Sodium hydride (60% dispersion in oil, 175 mg) is suspended in dimethylformamide (10 ml), and thereto is added dropwise a solution of ethyl 1-benzyl-5-(1,3-dithiol-2-ylidene)-4,6-dioxopiperidine-3-carboxylate (1.3 g) in dimethylformamide (30 ml) under ice-cooling. The mixture is stirred at the same temperature for 20 minutes. Methyl iodide (0.7 ml) is added to the mixture, and the mixture is stirred under ice-cooling for 1.5 hours and then at room temperature for 1.5 hours. The mixture is poured into water and extracted with ethyl acetate. The extract is washed with a saturated sodium chloride solution, dried and then distilled to remove the solvent. The residue is recrystallized from a mixture of ethyl acetate and n-hexane to give ethyl 1-benzyl-5-(1,3-dithiol-2-ylidene)-3-methyl-4,6-dioxopiperidine-3-carboxylate (1.15 g, 85.2%) as pale yellow needles, m.p. 111°-113° C.
