Terpenoid chemistry. XVII. (–)-Ngaoine, a toxic constituent of Myoporum deserti. The absolute configuration of (–)-Ngaione

Abstract

Varieties of the shrub Myoporum deserti A. Cunn. (Ellangowan poison bush), which causes heavy losses of stock in inland Australia, yield at least nine different essential oils, most of which are toxic. One oil from plant material of the Longreach chemical race from Blackall, Clermont, Hannaford, Longreach, Meandarra, Moree, and Roma contains the toxic furanoid sesquiterpene ketone, (-)-ngaione, as the principal constituent while two others (the Mitchell and the Blackall races) contain lesser proportions of ngaione. (-)-Ngaione comprises over half of the essential oil of a "greenish leaf" variety of Eremophila latrobei F. Nuell. and a low proportion of the oil of a "silver leaf" variety of the same species. The trans isomer of ngaione [(-)-epingaione], present in low concentration in some of these essential oils, is formed from ( -)-ngaione during fractional distillation or more rapidly by base-catalysed epimerization. "Isongaione acetate" prepared from (-)-ngaione by treatment with acetic anhydride-sodium acetate is shown to be a mixture of a, β- and β, γ-unsaturated ketones and obtainable in optically active form contrary to literature statements. Oxidation of the optically active β,γ-unsaturated ketone yields (-) acetylmalic acid from which it follows that (-)-ngaione is l(S),4(R)-β-(4,8-dimethyl-6-oxo-1,4-epoxynonyl)furan, ipomeamarone is the 1(R),4(S) enantiomer, and (-)-epingaione is the 1(S),4(S) isomer.