The general procedure for the synthesis of 3,4-dichloroisothiazole-5-carboxylic acid from 3,4-dichloro-5-cyanoisothiazole was as follows: 42.4 g of crude 3,4-dichloro-5-cyanoisothiazole was placed in three 500 mL round-bottomed flasks, and diluted by adding 100 mL of methanol and 30 g of 45% sodium hydroxide solution. The reaction was stirred at 40°C for 2 h. The progress of the reaction was monitored by thin layer chromatography (TLC) until 3,4-dichloro-5-cyanoisothiazole was completely disappeared. After completion of the reaction, the residual methanol was removed by concentration under reduced pressure. Subsequently, the pH of the reaction solution was adjusted to 3 with concentrated hydrochloric acid, the precipitate was collected by filtration and washed with cold water to afford 17.6 g of white solid 3,4-dichloroisothiazole-5-carboxylic acid (II). The product can be used directly in the subsequent reaction without further purification. Depending on the experimental requirements, the reaction can be scaled up or down to prepare different amounts of 3,4-dichloroisothiazole-5-carboxylic acid (II). This synthesis method has the same effect as the commercially available compounds.
