Terpenoid chemistry. XXI. (–)-Epingaione, (–)-Dehydrongaione, (–)-Dehydroepingaione, and (–)-Deisopropylngaione, toxic furanoid sesquiterpenoid ketones from Myoporum deserti

Abstract

The isolation and properties of four furanoid sesquiterpene ketones from various specimens of the stock-poisoning shrub Myoporum deserti are described. (-)-Epingaione [(-)-(1S,4S)-β-(4,8-dimethyl-6-oxo-1,4- epoxynonyl)furan] may be epimerized to (-)-ngaione either thermally or by base. (-)-Dehydrongaione [(-)-(1S,4R)-β-(4,8-dimethyl-6-oxo-1,4- epoxynon-7-enyl)-furan] is hydrogenated to (-)-ngaione and is equilibrated either thermally or by base with the unconjugated isopropenylic non-8-enyl isomer and (-)-dehydroepingaione [the (1S,4S)- diastereomer], which is hydrogenated to (-)-epingaione. Treatment of dehydrongaione and its epimer with aqueous alkali yields (-)- deisopropylngaione and its epimer by a retroaldol reaction.     The argument used by Matsuura¹ to deduce the absolute stereochemistry of (+)-ngaione (ipomeamorone) is invalid since, contrary to the literature report, the ipomic lactone obtained from the oxidation of ngaione is not optically active.     Specimens of M. deserti have been found which yield essential oils containing each of the four furanoid ketones (-)-ngaione, (-)- epingaione, (-)-dehydrongaione, and (-)-dehydroepingaione free or virtually free from the others. The presence of a few percent of (-)- deisopropylngaione in one (dehydrongaione-rich) M. deserti oil has been demonstrated.     (+)-Ngaione, (-)-epingaione, and a mixture of (-)-dehydrongaione and (-)-dehydroepingaione all cause the typical (-)-ngaione liver pathology when injected intraperitoneally into mice. M. deserti plants containing the latter three ketones must be regarded as potentially capable of inducing M. deserti poisoning in stock. A mixture of (-)- deisopropylngaione and (-)- deisopropylepingaione causes both kidney and liver lesions.