Etude chimique des fractions enrichies en saponines de Acacia polyacantha Willd. (Fabaceae) et Chytranthus macrobotrys Gilg Excell & Mendonça (Sapindaceae) : évaluation de leur activité antifongique

Abstract

The present work focuses on the chemical study of saponins enriched fractions from two Cameroonian medicinal plants, namely Acacia polyacantha (Fabaceae) and Chytranthus macrobotrys (Sapindaceae), followed by the evaluation of their antifungal activity against seven phytopathogenic fungi. The phytochemical study of the roots and stem barks of A. polyacantha using modern chromatographic methods such as HPLC, MPLC, CC, VLC etc, led to the isolation of nine pure compounds, while the phytochemical study of the stem barks of C. macrobotrys led to the isolation of seven pure compounds. These compounds were fully characterized and elucidated through the interpretation of modern NMR spectroscopic and mass spectrometric analyses ( H, C, COSY, HSQC, HMBC, TOCSY and NOESY, HRESI- MS) in comparison with literature data. This led to the discovery of nine triterpenoid saponins named Polyacoside A-F (45, 46, 48, 49, 50, 51), Rubelloside A (53), Elatoside E (56) and Kalinéanoside B (57) ; one phenolic compound derived from vanillic acid, Acapolyoside (59); two terpenoids, oleanolic acid (61) and lup-20(29)-ene-3α,23-diol (62), one sugar, Sucrose (63) and two phytosterols, stigmastérol (64) and stigmastérol 3-O-β-D-glucopyranoside (65). From all the isolated compounds, seven are new derivatives among which six triterpenoids saponins, Polyacoside A-F and the phenolic compound, Acapolyoside. These new derivatives represent an important contribution to the chemotaxonomy of species of Acacia genus, since all the new saponins derivatives have the particularity of having a methoxycinnamoyl group attached to the C-21 position of machaerinic acid; furthermore, the phenolic compound also has the particularity of having two substructures forming cyclisation links between them. The two main saponins enriched fractions were evaluated for their antifungal activity using the Petri dish supplementation method against seven (07) phytopathogenic fungi strains. The saponins enriched fraction APR obtained from of A. polyacantha showed good activity against the Aspergillus fungi genus with IC50 values of 27,38 ± 2,01 and 33,78 ± 3,01 µg/mL as well as a moderate activity against Pythium myriotylum (IC50 value 174,16 ± 1,94 µg/mL), while the saponins enriched fraction CMT obtained from C. macrobotrys showed no activity against all the phytopathogenic fungi tested,.