3.1.3. Flavonols

In the A ring, many studies have confirmed that hydroxylation at positions 5 and 7 together are critical for the antibacterial activity of flavonols against *S*. *aureus* strains [56]. In addition, hydroxylation on the B and C rings also increased the antimicrobial activity of these compounds. A comparison of compounds with the same structure showed that kaempferol with a hydroxy group at C-4 had less activity than galangin (without OH at C-4) against *S*. *aureus* (Figure 1D) [47]. The number of glycosyl groups instead of the hydroxy group at position 3 has been found to have a significant effect on antibacterial activity. For example, among the compounds extracted from *Maytenus buchananii*, quercetin-3-O-[ α-L-rhamnopyranosyl-(1 → 6)-β-D glucopyranoside] with a disaccharide group at the same position was the better inhibitor of *S*. *aureus* growth than amentoflavone-7,4- dimethyl-ether with monosaccharide group (quercetin-3-O-β-D-glucopyranoside) [57]. Substitution of the methoxy group at position 3 decreased the antimicrobial activity. For example, piliostigmol (with OMe and Me groups at positions 6 and 7 of the A ring and OH at position 3) was more active against *S*. *aureus* than 6-C-methylquercetin-3,3,7-trimethyl ether (with OMe at the C-3 position) [58].
