Methyl 2-amino-4-bromobenzoate (4.5 g, 20 mmol) was stirred in 20% hydrochloric acid (30 mL) until completely dissolved. The solution was cooled to 0 °C, the reaction temperature was controlled not to exceed 5 °C, and a solution of sodium nitrite (1.4 g, 0.020 mol) in water (20 mL) was added dropwise. The mixture was stirred at 0°C for 45 min. Meanwhile, sulfur dioxide was passed into a mixture of acetic acid (50 mL) and water (5 mL) to saturation at 0 °C, followed by the addition of copper (I) chloride (2.0 g, 0.020 mol). The mixture was cooled to 0 °C and the diazonium salt solution was added dropwise over 30 min with vigorous stirring. The reaction mixture was stirred at 0 °C for 1 h. After stirring, the mixture was slowly warmed to room temperature and stirring was continued for 2 h. The mixture was then cooled to room temperature. The mixture was poured into ice water (250 mL) and extracted with EtOAc (3 x 50 mL). The organic phase was washed with saturated NaHCO3 solution and dried over anhydrous Na2SO4. The solvent was concentrated under reduced pressure to give an oily residue, which was dissolved in tetrahydrofuran (40 mL) and cooled to 0 °C. Cold (0°C) 28% ammonium hydroxide solution (40 mL) was added in batches and the reaction temperature was controlled to be below 10°C. The mixture was warmed to room temperature and stirred for 1 hour. The solvent was removed under reduced pressure and the residue was dissolved in saturated aqueous sodium bicarbonate solution (40 mL) and washed with ether (50 mL). The aqueous layer was acidified to pH 1 with concentrated hydrochloric acid, the precipitate was collected by filtration and dried under vacuum to give 6-bromo-1,1-dioxo-1,2-dihydro-1λ6-benzo[d]isothiazol-3-one (500 mg, 10% yield).1H NMR (400 MHz, DMSO) δ 8.44 (d, J=1.5, 1H), 8.04 (dd, J=8.1,1.5, 1H), 7.81 (d, J=8.1,1.5, 1H), 7.81 (d, J=8.1,1.5, 1.1.5, 1.1.5, 1H). 1H), 7.81 (d, J=8.0,1H).
