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After 5.28 g (0.04 mol) of dimethyl malonate and 3.66 g (0.048 mol) of carbon disulfide were dissolved in 25 ml of dimethylsulfoxide, 10.9 g of 45% potassium hydroxide aqueous solution was dropwise added to the solution under ice cooling. The mixture was stirred at room temperature for 20 minutes. The resulting reaction solution was dropwise added to a mixture of 19.6 g of 40% chloroacetaldehyde and 2.88 g of glacial acetic acid below 5° C. The mixture was stirred at the same temperature for 30 minutes. The reaction solution was poured onto ice water. The mixture was extracted twice with ethyl acetate followed by washing with water. After drying over magnesium sulfate, the solvent was distilled off under reduced pressure to give dimethyl 3-hydroxy-1,3-dithiol-2-ylidenemalonate. The thus obtained compound and 12.2 g (0.12 mol) of triethylamine were dissolved in 20 ml of dioxan and 6.9 g (0.06 mol) of methanesulfonyl chloride was slowly added dropwise to the solution. After completion of the dropwise addition, the mixture was stirred at room temperature for 10 minutes and then heated to reflux for 10 minutes. The solution was poured into ice water and the mixture was extracted with ethyl acetate. After drying over magnesium sulfate, the solvent was distilled off under reduced pressure. The residue was purified by silica gel column chromatography (n-hexane: ethyl acetate=2:1) to give 4.0 g (yield 43%) of crystals showing a melting point of 125° to 129° C.

1. customThe resulting reaction solution
2. stirringThe mixture was stirred at the same temperature for 30 minutes
3. additionThe reaction solution was poured onto ice water
4. extractionThe mixture was extracted twice with ethyl acetate
5. washby washing with water
6. dry with materialAfter drying over magnesium sulfate
7. distillationthe solvent was distilled off under reduced pressure