Hydrochloric Acid → 1-(4-Hydroxynaphthalen-1-yl)thiolan-1-ium chloride

In 50 g of methanol were dissolved 10 g (0.069 mol) of 1-naphthol and 7.2 g (0.069 mol) of tetramethylene sulfoxide. The solution was cooled to −16° C. An excess of hydrogen chloride gas was fed to the solution at a temperature below 20° C. Nitrogen gas was bubbled to expel the excess of hydrogen chloride. The reaction solution was concentrated and combined with water and diisopropyl ether, from which a water layer was separated. It was an aqueous solution of 4-hydroxynaphthyl-1-tetrahydrothiophenium chloride. This aqueous solution was used in the subsequent step without further isolation.