[H-].[Na+].[CH2:3]([O:5][C:6](=[O:19])[CH:7]=[C:8]([CH3:18])[CH2:9]P(OCC)(OCC)=O)[CH3:4].[Br:20][C:21]1[CH2:25][CH2:24][CH2:23][C:22]=1[CH:26]=O>CN(C=O)C>[CH2:3]([O:5][C:6](=[O:19])/[CH:7]=[C:8](\[CH3:18])/[CH:9]=[CH:26]/[C:22]1[CH2:23][CH2:24][CH2:25][C:21]=1[Br:20])[CH3:4]2.03 g of NaH (50% in mineral oil) was suspended in 120 ml of DMF. 12.9 g of 4-(diethoxy-phosphinyl)-3-methyl-but-2-enoic acid ethyl ester was added at 0°. The mixture was stirred for 15 min at 0° and for 30 min at RT. After recooling to 0°, 5.72 g of 2-bromo-cyclopent-1-enecarbaldehyde, dissolved in 11 ml of DMF, was added drop by drop and allowed to react for 10 min. at 0° and for 2 h at RT. The mixture was then poured onto crushed ice, extracted with EtOEt, washed with sat. NaCi-solution, dried over Na2SO4, and evaporated to dryness. Purification of the residue by flash chromatography (silica gel, hexane/AcOEt=97/3) and crystallization from hexane/trace amounts of AcOEt yielded finally 3.408 g of pure (2E,4E)-5-(2-bromo-cyclopent-1-enyl)-3-methyl-penta-2,4-dienoic acid ethyl ester as yellowish crystals of mp. 85°-86°.
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