Acetic Acid → 2,2-Dimethyl-5-(1-methylethylidene)-1,3-dioxane-4,6-dione

Morpholine (261 mg, 261 μL, 2.99 mmol) and acetic acid (200 mg, 191 μL, 3.32 mmol) were added to a solution of 2,2-dimethyl-1,3-dioxane-4,6-dione (24.0 g, 166.17 mmol) in acetone (120 g) at room temperature under argon atmosphere, and the mixture was stirred at room temperature for 18 hr. The reaction mixture was concentrated under reduced pressure. To the obtained residue was added saturated aqueous sodium hydrogencarbonate solution (200 mL), and the mixture was extracted with toluene (200 mL). The organic layer was washed with 10% brine (200 mL), and the solvent was evaporated under reduced pressure to give 5-isopropylidene-2,2-dimethyl-1,3-dioxane-4,6-dione as a white solid. The obtained white solid was subjected to azeotropy with toluene (100 mL) (×2).