Etude chimique de deux plantes médicinales camerounaises : Anonidium mannii (Oliver) Engler et Diels (Annonaceae) et Donella welwitschii (Engl) Pierre ex Aubr. et Pelegr. (Sapotaceae) et évaluation de leurs activités antibactériennes.

Abstract

The present work reports on the chemical study of the dichloromethane-methanol (1:1) extracts of the stem and root barks of Anonidium mannii (Oliver) Engl. et Diels (Annonaceae) and liana of Donella welwitschii (Engl) Pierre ex Aubr et Pellegr. (Sapotaceae), together with the evaluation of the antibacterial activities of the extracts and some isolates. Fourteen (14) and thirteen (13) compounds were isolated from the extracts of A. mannii and D. welwitschii respectively using different chromatographic methods. The structures of these twenty-seven compounds were determined by usual spectroscopic methods such as (1D and 2D) NMR, MS, and IR. Three isolated compounds are described here for the first time: Two tetracyclic triterpenes Anomanol A, B, one isoquinoline alkaloid, 9-hydroxy-8- methoxyphenanthridin- 6(5H) –one. In addition, the alkaloid 9,8-dimethoxyphenanthridin- 6(5H)–one previously known as a synthetic compound is herein reported as naturally occurring for the first time. All these compounds were isolated from A. mannii. The twenty-three other compounds are classified as follows: - Three alkaloids : manniindole, Arborinine and oxoanolobine; - Eleven triterpenes: β-amyrin, β-amyrin acetate, taraxerol, α-amyrin acetate , taraxeryl acetate, polycarpol, erythrodiol, lanosta-7,9(11), 24-triene-3 ß,21-diol, ursolic acid, oleanolic acid, diopyric acid B; - Six steroids : β-sitosterol, β-sitosterol 3-O-β-D-glucopyranoside, spinasterol, spinaterol 3-O-β-D-glucopyranoside, stigmasterol and stigmasterol 3-O-β-D- glucopyranoside; - Three phenolic compounds: 3,4-dihydroxybenzoic acid, vanillic acid, 3-C-β-D- glucopyranosyl-2,4’,4,6-tetrahydroxylbenzophenone. The crude extracts of root and stem barks as well as sixteen isolated compounds were evaluated for their antibacterial activities against thirteen strains of multidrug-resistant includind nine Gram-negative and three Gram-positive. Broth microdilution method was used for the determination of Minimal Inhibitory Concentration with Ciprofloxacin and chloramphenicol as reference drugs. The results showed that root extract was the most active with a MIC values at 64 μg/mL on the Klebsiella pneumoniae (ATCC 11296) strain. Moreover, the sixteen compounds tested were active on at least one isolate of all the germs .(E. coli, E. aerogenes, K. pneumoniae, P. stuartii, P.s aeruginosa and S. aureus). However, diospyric acid B exhibited significant activity with MIC of 4 or 8 μg/mL on E. coli AG102, E.aerogenes ATCC13048, K. pneumoniae KP55, P. stuartii PS2636 et S. aureus MRSA3. This compound was more effective than chloramphenicol on E. coli AG100Atet with MIC value of 4 μg/mL. Erythrodiol and oleanolic acid exhibited significant activity with MIC value of 8 μg/mL on E.aerogenes EA27, K. pneumonia ATCC11296 E. coli AG102, P. stuartii PS2636 and S.aureus MRSA6. Anomanol A showed moderate activity with MIC values of 32 and 16 μg/mL on E. coli AG100Atet and K. pneumoniae ATCC 11296 respectively. Anomanol B and 9-hydroxy-8-methoxyphenanthridin-6(5H)-one showed weak activity. These results show that diospyric acid B, anomanol A, and oleanolic have antibacterial potentials thus can be exploited in the design of drugs against bacterial infections hence justifying the traditional use of A.mannii and D. welwitschii against abdominal pain