Étude des constituants chimiques et évaluation de quelques activités biologiques de deux plantes médicinales camerounaises : Rinoria oblongifolia C.H. Wright (Violaceae) et Sarcocephalus pobeguinii Hua ex Pobég (Rubiaceae)

Abstract

This thesis is written in the framework of the research axis within the Laboratory of Natural Products and Valorization that is led by Professor Nkengfack. The main objective of the work was to identify natural bioactive molecules with cytotoxic and antibacterial potentials from Cameroonian medicinal plants belonging to the Violaceae and Rubiaceae families. Our study focused on two plants, Rinorea oblongifolia and Sarcocephalus pobeguinii from the genus Rionrea and Sarcocephalus, respectively. The choice of these species was based on their various therapeutic uses in traditional medicine and chemotaxonomic data reported in the literature. From our phytochemical investigations of extracts from these two plants, 29 compounds were isolated using chromatographic techniques. Of these 29 compounds; 11 were isolated from Sarcocephalus pobeguinii, and the 18 others from Rinorea oblongifolia. Among all the isolated compounds, three were described for the first time in this study; they included two derivatives of coruleoellagic acid 53',4,5,5'-tetraméthylcoruleoellagic acid and 3', 4,4 ', 5, 5'-pentaméthylcoruleoellagic acid) and a new friedelane-type triterpene derivative (rinol). The structures of all these compounds were elucidated by using spectroscopic techniques including NMR 1D and 2D as well as mass spectrometry (FAB, ESI) and by comparison of their data with those reported in the literature. To confirm their chemical structures, chemical transformations such as methylation and acetylation were carried out on some isolated compounds. Regarding the biological aspect of the work, crude extracts and some of the pure compounds isolated were evaluated, firstly for their antibacterial activities against several pathogenic strains of microorganisms and secondly for their cytotoxic potential with respect to five cancer cell lines (MCF-7, HeLa, Caco-2, A549, HepG2). For the antibacterial activity, the results obtained showed that, from all the tested compounds, only 3,3 ', 4,4', 5'- pentamethylcoruleoellagic acid was active against the bacterial strain Haemophilus influenza, with a MIC value of 23.21 µM. Regarding cytotoxic activity, the results obtained indicate that the extracts from the leaves exhibit cytotoxic activities against MCF-7, HeLa and Caco-2 cancer cells, while betulinic and hederagenin showed better activities with regard to the HepG2 cancer strain with inhibitory concentration (IC50) values ranging from 1.68 to 3.30 μM.