3,3-Dimethyl- 2, 4-dioxa-9-thiaspiro[5.5]undecane-1,5-dione-9, 9-dioxide 1 (20.0 g, 76.3 mmol) and then 6 N hydrochloric acid (60 mL) and Antifoam 204 (200 mg, 1 wt % relative to 1) were charged to a 250-mL, three-neck round-bottom flask, equipped with a mechanical stirrer and a thermocouple. The reaction was then heated to reflux, and the distillate was collected until the internal temperature reached 100 eC; after that, the reaction was allowed to reflux for an additional 8 hours or until LC/MS showed a complete conversion of 1 to 2. The reaction was then cooled to room temperature and held for ~12 h when the product crystallized from the solution. The slurry was cooled to 0 °C for 1 h and then filtered. The cake was dried in a vacuum oven at 60 eC, with nitrogen bleed, overnight to give the product as a white, crystalline solid 1,1-Dioxo-tetrahydrothiopyran-4-carboxylic acid (8.84 g, 49.6 mmol, 65% yield.
