Phytochemical studies of Diospyros gilletii De Wild and Diospyros fragrans Gürke (Ebenaceae), chemical transformations and antibacterial, antioxidant and cytotoxic activities of extracts and isolated compounds

Abstract

The present thesis focused on the isolation, characterization and chemical transformations of some secondary metabolites of the species Diospyros gilletii De Wild and Diospyros fragrans Gürke, two Cameroonian medicinal plants belonging to the Ebenaceae family, followed by a subsequent evaluation of the antioxidant, antibacterial and cytotoxic properties of extracts and isolates. From these two plants, thirty-four compounds were isolated using liquid phase chromatographic methods, twenty eight of which were fully characterized and belong to various classes of secondary metabolites. These include five (05) isocoumarins among which two (02) new assigned as 4-O-p-hydroxybenzoylnorbergenin and 11-O-(E)- cinnamoylnorbergenin and three known identified as norbergenin, 4-O galloylnorbergenin and 11-O-p-hydroxybenzoylnorbergenin; one (01) new naphthalene derivative, the 3,6-dihydroxy-8-methyl-3,4-dihydronaphthalen-1(2H)-one or fragranone, Twelve (12) pentacyclic triterpenes identified as ursolic acid, oleanolic acid, corosolic acid, rotundic acid, myrtifolic acid, betulinic acid, vismiaefolic acid, uvaol, lupeol, betulin, hederagenin and β-amyrin acetate; one (01) polyterpene, the α-tocopherol; one monoglyceride (01), the 1-O-(28-hydroxyoctacosanoyl) glycerol; one carotenoid, the luteine; four sterols (04) identified respectively as a mixture of stigmasterol + β-sitosterol and a mixture of 3-O-β-D-glucopyranoside of stigmasterol + β-sitosterol; three (03) polyols, identified as quercitol, 5-O-methyl-myo-inositol and methyl-β-D-glucopyranoside. The structures of these compounds were established by means of spectroscopic techniques including IR, UV, MS, NMR 1D and 2D, by X-ray diffraction for some and by comparison of their spectral data with those reported in the literature. 4-O-p-hydroxybenzoylnorbergenin, ursolic acid, betulinic acid and vismiaefolic acid underwent chemical transformation by acetylation and allylation, leading to their acetylated and allylated derivatives, among which those of 4-O-p-hydroxybenzoylnorbergenin were new derivatives. Crude extracts and some of the isolated and hemisynthetic compounds were evaluated for their antibacterial activities against five strains of microorganisms using agar disk diffusion and microdilution method, for their cytotoxic effect on human cervix carcinoma cell line KB-3-1 and human colon cancer cell line HT-29 using resazurin reduction assay and for their antioxidant activities using DPPH method. The results from antibacterial activities showed that the most active compounds were myrtifolic acid and acetylated betulinic acid against Bacillus subtilis with MIC values of 31.3 and 250 μg/ mL respectively. The results from cytotoxic activities showed that corosolic acid was the most active compound on both cancer cells KB-3-1 and HT-29 with IC50 values of 14.6 and 16.5 μM respectively. The antioxidant activity showed that all the five isocoumarins exhibited good activities with IC50 values between 8.2-144 μg/mL.