trans-2-Methyl-2-butenal → 3,5,7-Trimethyl-2E,4E,6E,8E-decatetraene

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Reaction Steps

(1-Methyl-(E)-2-butenyl)triphenylphosphonium bromide from Example 9 (0.62 g, 0.0015 mole) was added to a dry flask with 5 ml tetrahydrofuran. The flask was equipped with magnetic stirrer and septum; the reaction was carried out under nitrogen. The salt did not dissolve completely but became a sticky suspension. The mixture was cooled over ice, and butyllithium (2.5M in hexane) was added dropwise, with stirring, until the color change became permanent; then an additional 0.0015 mole was added. The solid in the flask dissolved as it was converted to the ylide. One hundred milligrams of 2,4-dimethyl-(E,E)-2,4-hexadienal (0.0008 mole), compound 4, R1 =H, prepared previously by Pattel and Pattenden (1985), was added to the Wittig reagent, and the mixture was allowed to warm to room temperature. The mixture was stirred for 2 hr, and it was again cooled over ice. Water was added dropwise until the red color of the solution had disappeared, and ca. 2 ml more water was added. The mixture was diluted with hexane and the organic layer dried over sodium sulfate. The solvent was removed and the product passed through a silica column with hexane. By capillary GC the product was 61% the (E,E,E,E) isomer, ca. 31% the (E,E,Z,E) isomer, and ca. 8% by-products, after clean-up on silica. Further purification on the AgNO3HPLC column yielded the (E,E,E,E) isomer in >97% purity.

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