Adenosma bracteosum and Vitex negundo are natural sources of methoxylated flavonoids. Little is known about the α‐glucosidase inhibition of multi‐methoxylated flavonoid derivatives. Eighteen natural flavonoids were isolated from A. bracteosum and V. negundo. Seven halogenated derivatives were synthesized. Their chemical structures were elucidated by extensive NMR analysis and high‐resolution mass spectroscopy as well as comparisons in literature. All compounds were evaluated for their α‐glucosidase inhibition. Most compounds showed good activity with IC50 values ranging from 16.7 to 421.8 μM. 6,8‐Dibromocatechin was the most active compound with an IC50 value of 16.7 μM. A molecular docking study was conducted, indicating that those compounds are potent α‐glucosidase inhibitors.