Phenolics from Mikania micrantha and Their Antioxidant Activity.

A phytochemical study on the aerial parts of Mikania micrantha led to the isolation of two new phenolic compounds, benzyl 5- O -β-d-glucopyranosyl-2,5-dihydroxybenzoate ( 1 ) and (7 S ,8 R )- threo -dihydroxydehydrodiconiferyl alcohol 9-acetate ( 2 ), together with twelve known compounds, benzyl 2- O -β-d-glucopyranosyl-2,6-dihydroxybenzoate ( 3 ), 4-allyl-2,6-dimethoxyphenol glucoside ( 4 ), (+)-isolariciresinol ( 5 ), icariol A₂ ( 6 ), 9,10-dihydroxythymol ( 7 ), 8,9,10-trihydroxythymol ( 8 ), caffeic acid ( 9 ), p -coumaric acid ( 10 ), ethyl protocatechuate ( 11 ), procatechuic aldehyde ( 12 ), 4-hydroxybenzoic acid ( 13 ), and hydroquinone ( 14 ). Their structures were elucidated on the basis of extensive spectroscopic analysis. Except 8 and 9 , all the other compounds were isolated from this plant species for the first time. The antioxidant activity of those isolated compounds were evaluated using three different assays. Compounds 1 , 2 , 3 , 9 , 10 , 13 , and 14 demonstrated significant 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) free radical cation scavenging activity ranging from SC50 0.31 to 4.86 µM, which were more potent than l-ascorbic acid (SC50 = 10.48 µM). Compounds 5 , 9 , 11 , and 12 exhibited more potent 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity (SC50 = 16.24-21.67 µM) than l-ascorbic acid (39.48 µM). Moreover, the ferric reducing antioxidant power (FRAP) of compounds 2 , 5 , 9 , and 11 were discovered to be also comparable to or even more potent than l-ascorbic acid.