Chemical prospecting of medicinal plants for drug discovery.
African traditional medicine relies largely on the abundant African flora estimated at several tens of thousands of species. These plants, like other living organisms, produce natural products which are organic molecules exhibiting a remarkable wide range of chemical diversity and a multiplicity of biological properties. Over the past 20 years, interest in drugs of plants origin has been reviving and growing steadily. Among the broad spectrum of natural products that are showing promise as possible leads to useful therapeutic agents are the terpenoids. In the present study, selected African medicinal plants were investigated for the presence of extractable and exploitable terpenoids as leads or raw materials for producing more potent bioactive compounds for pre-clinical drugs discovery programme for chemoprotective agents against cancer, HIV/AIDS, diabetes, hypertension, malaria and other chronic diseases. The plants investigated in this study included Callistemon salignus, C. viminalis, Melaleuca bracteata var. revolution gold, M. bracteata var. revolution green, M. trichostachya var. compata, Syzygium aromaticum and Tectona grandis. These plants were subjected to two separate regimes of phytochemical extraction protocols namely volatile and non-volatile-extraction protocol. The Callistemon species and Melaleuca species upon hydrodistillation afforded essential oils. The gas chromatographic and mass spectrometric (GC-MS) analysis of these essential oils reveals that 1,8-cineole was the major constituent of the Callistemon oils. Similarly, 1,8-cineole was the major constituents of the essential oil of M. trichostachya var. compata, while methyl eugenol was the predominant constituent of the oils of Melaleuca bracteata var. revolution gold, and M. bracteata var. revolution green. Antibacterial investigation of the essential oils showed that they possess strong to moderate inhibitory effect against selected bacteria. In the non-volatile extraction protocol, various parts of the plants were sequentially extracted with organic solvents to obtain crude extracts which were subjected to fractionation and purification protocols (chromatographic techniques and re-crystallization). The crude extracts from the leaves of the Callistemon and Melaleuca species gave a crystalline mixture of betulinic acid and oleanolic acid in an appreciable yield. The crude extract from the cloves of Syzygium aromaticum yielded oleanolic acid as the major extractive and maslinic acid as minor extractive. The crude extracts from Tectona grandis afforded betulinic acid in an appreciable yield. The elucidation of the structures of the pure extractives was achieved by extensive 1D and 2D nuclear magnetic resonance (NMR) spectroscopy as well as infra-red spectroscopy (FT-IR) and mass spectrometry (MS). Betulinic acid and oleanolic acid were chosen as seed molecules for making known and unknown derivatives for lead optimization study. The semi-synthesized compounds were 3- acetoxyoleanolic acid, 3-acetoxyoleanolic hydrazide, 3-acetoxyloleanolic hydrazone, 3-succinyl oleanolic acid, 3-acetoxybetulinic acid, 3-succinylbetulinic acid and maslinic acid di-acetate.