The Australian desert shrub Eremophila longifolia (R. Br.) F. Muell. (Schrophulariaceae), as a potential source of karahanaenone and other terpenoids

Abstract

Eremophila longifolia (R. Br.) F. Muell. (Schrophulariaceae) is a shrub endemic to the arid and semi-arid regions of Australia. The plant is used in contemporaneous indigenous traditional medicine and has been cited extensively in medical ethnobotanical literature [1]. This work forms part of an ongoing project investigating variation in essential oil yield and composition from this species. Here we report on the discovery of two novel essential oil chemovarieties, one rich in isomenthone (5-methyl-2-propan-2-ylcyclohexan-1-one), and the other yielding large quantities of the monocyclic heptene ketone karahanaenone (2,2,5,-trimethyl-4-cyclohepten-1-one), Yield and composition was investigated for hydrodistilled leaf essential oils of specimens occurring in semi-arid areas in the state of NSW. Essential oil composition was investigated using a combination of GC/GC-MS. The identity of karahanaenone was established by NMR spectroscopy. Isomenthone type specimens produced essential oils (3.12% –5.65% w/w yield) containing 61.1% –86.7% isomenthone. Karahanaenone type specimens produced essential oils (0.46% –1.90% w/w yield) containing 81.0% –82.2% karahanaenone. This relatively rare natural product and its derivatives are of considerable value to the flavour and fragrance industry and several methods of synthesis from a variety of precursors have been developed [2–5]. Given the potential commercial value of these essential oils as a source of naturally occurring karahanaenone and other compounds, we believe further investigation is warranted into the geographic distribution of these varieties, along with assessment of commercial viability as an agricultural crop for oil production in Australia.