[CH3:1][S:2]([CH3:4])=O.[H-].[Na+].[F:7][C:8]1[CH:13]=[CH:12][C:11]([CH2:14][C:15]([O:17][CH:18]([CH3:20])[CH3:19])=[O:16])=[CH:10][CH:9]=1.BrCCBr.[C:25](=S)=[S:26]>O.CCOCC>[S:2]1[CH2:4][CH2:25][S:26][C:1]1=[C:14]([C:11]1[CH:10]=[CH:9][C:8]([F:7])=[CH:13][CH:12]=1)[C:15]([O:17][CH:18]([CH3:20])[CH3:19])=[O:16]
Dry dimethyl sulphoxide (180 ml) is added to sodium hydride (9.0 g, 55% dispersion in mineral oil). After 30 minutes, isopropyl 4-fluorophenylacetate (17.7 g) and carbon disulphide (8.2 g) are added. The mixture is stirred for two hours at room temperature. 1,2-Dibromoethane (18.6 g) is then slowly added. The reaction is exothermic and the temperature is maintained below 30°. The mixture is stirred for one hour, and then ether (200 ml) is added. After cooling to 0° water (200 ml) is slowly added. The two phases are separated. The aqueous phase is extracted with ether (2×100 ml) and the combined organic solutions are washed with water (2×50 ml). After drying over magnesium sulphate, the solvent is evaporated in vacuo and the residue is recrystallised from hexane. The product is dried in vacuo to constant weight to yield isopropyl (1,3-dithiolan-2-ylidene)-(4-fluorophenyl)-acetate, m.p. 106°-107°.
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