Détection et isolement des constituants chimiques des fruits d’Alchornea cordifolia Schum. & Thonn. Muell. Arg. (Euphorbiaceae) et activités antimicrobiennes des composés isolés

Abstract

The research of bioactive secondary metabolites from natural resources proves sometimes exasperating in the absence of a guide like the LC-MS for detection. It is in this context the present work was first of all focused on the detection, then the isolation of secondary metabolites of Alchornea cordifolia (Euphorbiaceae), and finally the evaluation of antimicrobial activity of isolated compounds. From the CH2Cl2/MeOH crude extract of the fruits of Achornea cordifolia, we have detected and identified by LC-MS technique, compounds not yet described in the literature from the plant. Among these compounds, two (02) were new derivatives and ten (10) were isolated for the first time from A. cordifolia by usual chromatographic methods such as column chromatography (CC), thin layer chromatography (TLC). The structures of these isolated compounds were characterized by spectroscopic techniques (MS, 1D and 2D NMR) and by comparison with the literature data. The two new compounds were alchornoic acid derivatives namely deepoxyalchornoic acid and bisalchornoicester. Ten (10) were isolated for the first time included 2,3-dihydroxypropyloctacosanoate, 1-tricosanol, 1-triacontanol, n-hexacosanoic, n-octacosanoic, dotriacontanoic, 4-O-methylgallic and shikimic acids, kaempférol-3-O-galactopyranoside, lupeol, betulinic acids. The remaining five (05) isolated compounds have been previously described in the literature. These include: alchornoic acid, methyl gallate, gallic acid, friedelin, epifriedelinol, in addition to the mixture of β-sitosterol and stigmasterol. Four of these compounds which were isolated in large amount, were screened for their antibacterial activities against six Gram-positive bacteria strains, among which three Escherichia coli strains (ATCC 25922, clinical, ATCC 35218), two Klebsiella pneumonia strains (ATCC 700603, ATCC 13883) and one Citrobacter braakii strain (clinical), with ciprofloxacin for the positive control. The results of these assays revealed significant inhibitory activities of methyl gallate (20⎯25 mm of ZOI) and shikimic acid (12⎯15 mm of ZOI) against all the tested strains. Gallic acid was only active on the Citrobacter braakii strain (ZOI = 10 mm) and the new compound deepoxyalchornoic acid significantly inhibited the growth of Citrobacter braakii (ZOI = 20 mm) and Escherichia coli (ZOI = 25 mm).