Studies On Bioactive Stilbenoids From Vitaceous Plants

Stilbenoids, a kind of important natural polyphenols, have been considered to be a potential source of natural medicine. In recent years, they have received considerable attention in the chemical and biological fields, owing to their structural complexity as well as their diverse bioactivities. The present dissertation described phytochemical investigation on two vitaceous plants, Parthenocissus laetevirens and Vitis chunganeniss, for the first time. This study focused on stilbenoids, especially resveratrol oligomers, from vitaceous plants, including the following five subjects:1. Discovery of natural products with novel structuresTwenty one stilbene oligomers have been isolated from these two vitaceous plants. The structures of the compounds were elucidated by spectroscopic analysis, including MS,1D and 2D NMR experiments, and comparison with data in literatures. Eight of them were new compounds, five of which possessed of novel carbon skeletons. Laetevirenol A and B were the first natural occurring resveratrol dimer and trimer with a phenanthrene structure, respectively. While laetevirenol F and G were the first example of stilbene tetramers from the genus Parthenocissus. In addition, chunganenol was a stilbene hexamer with novel and complex structure, which was the first stilbene hexamer from the Vitaceae family. It was also the first example of two trimers connected by CH2.2. Biomimetic synthesesHorseradish peroxidase, AgOAc and UV irradiation have been applied to the biomimetic syntheses of stilbene oligomers, where stilbene trimers were enzymatically synthesized for the first time. The results of the biomimetic transformations revealed a possible biogenetic route of stilbene oligomers from P. laetevirens. Our study demonstrated a free radical mechanism in the enzyme catalyzed oligomerization of stilbenes, which was able to explain the substrate selectivity in the biomimetic syntheses of stilbene trimers. 3. Absolute configurationStilbene oligomers usually possess of complex structure with many chiral centers, which are difficult to be determined. We succeeded in determination of the absolute stereochemistry of eight stilbene oligomers by CD method and biosynthetic relationship.4. Antioxidant activitiesAntioxidant activities of stilbenoids isolated were screened by DPPH assay. Most of them exhibited strong antioxidant activities. 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 P. laetevirens.Furthermore, the scavenging and quenching effects of pallidol, chunganenol, and vitisin A on reactive oxygen species (ROS) were investigated by EPR spin-trapping technique. The results demonstrated that they were potent and selective singlet oxygen quencher.5. Development of new preparative separation methodCounter-current chromatography in combination with step-wise elution mode was developed and applied to the preparative separation and purification of five antioxidative stilbene oligomers from the crude samples of two vitaceous plants in a one-step separation.