Antimicrobial flavonoids and diterpenoids from Dodonaea angustifolia

The surface exudates of the leaves of Dodonaea angustifolia from Ngong forest population (6km from Nairobi city center, Kenya) demonstrated antimicrobial activity against Gram-negative (Escherichia coli), Gram-positive (Staphylococcus aureus and Bacillus pumilus) bacteria and the fungus Sacchromyces cerevisiae. Chromatographic separation of the exudates yielded eight methylated flavonoids; 5-hydroxy-3, 4′,7-trimethoxyflavone (1), 3,5-dihydroxy-4′,7-dimethoxyflavone (2), santin (3), kumatakenin (4), rhamnocitrin (5), isokaempferide (6), 3,4′,5,7, tetrahydroxy-6-methoxyflavone (7), pinocembrin (8); two clerodanes, dodonic acid (9) and 2β-hydroxyhardwickiic acid (10) and one labdane; (ent-3β,8α)-15,16-epoxy-13(16),14-labdadiene-3,8-diol (11) diterpenoids. The flavonoid aglycones; 6, 7 and the clerodane diterpenoids; 9 and 10 and labdane diterpenoid, 11 were isolated for the first time from this plant species. The structures of the isolated compounds were identified using ultraviolet (UV), mass spectroscopy (MS), one dimension (1D) and two dimension (2D) nuclear magnetic resonance (NMR) spectroscopy and by comparison of the spectral data with literature. The quercetin derivative, 3,4′,5-trihydroxy-3′,7-dimethoxyflavone (12) showed broad spectrum antibacterial activities against E. coli and B. pumilus with minimum inhibition concentration (MIC) values less than 31.25μg/well and against S. aureus with MIC below 62.5μg/well. This compound showed poor antifungal activity against S. cerevisiae (MIC<500μg/well). Good antifungal activities were observed for 5,4′-dihydroxy-7-methoxyflavanone (13) and hautriwaic acid lactone (14) against S. cerevisiae with MIC values less than 7.8μg/well. The most active antifungal compound was 5,7-dihydro-3,4′,6-trimethoxyflavone (3, santin) with an MIC value less than 3.9μg/well against S. cerevisiae. The rest of the compounds exhibited weak to moderate activities. For comprehensive structure activity relationship studies (SAR), hautriwaic acid lactone (14), hautriwaic acid (15), penduletin (16) isolated from the surface exudates of D. angustifolia from Voi (200km from Mombasa city center, Kenya) and 12 and 13 from Senecio roseiflorus isolated earlier were included in the bioassays.