[CH2:1]([N:8]1[CH2:13][CH2:12][C:11](=[O:14])[CH2:10][C:9]1=[O:15])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.[C:16](=[S:18])=[S:17].[OH-].[K+].[CH2:21](Br)[CH2:22]Br>CS(C)=O.O>[CH2:1]([N:8]1[CH2:13][CH2:12][C:11](=[O:14])[C:10](=[C:16]2[S:18][CH2:22][CH2:21][S:17]2)[C:9]1=[O:15])[C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=1
1-Benzyl-2,4-dioxopiperidine (2.03 g) and carbon disulfide (760 mg) are dissolved in dimethylsulfoxide (20 ml), and thereto is added dropwise a solution of potassium hydroxide (1.1 g) in water (3 ml) at room temperature. The mixture is stirred for 30 minutes and thereto is added dropwise a solution of 1,2-dibromomethane (1.87 g) in dimethylsulfoxide (3 ml). The mixture is stirred at room temperature for 2 hours and further at 60° C. for 1 hour. The reaction mixture is poured onto water and extracted with benzene-ethyl acetate. The extract is washed with water, dried and distilled to remove the solvent. The residue is purified by silica gel column chromatography (solvent, benzene:ethyl acetate=8:2) and is recrystallized from ethanol to give 1-benzyl-3-(1,3-dithiolan-2-ylidene)-2,4-dioxopiperidine (1.08 g) as pale yellow needles. m.p. 148°-150° C.
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