Etude phytochimique de trois plantes médicinales du Cameroun à activités antimicrobiennes: Ficus polita Vahl, Ficus exasperata Vahl (Moraceae) et Albizia ferruginea (Guil. & Perr.) Benth. (Fabaceae), et quelques transformations chimiques sur l’acide bétulinique.

Abstract

The first part of this work concerns the phytochemical study of three Cameroonian plants: Ficus polita Vahl, Ficus exasperata Vahl (Moraceae) and Albizia ferruginea Guill. & Perr. (Fabaceae). Next is the chemical transformations of some derivatives of betulinic acid and finally with evaluation of the antimicrobial activity of the extract, fractions and secondary metabolites isolated from Ficus polita Vahl and Albizia ferruginea Guill. & Perr. The first chapter is the literature survey of the genera Ficus and Albizia. The second chapter describes the isolation, the purification of compounds from the three plants, their characterisation. The compounds were isolated by usual chromatographic methods (CC, flash chromatography) and characterised by routine spectroscopic techniques including 1H and 13C (1D and 2D) NMR, mass spectrometry, IR and UV spectroscopy. Twenty-six compounds have been obtained and the structure have been completely elucidated with two described for the first time. These new compounds include a cerebroside: (2R, 9Z)-2-hydroxy-N-{(1S,2S,3S,4S)-1-[(β-D-glucopyranosyl)methyl]-2,3,4-trihydroxyoctacosan-1-yl}-9-pentamide for which the trivial name politamide has been given; and a flavonoid: (2R,3R,4R)-4-(4-hydroxyphenyl)-4-methoxychroman-3,7,8-triol called albiziaflavane A. The twenty-four other compounds comprise thirteen pentacyclic triterpenoids: (lupeol acetate, lupeol, 16-hydroxylupeol, betulin, betulinic acid, β-amyrin, α-amyrin, ursolic acid, oleanolic acid, β-amyrin acetate, euphol-3-O-cinnamate, fridelan-3-one, and taraxerol), four sterol derivetives (β-sitosterol, stigmasterol, β-sitosteryl 3-O-β-D-glucopyranoside and spinasterol), three saponins (julibroside A1, julibroside A2 and julibroside A3), two coumarines (psoralen and bergapten), two flavanes (4’,7-dihydroxyflavan-3,4-diol), a stilben (trans-piceid) and a xanthone (lichexanthone). In addition to the phytochemical aspect, three derivatives of betulinic acid were made, one new: 3β-acetoxy-28-(1’’,3’’,4’’-oxadiazole)lup-20(29)-en and the other benzyl 3β- hydroxylup-20(29)-en-28-oate, acetate of betulinic acid are known compounds. The antimicrobial activities of compounds from A. ferruginea and of the methanol extract from the roots of F. polita, fractions, and some compounds isolated from it have been evaluated. The results show that trans-piceid, euphol-3-O-cinnamate, albiziaflavane A and julibroside A3 show weak or moderate selective activities. In fact trans-piceid presented the highest inhibitory activity (MIC 64 µg/mL) similar to that of the reference antibiotic on Pseudomonas aeruginosa. Chapter three presents the experimental condition used for extraction, the isolation of natural products, synthesis, physical and spectroscopic data as well as the methods of evaluation of the antimicrobial activities of different samples tested.