Antibacterial flavonoids and other compounds from the aerial parts of Vernonia guineensis Benth. (Asteraceae)†

An extensive phytochemical study of the aerial parts of Vernonia guineensis Benth. (Asteraceae) led to the isolation of a new flavone, vernoguinoflavone (1) and a naturally isolated glycerol ester, bisarachidicester (2), together with eighteen known secondary metabolites including quercetin (3), luteolin (4), vernopicrin (5), vernomelitensin (6), β-amyrin (7), oleanolic acid (8), ursolic acid (9), lupeol (10), betulinic acid (11), β-carotene (12), a mixture of stigmasterol (13) and β-sitosterol (14), β-sitosterol-3-O-β-D-glucoside (15), 2,3-dihydroxypropyl heptacosanoate (16), pentacosanoic acid (17), docosan-1-ol (18), tritriacontan-1-ol (19), and heptatriacontan-1-ol (20). Compounds 3, 4, 7-9, 12 and 16-20 are reported herein for the first time from this species. The structures of these compounds were elucidated based on extensive spectroscopic analyses, particularly 1D and 2D NMR, and HR-ESI-MS and by comparison of their data with those reported in the literature. The crude extract, fractions and some isolated compounds were evaluated for their antibacterial activity against Gram-negative bacteria: Escherichia coli (ATCC 25922), Shigella flexineri (NR 518), Salmonella Muenchen, Salmonella typhimurium and Salmonella typhi (ATCC 19430). All the tested compounds demonstrated inhibitory activities against the tested enteric bacteria with MIC values ranging from 3.12 to 100 µg/mL. Flavonoids 1, 3 and 4 isolated from the most active fraction demonstrated the best bioactivities against Escherichia coli, Salmonella Muenchen and Salmonella typhimurium with MIC values ranging from 3.12 to 25 µg/mL.

Keywords: Asteraceae; Vernonia guineensis; antibacterial; flavone; glycerol ester.