<jats:p>Certain samples of essential oils from Myoporum deserti, M. betcheanum, M. acuminatum, Eremophila maculata, and E. latrobei have been shown to contain myoporone and/or 10,ll-dehydromyoporone. The ambiguity regarding the stereochemistry of myoporone (previously known from M, bontioides) has been resolved,(-)-myoporone being shown to possess the S configuration by degradation to (S)-(+)-2-methylglutaric acid. (-)-Dehydromyoporone, m.p. 32.5-33.5, [x]D ? 15�, has the S configuration as shown by degradation to (S)-(-)-3-methyladipic acid and hydrogenation to (S)-(-)-myoporone, m.p. 17.5-19�, [x]D-5.3�. Both ketones activate the typical ngaione pathology on intraperitoneal injection into mice but myoporone is not toxic to sheep. Myoporone from Myoporum species shows wide variations in optical purity.