Constituants chimiques des écorces du tronc de deux plantes du Cameroun: Pauridiantha callicarpoides Hiern (Rubiaceae et Entandrophragma cylindricum Sprague (Meliaceae) ; Transformations chimiques et activités anti-inflammatoires de quelques composés isolés.

Abstract

The present work deals with the study of chemical constituents of two Cameroonian medicinal plants, namely Pauridiantha callicarpoides (Rubiaceae) and Entandrophragma cylindricum (Meliaceae). Twenty six compounds have been isolated from the dichlromethane-methanol extracts of these two plants using conventional chromatographic techniques (CC, TLC, HPLC). These isolated compounds have been classified as follows: - 02 alkaloids: javaniside and 7-epi-javaniside which is a new oxindole derivative. - 01 phenolic acid: vanillic acid or 4-hydroxy-3-methoxybenzoic acid. - 04 coumarins: scopoletin or 7-hydroxy-6-methoxycoumarin, hymexelsin or xeroboside, 7,7'- dihydroxy-6,6'-dimethoxy-3,3'-biscoumarin or 3,3’-biscopoletin and 7,7'-dihydroxy-6,6'- dimethoxy-8,8'-biscoumarin. - 02 flavonoids: (+) -Catechin and epicatechin or epicatechol. - 01 iridoid: sweroside. - 01 limonoid: andirolide G. - 04 steroids: a mixture of β-sitosterol and stigmasterol; a mixture of β-sitosterol 3-O-β-Dglucopyranoside and stigmasterol 3-O-β-D-glucopyranoside. - 01 carbohydrate: 6-O-hexopyranosylhexopyranose. - 10 terpenoids: sapelenin A, (14E,18E)-2,6,10,15,19,23-hexamethyltetracosa-14,18,22-triene- 2,3,6,7,10,11-hexaol, sapelenin B, sapelenin C, sapelenin D, ekeberin D2, and four new derivatives for which we have given the name sapelenins G-J. The structures of these compounds were fully characterized on the basis of their physical (melting point, optical rotation) and spectroscopic data (1D and 2D NMR, MS, IR, UV). Some of the structures were determined by comparison, either of their spectral data with those reported in the literature for analogues or by chemical transformations such as hydrolysis, acetonidation and esterification. Thus, the hydrolysis reactions carried out on hymexelsin and sapelenin H led to scopoletin and ekeberin D2 respectively, which have been already isolated from P. callicarpoides and E. cylindricum. The acetonidation reactions on ekeberin D2 and sapelenin I afforded acetonide of ekeberin D2 and acetonide of sapelenin I which are two new derivatives. The esterification reactions on ekeberin D2, acetonide of ekeberin D2, sapelenin H, sapelenin I and acetonide of sapelenin I yielded eight new derivatives: diacetylekeberin D2, diacetylsapelenin I, and the corresponding Mosher’s esters products. The structures of the products were confirrmed based on the spectroscopic data and by comparison with those of the starting materials. The bioguided investigation carried out on the extract of the stem bark of E. cylindricum have led to the isolation of four new acyclic triterpenes which have been evaluated in vitro for their anti-inflammatory activities through the PGE2 secretion inhibition and the IL-17 secretion inhibition by human PBMCs, and for their cytotoxic activities against human PBMCs. The results obtained show that: - Sapelenins G-J inhibite the secretion of PGE2 with the MIC of 0,001; 0,001; 10,0 and 0,01 μg/mL respectively. - Sapelenin G possesses a strong anti-inflammatory activity through the reduction of the IL-17 secretion by PBMCs stimulated with PHA and presents no cytotoxic effect on PBMCs. - Sapelenin H possesses neither anti-inflammatory activity through the reduction of the IL-17 secretion by PBMCs stimulated with PHA, nor cytotoxic effect against PBMCs. - Sapelenins I and J show cytotoxic effect in dose-dependent manner on the PBMCs, hence their data on the suppression of IL- 17 secretion were invalid at higher concentrations.