The structure of orobanchol, a new seedgerminationstimulant for clover broomrape (Orobanche minor), was proposed as 5a (tentative absolute configuration) on the basis of GC-MS comparison of the natural product with several synthetic compounds [(±)-4a–(±)-4h, (±)-5a and (±)-5b]. All of the synthetic compounds showed significant seedgermination activities for Orobanche minor and witchweed (Striga asiatica)