New constituents from the roots of Oenothera biennis and their free radical scavenging and ferric reducing activity

•Isolation of new compounds from the roots of Oenothera biennis.•Two new compounds identified as dihydroxyprenyl xanthone acetylated and cetoleilyl diglucoside.•Study of antioxidant activities of new and previously isolated compounds.•The compounds were investigated for the antioxidant effects using several different methods.

In an effort to discover biologically active molecules from plants, the present study reports isolation of two new compounds [dihydroxyprenyl xanthone acetylated (1) and cetoleilyl diglucoside (2)] along with four known compounds [oenotheralanosterol A (3) and B (4), oenotheraphenoxylactone (5), and oenotheraphytyllactone (6)] from the roots of Oenothera biennis and their free radical scavenging and ferric reducing activity. The structures of the new compounds (1-2) were elucidated by 1D and 2D NMR spectral methods, viz.: COSY, HSQC, HMBC, and DEPT aided by FABMS, ESIMS, IR spectroscopy, and elemental analysis. Compounds 1-6 along with mixture of oenotheralanosterol A and B (7) and dihydroxyprenyl xanthone (8) were evaluated for both scavenging and reducing potential using an array of in vitro assay system. Compound 7 displayed 69.83% inhibition of DPPH free radical formation whereas the ferric reducing antioxidant power of compound 5 was found maximum which showed significant relation with the presence of high phenolics and NO scavenging. Highest flavonoid content was observed in compound 8 which also scavenges NO radical formation maximally whereas compound 3 exhibit better total antioxidant capacity.