[CH3:1][C:2]1([CH3:14])[C:7]2[S:8][C:9](=S)[S:10][C:6]=2[C:5](=[O:12])[C:4](=[O:13])[O:3]1.C(Cl)Cl.C(Cl)Cl.CC(O)=[O:23]>C([O-])(=O)C.[Hg+]>[CH3:1][C:2]1([CH3:14])[C:7]2[S:8][C:9](=[O:23])[S:10][C:6]=2[C:5](=[O:12])[C:4](=[O:13])[O:3]1Mercury acetate (181 mg, 0.568 mmol) was added to a stirred solution of 4,4-dimethyl-2-thioxo-4H-[1,3]dithiolo[4,5-c]pyran-6,7-dione (4) (100 mg, 0.406 mmol) in CH2Cl2/AcOH (3:1). The reaction was followed by TLC (silica, CH2Cl2) and after ca. 30 min the mixture is filtered through a celite pad to remove the mercury salts. The resulting solution was washed first with water (3×15 mL) and then with sat. aqueous NaHCO3 (5×10 mL), and dried over MgSO4. Removal of the solvent under reduced pressure afforded pure 4,4-dimethyl-4H-[1,3]dithiolo[4,5-c]pyran-2,6,7-trione (5) a pale beige solid. Yield: 65 mg (70%). 1H-NMR spectrum in CDCl3 (ppm): 1.88 (s, 6H). 13C-NMR spectrum in CDCl3 (ppm): 184.5, 163.5, 160.4, 153.5, 128.3 84.2, 31.8. Selected IR (neat, cm−1): 1746, 1693, 1641, 1552, 1453. MS-ESI calculated for C7H8O4S2 [M+H]+230.98; found 230.78; calculated for C7H8O4S2Na [M+Na]+) 252.96; found 252.79.
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