The synthesis of 5-methoxy-3-(8′Z,11′Z)-pentadeca-8′,11′,14′-trienylbenzene-1,2,4-triol (18), a germination stimulant for Striga asiatica(witchweed), isolated from Sorghum bicolor, is reported. The key step was a (Z)-stereoselective Wittig reaction between the ylide derived from 3-(Z)-hepta-3,6-dien-1-yltriphenylphosphonium iodide (7) and 8-(2,3,6-triacetoxy-5-methoxyphenyl)octanal (16). The synthesis of the structurally related catechol, 3-(8′Z,11′Z)-pentadeca-8′,11′,14′-trienylbenzene-1,2-diol (22), which occurs in the Anacardaceous species,Toxicodendron radicans(poison ivy), is also reported.