Flavonoids from Pterocaulon alopecuroides with Antibacterial Activity

Three flavonoids, (2R,3R)-5,4'-dihydroxy-3'-O-methyl-7-(gamma,gamma-dimethylallyloxy)dihydroflavonol 1, (2R,3R)-5,7,4'-trihydroxy-3'-O-methyl-6-(alpha,alpha-dimethylallyl)dihydroflavonol 2, and (2R,3R)-5,7,4'-trihydroxy-6-(alpha,alpha-dimethylallyl)dihydroflavonol 3, together with three known flavonoids (4-6), were isolated from the aerial parts of Pterocaulon alopecuroides. The structures of the compounds were determined by mass and by 1 D and 2 D NMR spectroscopy. Screening of the antibacterial activity of all six compounds was conducted by a disc diffusion test against Bacillus cereus, Bacillus subtilis, Salmonella typhimurium and Proteus mirabilis. The minimum inhibitory concentration (MIC) of the active compounds (2, 3, 4, 6) was determined by a microdilution assay. These compounds were active only against both Gram (+) bacteria with MIC values <= 200 mu g/mL.