Five new stilbene oligomers, laetevirenol A-E (4-8), were isolated from Parthenocissus laetevirens, together with three known stilbene oligomers (2, 3, and 9). The structures of the new compounds were elucidated by spectroscopic analysis, including 1D and 2D NMR experiments. Afterward the absolute configurations were determined. Biomimetic transformations revealed a possible biogenetic route, where stilbene trimers were enzymatically synthesized for the first time. In addition, their antioxidant activities were evaluated by 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. The results showed that stilbene oligomers with an unusual phenanthrene moiety exhibited much stronger antioxidant activities. Thus, the photocatalyzed cyclization of stilbenes was supposed to be an antioxidant activity promoting transformation, which was hypothesized to play a role in the antioxidative defense system of the plant.
Preparative high-speed counter-current chromatography (HSCCC) was successfully applied to the isolation and purification of three stilbene oligomers from Vitis chunganeniss using stepwise elution with a pair of two-phase solvent systems composed of n-hexane-ethyl acetate-methanol-water at (2:5:2:5, v/v) and (1:2:1:2, v/v). The preparative HSCCC separation was performed on 800 mg of crude sample yielding hopeaphenol (21.1 mg), amurensin G (37.2 mg) and vitisin A (95.6 mg) in a one-step separation, with purities over 95% as determined by HPLC. The structures of these three compounds were identified by MS, (1)H NMR and (13)C NMR. In addition, their antioxidant activities were screened by DPPH assay, where vitisin A showed strong antioxidant activity. Further EPR experiments with spin-trapping technique demonstrated that vitisin A is a potent and selective singlet oxygen quencher, which may be used in singlet oxygen-mediated diseases as a pharmacological agent.
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