Contribution à l’étude chimique de deux plantes médicinales camerounaises (Boswellia dalzielii Hutch. et Echinops gracilis O. Hoffm) et l’évaluation de l’activité biologique de quelques extraits et composés isolés.

Abstract

The present work focuses on the chemical and biological investigation of Cameroonian medicinal plants Boswellia dalzielii (Burseraceae) and Echinops gracilis (Asteraceae), by searching for compounds that could combined various biological activities at once for an optimal therapeutical effect. This is in view of contributing to the fight against microbial infectious diseases. Thus, by means of various liquid phase chromatographic techniques such as Column Chromatography (CC), Thin Layer Chromatography (TLC) and Preparative Thin Layer Chromatography (Prep. TLC), seventeen compounds were isolated from methanol (MeOH), methylene chloride (CH2Cl2) and ethyl acetate (EtOAc) extracts from stem bark of B. dalzielii, roots and aerial parts of E. gracilis. These include five flavonoids (7-hydroxy-4’-methoxy-flavone-6-O-β-Dglucopyranosid), quercetin, apigenin-7-O-(4''-feruloyl)-β-D-glucopyranosid, apigenin-7-O- (4"-trans-p-coumaroyl)-β-D-glucopyranosid, apigenin-7-O-β-D-glucopyranosid), four phenylpropanoids (Erythrinasinate, Vogelate, ferulic acid, p-coumaric acid), two pentacyclic triterpenes (ursolic acid and oleanolic acid), two stilbenes (4'-methoxy-E-resveratrol-3-O-β-Dglucopyranosid and 4’-methoxy-(E)-resveratrol-3-O-('-O-rhamnopyranosyl)-xylopyranoside), one steroid (β-sitostérol-3-O-β-D-glucopyranosid), two benzoic acid derivatives (methyl gallate and gallic acid), and one glucopyranosid (α-D-glucopyranosid). The structures of these compounds were determined by interpretation of their 1D (1H, 13C, DEPT) and 2D (HSQC, HMBC, COSY, NOESY) NMR data and in some cases by comparison of their data with those reported in literature or by comparative TLC with available authentic samples. Of the seventeen isolated compounds, one flavonoid apigenin-7-O-(4"-feruloyl)-β-D-glucopyranosid and one stilbene 4’-methoxy-(E)-resveratrol-3-O- (2''-O-rhamnopyranosyl)-xylopyranosid are reported herein for the first time. some other known compounds identified as erythrinasinate, vogelate, ferulic acid, p-coumaric acid, ursolic acid, oleanolic acid, quercetin, apigenin-7-O-(4"-trans-pcoumaroyl)-β-D-glucopyranosid and apigenin-7-O-glucopyranosid were isolated for the first time from E. gracilis. Some extract and isolated compounds were tested for their antibacterial, antioxidant, anti-inflammatory and cytotoxic activity. The results are as follows: The new compound apigenin-7-O-(4"-feruloyl)-β-D-glucopyranosid isolated from aerial part of E.gracilis exhibited significant activity against Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae with a minimum inhibition concentration (MIC) value of 31.25, 15.63 and 31.25 μg/mL respectively. The EtOAc extract, the new compound apigenin-7-O-(4"-feruloyl)-β-D-glucopyranosid and apigenin-7-O-(4”-trans-p-coumaroyl)-β-D-glucopyranosid, from aerial part of E. gracilis, showed the ability to scavenge 2,2′-Azino-bis-(3-Ethylbenzothiazoline-6-Sulfonic Acid) radical cation (ABTS), with scavenging concentration (SC50) values of 13.59 ± 0.84 µg/mL, 108.25 ± 4.32 µg/mL, and 28.54 ± 2.18 µg/mL respectively Ethyl acetate extract from roots of E. gracilis showed potent inhibitory activity against protein denaturation with an IC50 value of 125.54 µg/mL. Erythrinasinate and vogelate exhibited an anti-inflammatory activity with an IC50 value of 469.43 and 413.71 µg/mL, respectively. In view to cease the opportunity to determine selectivity index and anticancer activity, apigenin-7-O-β-D-glucopyranosid and apigenin-7-O-(4"-feruloyl)-β-D-glucopyranosid displayed a significant cytotoxic effect against cervical cancer cells HeLa at 75 µM after 24 hours with a viability percentage of cancer cells less than 1% for apigenin-7-O-β-Dglucopyranosid and less than 10% for apigenin-7-O-(4"-feruloyl)-β-D-glucopyranosid.