Isolement et caractérisation des constituants chimiques des feuilles de deux plantes de la famille des Rubiaceae : Tricalysia atherura N. Hallé et Tricalysia oligoneura K. Schum; évaluation de leurs activités sur certaines parasitoses (paludisme et leishmaniose)

Abstract

The phytochemical study of the leaves of T. atherura N. Hallé and T. oligoneura K. Schum led to the isolation and characterization of twenty-one (21) compounds, seven (7) of which are newly described. The structures of all these compounds were established through extensive analysis of 1D (1H, 13C) and 2D (COSY, HSQC, HMBC and NOESY) NMR spectra coupled with highresolution (HR-ESI-MS) and low-resolution (IE-MS) mass spectrometry, and by comparison with literature data. The MeOH extract of T. atherura leaves led to the isolation of 11 compounds, four (04) are newly described, they are: two ursane-type glycosilated triterpenes [atherurosides A and B (146- 147)]; an akuammidine-type indolic alkaloϊd, atheruramine (148) and a hydrobenzoϊn, tricalydioloside (149). The seven other compounds already described in the literature have been grouped into different classes, including four triterpenes [betulinic acid (151), ursolic acid (154), rubrinol (155); 19α- hydroxy-α- amyrin (156)]; two alkaloϊds [a diterpenic alkaloϊd, tricalysiamide D (153); an indolic alkaloϊd, strictosidinic acid (150)] and a diterpene, tricalysiolide B (152). The MeOH extract of T. oligoneura leaves led to the isolation of ten (10) compounds, including three (03) new derivatives: an oleanan-type glycosylated triterpene, oligoneurin (159); an nparaffinic alcohol, nonacosan-1,10-diol (157); and a phytosphingosine-type ceramid, oligoneuramide (158). The other seven (07) compounds already described in the literature were grouped into different classes including two sugars [β-D- fructofuranosyl- α-D- glucopyranosyl (151) and D-mannitol (162)]; a lignan, asarinin (164); a fatty acid, oleic acid (165)]; two steroϊds [β-sitosterol (163) and β-sitosterol-3-β-D- glucopyranoside (160)] and a triterpene, aridanin (166). Concerning the antiparasitic activities of the crude extracts and some of the pure compounds isolated, they were evaluated on the one hand for their in vitro antiplasmodial activities on CQS and CQR of Plasmodium falciparum strains 3D7 and Dd2 using the Sybr-green- I fluorescencebased method and on the other hand, for their antileishmanial activities on Leishmania donovani 1S promastigotes (MHOM/SD/62/1S) using the Resazurin colorimetric assay-based method. With regard to antiplasmodial activity, the crude extract of Tricalysia atherura leaves showed good antiplasmodial activity against Plasmodium falciparum strains 3D7 and Dd2, with IC50 values of 7.923 ± 0.530 and 4.196 ± 0.278 μg/mL respectively. With regard to antileishmanial activity, oligoneurin (159) and oligoneuramide (158) exhibited moderate leishmanicidal activities with IC50 values of 21.10 and 23.87 μg/mL respectively. In addition, the crude extract showed good activity on the same strain with an IC50 value of 13.62 μg/mL and a relatively low selectivity (SI = 5.05), which would indicate a plausible toxicity of the plant for human consumption.