Phosphoric Acid → 3-Chloro-4,5,6,7-tetrahydro-1,2-benzisothiazole

A mixture of 3-hydroxy-4,5,6,7-tetrahydro-1,2-benzisothiazole 22a (4.74 g), pyridinium hydrochloride (12.7 g), phosphoric acid (2.1 g) and phosphorus oxychloride (25 mL) was stirred at 90° C. for 5 h. The reaction mixture was evaporated and ethyl acetate (130 mL) was added to the residue. A saturated solution of sodium hydrogencarbonate (130 mL) was added and after 10 min of stirring the phases were separated. The aqueous phase was extracted with ethyl acetate (2×150 mL) and the combined organic phases were dried and evaporated to give an oil. Column chromatography on silica gel (eluent:toluene-ethyl acetate 1:1) gave the title compound as a yellow oil (2.8 g).