuric Acid → 6H-thiazolo[5,4-e]indole-7,8-dione

To a 1-L flask was added a magnetic stir bar, 85 g of sodium sulfate, and 100 mL of water. The mixture was magnetically stirred until all the solids were dissolved. To the resultant aqueous solution was added a solution of 6-aminobenzothiazole (4.96 g, 33.0 mmol) in 50 mL of 1 N aqueous hydrochloric acid and 10 mL of ethanol. The mixture was stirred, and chloral (6.0 g, (36 mmol) was added. To the resultant solution was added a solution of hydroxyl amine hydrochloride (7.50 g, 108 mmol) in 30 mL of water. The final mixture was heated with stirring to a gentle boil until all solids disappeared, and heating was continued for an additional 15 min. The flask was removed from the heat, and the solution was poured onto 500 g of ice. The mixture was stirred as the product precipitated from solution. The precipitate was collected by suction filtration, washed thoroughly with water, filtered, and air dried to provide 6.9 g (94%) of N-benzothiazol-6-yl-2-hydroxyimino-acetamide: 1H NMR (DMSO-d6): 67 12.2 (s, 1H), 10.4 (s, 1H), 9.2 (s, 1H), 8.5 (s, 1H), 7.9 (d, 1H), 7.7 (m, 1H), 7.7 (s, 1H); APCI−MS m/z 220 (M−H)−. To a 1-L 3-neck round bottom flask was placed a magnetic stir bar and 100 ml of concentrated sulfuric acid. The flask was fitted with a thermometer to monitor the temperature of the reaction. The sulfuric acid was heated to 100° C., and 10.0 g (45.2 mmol) of N-benzothiazol-6-yl-2-hydroxyimino-acetamide was added slowly. The solution was heated for ˜1 hour, and the reaction mixture was poured into 750 g of ice and water. The residual reaction mixture in the reaction vessel was washed out with an additional 20 mL of cold water. The aqueous slurry was stirred for about 1 hour and filtered. The solid was washed thoroughly with water, filtered, and air dried to yield 4.3 g (46%) of 6-H-1-thia-3,6-diaza-as-indacen-7,8-dione: 1H NMR (DMSO-d6): δ11.1 (s, 1H), 9.2 (s,1H), 8.2 (d,1H), 7.0 (d,1H); APCI−MS m/z 203 (M−H)−.