The Wittig condensation of 6,10,14-trimethylpentadeca-3,5,9,13-tetraen-2-one (I) with triethyl phosphonoacetate (II) by means of NaH in refluxing hexane gives ethyl 3,7,11,15-tetramethyl-2,4,6,10,14-hexadecapentaenoate (III), which is then hydrolyzed with KOH in hot isopropanol.