Terpenoid chemistry. XVIII. Myodesmone and isomyodesmone, toxic furanoid ketones from myoporum deserti and m. acuminatum

Abstract

Two furanoid sesquiterpene ketones (designated myodesmone and isomyo- desmone) have been isolated from the essential oils of certain varieties of Myoporum deserti A. Cunn. and M. acuminatum R. Br. and have been shown to be toxic to mice, rats, and sheep.     (-)-Isomyodesmone, b.p. 107-109°/1 mm, m.p. 46.5-47°, [α]D -193°, is shown to be 1S,5S-2-(β-furyl)-5-methylcyclopent-2-enyl isobutyl ketone and is characterized as a dihydro derivative, m.p. 33-33.5°, and as a glycol, m.p. 173-173.5°. Brief treatment of isomyodesmone with base yields largely the epimeric epiisomyodesmone while prolonged treatment forms an equilibrium mixture of 1% (-)-isomyodesmone, 41% of (+)-epiisomyodesmone, and 58% of (-).myodesmone, oil, b.p. 107-109°/1 mm, [α]D -108°, which is 5S-2-(β-furyl)-5-methylcyclopentenyl isobutyl ketone. The proposed structures are supported by electronic, p.m.r., and mass spectral evidence and confirmed by the degradation of isomyodesmone to (+)-dihydrocitronellic acid and myodesmone to (+)-2- methylglutaric acid.     The toxicity to stock of the Jackson variety of M. deserti known from Jackson, Dulacca, Tchanning Creek, and Myall Park (and the Chinchilla variety of M. acuminatum) is attributed primarily to the presence of myodesmone and isomyodesmone which are present in closely equal amounts and together constitute 50-55% of the whole oil. Alternatively the plant may contain a toxic ketol precursor which is dehydrated to the myodesmones during steam distillation of the leaves.