Sulfuric Acid → 4,6-dimethyl-1H-indole-2,3-dione

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The procedure of Varma and Singh, Ind. J. Chem. 296:578-81 (1990), was adapted. To a solution of tribromoacetaldehyde CBr3CHO (6.80 g, 24 mmol, Aldrich) in water (60 mL) and 1N HCl (2 mL) was added successively sodium sulfate (6.20 g, 43 mmol), a solution of 3,5-dimethylaniline (2.54 g, 21 mmol) in 1N HCl (40 mL), and a solution of hydroxyamine hydrochloride (5.50 g, 79 mmol) in water (25 mL). The mixture was heated to vigorous boiling and kept boiling for 10 min then cooled in ice bath to room temperature. It was filtered, washed by water, dried to leave pale yellow solid. The solid was added portionwise into concentrated H2SO4 (50 mL) kept at 50° C. After the addition, the solution was heated to 80° C. for 15 min. It was cooled to room temperature, and added into ice water (450 mL). After standing for 0.5 h, it was filtered and washed with water and dried to leave a yellow solid. The solid was boiled with ethyl acetate (180 mL), filtered and cooled to room temperature. The crystalline solid was collected and dried to give the title compound (752 mg, 20%) containing a 2% impurity by NMR. Recrystallization from DMSO/H2O afforded pure title compound as needles: mp 220°-2° C. IR (KBr, cm-1) 3439, 3211, 1762, 1729, 1628. 1H NMR (DMSO-d6): δ2.821 (s, 3H); 2.380 (s, 3H); 6.517 (s, 1H); 6.673 (s, 1H); 10.931 (s, 1H). LRMS: calcd for C10H9NO2 m/z: 175; found: 175. Purity: >97.4% by HPLC.

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