Isolation and characterisation of secondary metabolites of two asteraceae species, Artemisia afra and Elytropappus rhinocerotis.

Abstract

In this study two medicinal plant species, namely Artemisia afra Jacq. ex. Willd and Elytropappus rhinocerotis Less. (L.f) (= Dicerotamnus rhinocerotis Koekemoer), both belonging to the family Asteraceae, have been investigated and different compounds isolated and characterised. Both species are important plants used in traditional medicine in general in Africa and particularly in South Africa. A. afra, commonly called "African wormwood" is one of 400 species belonging to the genus Artemisia and it is the only one indigenous to Africa. E. rhinocerotis is one of eight Elytropappus species which are all restricted to the Cape floristic region. The aim of this study was to investigate the phytochemistry of these species. In total fifteen compounds were isolated and characterised. From the E. rhinocerotis extract, four known compounds, labdanolic acid, methyl labdanolate, 6, 7- dimethoxycoumarin and a sesquiterpene viridiflorol were isolated. These compounds were not previously reported from E. rhinocerotis. Two different chemotypes of A. afra were studied and eleven compounds were isolated. These compounds include sesquiterpenes such as taurin, artesin, maritimin, artemin, norsantolinifolide, santolinifolide A, and reynosin, a flavonoid, 5- hydroxy-7,4-dimethoxyflavone, a coumarin called scopoletin or 7-hydroxy-6- methoxycoumarin and other aromatic compounds such as p-hydroxyacetophenone, and 2,4-dihydroxy-6-methoxyacetophenone. Except for taurin, scopoletin and 5-hydroxy-7, 4- dimethoxyflavone , none of these other compounds has been reported previously from A. afra. This study has shown that A. afra contains a large number of sesquiterpenoids, mostly from the eudesmane-type. Structural elucidation of different compounds was performed using mainly NMR spectroscopy. Other methods used for identification include LC-MS and infrared spectroscopy. The major compound, labdanolic acid, is known to selectively inhibit cyclooxygenase-2, an enzyme associated with inflammation. The presence of labdanolic acid in the plant may account for its traditional use as an anti-inflammatory.