Etude phytochimique des plantes du genre campylospermum (c. Dybowskii van tiegh, c. Vogelii hook. Fil. Farron, et c. Zenkeri van tiegh. Farron) ; modifications structurales et détermination de l’activité antiplasmodiale

Abstract

The aim of this work is to search secondary metabolites with potential antiplasmodial activities from three cameroonian species belonging to the genus Campylospermum (C. dybowskii Van Tiegh., C. vogelii Hook. Fil. Farron. and C. zenkeri Van Tiegh. Farron) in order to fight against the phenomena of resistance. This is to extract, isolate, and to elucidate secondary metabolites from these plants. Subsequently, hemisynthetic tests were carried out on serotobenine, as well as an evaluation of the antiplasmodial activity of the crude extracts, of the isolated compounds and of compounds derived from hemisynthesis. From the phytochemical study, a total of 22 different compounds have been isolated. From the stem barks of C. zenkeri a total of ten compounds were isolated in which two are new biflavonoids [(dihydrozenkerinol (75) and zenkerinol (76)], and eight knowed compounds [(linoleic acid (71), stearic acid (72), palmitic acid (73), serotobenine (18), β-sitostérol-3-O-β D-glucopyranoside (55), lophirone A (50), lophirone F (74), agathisflavone (45)]. The methanol extract of the leaves of C. dybowskii gave seven knowed compounds [Serotobenine (18), β sitostérol-3-O-β-D-glucopyranoside (55), 4'-O-méthylvitexine (77), 2''-O-acetyl-7-O methylvitexine (27), amentoflavone (35), robustaflavone (40), lanceoline C (60)]. The methanol extract of the roots of C. vogelii led to the isolation and identification of five knowed compounds [stigmasterol (78), β-sitostérol-3-O-β-D-glucopyranoside (55), serotobenine (18), campylospermine (22), ß-D-fructofuranosyl-α-D-glucopyranoside (79)] together with a new biflavonoïd [trans-3-hydroxyapigenyl-3'-O-4'''-dihydrokampférol (80)]. From the stems of C. vogelii five previously reported compounds were isolated [β-sitosterol (81), β-sitostérol-3-O-β D-glucopyranoside (55), 3-O-methylquercetin (83), betulinic acid (82), serotobenine (18)]. For a better understanding of structure-activity relationship, three new semisynthetic derivatives of serotobenine have been prepared [serotobenine monopropionate (84), serotobenine monopivalate (85), and 4'-O-cyclohexylserotobenine (86)]. Characterization of the compounds was done using a combinaison of spectroscopic techniques including UV, 1D NMR (1H, 13C, DEPT), 2D NMR (HSQC, HMBC, NOESY, COSY) and mass spectrometry. Some crude extracts, isolated compounds and serotobenine derivatives were evaluated for antiplasmodial activities against the chloroquine-sensitive (CQS) 3D7 and (CQR) Dd2 and 7G8 strains of P. falciparum. Among the pure compounds, campylospermine (22) was the most active against the (CQR) 7G8 strains (IC50 = 5.9155 ± 0.1612 μg/ml), compared to the DHA (IC50 = 0.002121 ± 0.0002 μg/ml) as template, under Sybr Green 1 Fluorescence-based assay. xxxiv Among crude extracts, C. zenkeri showed a significant activity (IC50 = 50 µg/mL) against Dd2 CQR strain of the P. falciparum compared to Artemisinin (IC50 = 15.46 µg/mL) as template. The in vitro antiplasmodial activity of two hemisynthetic derivatives were also evaluated on the 3D7 CQS strain of Plasmodium falciparum in a parasite lactate dehydrogenase (pLDH) assay. This reveals that the serotobenine monopivalate presents a moderate activity (36.6 μM) while serotobenine monopropionate which presents a weak activity (123 μM) compared to decursivine (3.37 μM) as template. It should be noted that previous studies have revealed that serotobenine showed no activity on a P. falciparum. Further studies should be devoted to the phytochemical study of the remaining fractions, at testing the other compounds and extracts of Campylospermum species for in-vitro antiplasmodial activity and cytotoxicity.