CC(OC1CCCCO1)(C)C#C[C:5](=[O:11])[C:6]([O:8]CC)=[O:7].[C:20]1([CH:26]2[CH2:30][S:29][C:28](=[S:31])[S:27]2)[CH:25]=CC=C[CH:21]=1.C(Cl)Cl>C1(C)C(C)=CC=CC=1>[CH3:25][C:20]1([CH3:21])[C:26]2[S:27][C:28](=[S:31])[S:29][C:30]=2[C:5](=[O:11])[C:6](=[O:7])[O:8]1Ethyl 5-methyl-2-oxo-5-(tetrahydro-2H-pyran-2-yloxy)hex-3-ynoate (2) (1.0 g, 3.7 mmol) and 4-phenyl-1,3-dithiolane-thione (2.0 g, 9.3 mmol) were dissolved in xylene (15 mL). The deep yellow solution was heated to 130° C. for ca. 4 h under Ar (the reaction was followed by TLC, eluent: CH2Cl2). The solvent was removed under reduced pressure and purification of the residue by chromatography (silica gel, CH2Cl2) provided 4,4-dimethyl-2-thioxo-4H-[1,3]dithiolo[4,5-c]pyran-6,7-dione (4) as a yellow solid. Yield: 0.44 g (48%). 1H-NMR spectrum in CDCl3 (ppm): 1.88 (s, 6H). 13C-NMR spectrum in CDCl3 (ppm): 205.3, 167.7, 162.6, 157.8, 153.6, 83.2, 31.9. Selected IR (neat, cm−): 1747, 1681, 1557, 1460. MS-ESI calculated for C7H8O4S2Na [M+Na]+ 268.96. found 268.79.
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