The preparation of the phosphonium salt and the dimethyl-phosphonate of (E)-5-bromo-3-penten-2-one ethylene ketal, 6 and 7, is described. These C5-isoprenoid synthons were condensed with several aldehydes and ketones giving mixtures of the corresponding E and Z-olefination products. Reaction of the anion of the phosphonate derivative with β-ionone gave 9-cis and 9-trans-β-C18-tetraenone ketals, 13 and 14, which by subsequent hydrolysis afforded 9-cis and 9-tras-β-C18-tetraenones, 15 and 16. A C8-isoprenoid synthon is also described. © 1981.