[CH2:1]([N:3]1[CH2:8][CH2:7][C:6](=[O:9])[CH2:5][C:4]1=[O:10])[CH3:2].[H-].[Na+].[I-].CS[C:16]1[S:20][CH:19]=[CH:18][S+:17]=1>CN(C)C=O>[CH2:1]([N:3]1[CH2:8][CH2:7][C:6](=[O:9])[C:5](=[C:16]2[S:20][CH:19]=[CH:18][S:17]2)[C:4]1=[O:10])[CH3:2]1-Ethyl-2,4-dioxopiperidine (1.06 g) is dissolved in N,N-dimethylformamide (50 ml), and the solution is added dropwise to a suspension of sodium hydride (60% dispersion in oil, 332 mg) in N,N-dimethylformamide (50 ml) under ice-cooling. The mixture is stirred for 20 minutes, and thereto is added 2-methylthio-1,3-dithiolium iodide (1.89 g). After stirring at room temperature overnight, the reaction mixture is poured onto water and extracted with ethyl acetate. The extract is washed with water, dried and distilled to remove the solvent. The residue is purified by silica gel column chromatography (developer: ethyl acetate) and then recrystallized from ethyl acetate-isopropyl ether to give 1-ethyl-3-(1,3-dithiol-2-ylidene)-2,4-dioxopiperidine (1.10 g) as pale yellow needles. m.p. 114.5°-117° C.
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