Current Organic Chemistry - Volume 4, Issue 10, 2000

African plants have long been the source of important products with nutritional and therapeutical value. Coffee originates from Ethiopia, Strophanthus species are strong arrow poisons and supply cardenolides for use against cardiac insufficiency, the Catharanthus roseus alkaloids are well-known antileukaemic agents - just to mention a few examples. Research is continuing on the vegetable material from this continent in an endeavour to find new compounds of therapeutic interest. An outline is presented here covering the results obtained by the Institute of Pharmacognosy and Phytochemistry of the University of Lausanne during 15 years work on African plants. The strategy employed for the study of these plants is outlined, covering all aspects from the selection of plant material to the isolation of pure natural products. Different bioactivities have been investigated: the search for new antifungal, molluscicidal and larvicidal agents has been the most important axis. Results are also included for antibacterial, cytotoxicity, anti-inflammatory testing.

The Meliaceae family is well represented in Southern and Eastern Africa and Madagascar. Plants range in size from magnificent forest trees to small shrubs. Certain species are important as timber trees and they have wide-ranging uses in ethnomedicine in the region, prompting extensive investigation. Twenty-two of the fifty-one genera of the Meliaceae occur in the geographic area under review, and, of these, the chemistry of forty-four species, from nineteen genera, has been investigated. Compounds isolated include limonoids, mono-, di-, sesqui-, and triterpenoids, coumarins, chromones, lignans, flavonoids and other phenolics.Relatively few of the compounds and extracts from these species have been screened for biological activity, probably due to the limited screening facilities available in the area. However, properties including cytotoxicity against tumour cell lines, insect anti-feedant and anti-malarial activity, and uterotonic activity suggest that further extensive biological screening of compounds from this family is warranted.The similar chemistry of the genera Ptaeroxylon and Cedrelopsis support their grouping together in the distinct family Ptaeroxylaceae. Examination of the chemistry of species from this family suggests a close relationship with the Cneoraceae family.

The genus Aloe (Asphodelaceae), with nearly 420 species confined mainly to Africa, has over the years proved to be one of the most important sources of biologically active compounds. Over 130 compounds belonging to different classes including anthrones, chromones, pyrones, coumarins, alkaloids, glycoproteins, naphthalenes and flavonoids have so far been reported from the genus. Although many of the reports on Aloe are dominated by A. vera and A. ferox, there have also been a number of fruitful phytochemical studies on many other members of the genus. In this review an attempt is made to present all compounds isolated to date from Aloe. The biogenesis and chemotaxonomic significance of these compounds are also discussed.

The genus Dorstenia contains many plants that are used as anti-snakebite, anti-infection and anti-rheumatic remedies in the medicinal plant therapy of many countries in Africa, Central and South America. The genus is now recognized as a rich source of prenyl and geranyl-substituted coumarins, chalcones, flavones and flavanones. The first examples of naturally occurring styrenes (2 and 3) were reported from D. barnimiana. The geranyl substituted furocoumarin (5) has been found in many species. D. poinsettifolia furnished the unusual 4-phenyl substituted dihydrocoumarin, and the rare geranyl and prenyl substituted chalcone (17). Prenylated flavonoids have so far been reported from African Dorstenia only. Compound 18 from D. proropens is the only example of a bis-geranylated chalcone in the literature. D. manni furnished the novel chalcone (13), 6-prenylchrysoeriol (22 R OMe), the geranyl derivative (43), eight diprenylated flavanones (28, 30-36) and one flavonol (29). These dorsmannins appear to be derived from 6,8-diprenyleriodictyol and a possible biosynthetic scheme for them is presented (Fig. 5). Compound 21 was reported from D. zenkeri and is believed to arise from 20. The latter is probably formed via an enzymatic Diels-Alder reaction of isobavachalcone (10) and its dehydroderivative (19). D. psilurus provided all the triprenylated compounds (37-42) found in Dorstenia and, of these, 42 is unique in having ring B of the flavonoid structure modified to a dienone. The pharmacological data on this genus are scanty. Extracts of D. multiradiata show antileishmanial activity. Extracts and/or compounds from other species show anti-inflammatory, analgesic, anti-oxidant and cytotoxic activities.