Hydroxyamine-O-sulfonic acid (730 mg, 6.5 mmol) was diluted in water (2.5 mL), cooled to 0°C, treated with 4,4-diethoxybut-2-acetylacetal (1 g, 6.5 mmol), and the reaction mixture was stirred at 0°C for 10 minutes. Then, an equivalent of solid NaHCO3 (545 mg, 6.5 mmol) was added, followed by an aqueous solution of NaSH (5 mL, 7.1 mmol, 1.4 M). The mixture was warmed to room temperature and maintained for 3 hours. After washing with water and diethyl ether, the organic phase was concentrated under vacuum to give pure isothiazole diethyl acetal (370 mg).
The acetal (370 mg, 1.98 mmol) was diluted in wet acetone (10 mL), treated with a catalytic amount of p-toluenesulfonic acid, and the mixture was heated under reflux for 1 hour. The mixture was cooled to 23 °C, partitioned between a saturated aqueous sodium bicarbonate solution and diethyl ether, and the organic phase was concentrated under vacuum to give the title compound. Yield (330 mg).
