Pyrroloquinolones B–F: Five unusual alkaloids from Vernonia glabra (Steetz) Vatke (Asteraceae)

Five unusual alkaloids featuring a pyrrolo[1,2-a]quinolone skeleton (pyrroloquinolones B–F, 1–5) were isolated from the ethanol extract of the whole plant of Vernonia glabra (Steetz) Vatke, along with sixteen known compounds. Their structures were established by means of spectroscopic (1D and 2D NMR, UV, IR, and ECD) and high resolution mass spectrometric techniques as well as by comparison of their spectroscopic data with those reported in the literature. The ethanol extract and some isolated compounds were assessed for their antibacterial activity against four bacterial strains. The extract was significantly active against Staphylococcus aureus ATCC1026 and S. epidermidis ATCC35984 (MIC = 64 μg/mL). All the tested compounds showed moderate activity against S. epidermidis (16 ≤ MIC ≤ 64 μg/mL). Furthermore, this is the first report on tricyclic pyrrolo[1,2-a]quinolone alkaloids from a plant source. A biosynthetic pathway for the formation of these compounds is also proposed. [Display omitted] •Five unusual alkaloids were isolated from Vernonia glabra (Steetz).•This is the first report of pyrrolo[1,2-a]quinolone alkaloids from a plant source.•The structures were established using spectroscopic and spectrometric data.•A biosynthetic pathway of the formation of these compounds is proposed.•Some of the isolated compounds were tested for their antibacterial activity.