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.