[H-].[Na+].[CH2:3]([O:5][C:6](=[O:19])[C:7]([PH2]=O)=[C:8]([CH3:16])[CH:9](OCC)OCC)[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](\[CH3:16])/[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. 4-(Diethoxy-phosphinyl-3-methyl-but-2-enoic acid ethyl ester (12.9 g) was added at 0° C. The mixture was stirred for 15 minutes at 0° C. and for 30 minutes at RT. After recooling to 0° C., 2-bromo-cyclopent-1-ene-carbaldehyde (5.72 g), dissolved in 11 ml of DMF, was added drop by drop and allowed to react for 10 minutes at 0° C. and for 2 hours at RT. The mixture was then poured into crashed ice. extracted with EtOEt, washed with sat. NaCl-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° C.
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