Isolation, Synthesis and Bioassay of Natural 6,8-Di-C-glycosylflavonoids

霍山石斛 (Dendrobium huoshanense) 為常用中藥，原產於中國安徽霍山，有退熱、清肝明日等療效。近年來從霍山石斛分離出含有芹菜素配醣體的活性成分，可幫助視網膜色素上皮細胞層 (Retinal Pigment Epithelium, RPE) 清除沉積廢物以維持視網膜的正常功能。然而，從天然物得到的種類有限，因此，我們希望藉由化學合成的方法建構一系列芹菜素配醣體的衍生物，並搭配生物活性檢測，探討結構與活性之間的關係。本篇論文著重於芹菜素配醣體的位向選擇性合成，首先使用柚皮素作為起始物進行選擇性地保護，再藉由兩次醣化作用依序將選定之配醣體引入二氫芹菜素衍生物的C-6和C-8位置，並透過官能基和保護基之轉換，得到具位向選擇性的芹菜素配醣體。此外，再經2‒碘醯基苯甲酸 (2-iodoxybenzoic acid, IBX) 與二硫亞磺酸鈉 (Na2S2O4) 的氧化–還原作用亦可得到木犀草素配醣體。透過上述過程合成的衍生物即能作為後續生物活性檢測之用。Dendrobium huoshanense is a traditional medicinal plant that has been proven to enhance body immunity, scavenge active oxygen metabolites, prevent ophthalmic disorders, inhibit inflammation and suppress the growth of cancer cells in recent pharmacological studies. Since apigenin 6,8-di-C-glycosides were isolated from this herb with limited amount and species in previous work, our study focused on the synthesis of the analogues of apigenin 6,8-di-C-glycosides. Posterior to selective benzylation, acetylation and reductive transformation of the starting material, (±)-naringenin, 6-C-glycosylation was promoted by trimethylsilyl trifluoromethanesulfonate (TMSOTf) via O-to-C glycoside rearrangement. Followed by hydrogenolysis, 8-C-glycosylation, benzylation and sequential oxidations with ceric ammonium nitrate (CAN) and pyridinium dichromate (PDC), dehydrogenation with iodine and the subsequent deprotection of benzyl and acetyl groups gave a series of apigenin 6,8-di-C-glycosides. In addition, luteolin 6,8-di-C-glycosides were obtained by employing a one-pot two-step hydroxylation with 2-iodoxybenzoic acid (IBX) and sodium dithionite (Na2S2O4) in a sequence. Therefore, this strategy created an access to the synthesis of 6,8-C-glycosylflavonoids bearing different glycosides at the designated positions with stereoselectivity. These analogues were further screened for their bioactivities, such as the anti-inflammatory activity, to establish the structure–activity relationship.Acknowledgements……………………………………………………………………ibstract in Chinese…………………………………………………………………iiibstract in English…………………………………………………………………vable of Contents…………………………………………………………………viindex of Figures……………………………………………………………………xndex of Schemes…………………………………………………………………xiindex of Tables……………………………………………………………………xivbbreviations………………………………………………………………………xvhapter 1. Introduction……………………………………………………………1.1 Flavonoids…………………………………………………………………1.1.1 General Classification of Flavonoids………………………………1.1.2 Glycosylflavonoids…………………………………………………3.2 C-Glycosylflavonoids………………………………………………………4.2.1 Naturally Occurring C-Glycosylflavonoids………………………4.2.2 Biological Properties of C-Glycosylflavonoids……………………6.2.3 Strategy of C-Glycosylation………………………………………11.2.4 Previous Syntheses and Reactions of C-Glycosylflavonoids……14.3 Retina……………………………………………………………………19.3.1 Retinal Pigment Epithelium (RPE)………………………………20.3.2 Relationship between the RPE and Oxidative Stress……………23.4 Dendrobium huoshanense………………………………………………27.4.1 Pharmacological Properties………………………………………28.4.2 C-Glycosylflavonoids in D. huoshanense…………………………29hapter 2. Results and Discussion………………………………………………31.1 Analysis of C-Glycosylflavonoids from D. huoshanense……………31.1.1 Isolation of Glycosylflavonoids from D. huoshanense…………31.1.2 Structural Determination of 6,8-Di-C-glycosylflavonoids………38.2 Synthesis of 6,8-Di-C-glycosylflavonoids Bearing Different Glycosides……41.2.1 Synthetic Strategy…………………………………………………41.2.2 Determination of the Structures of Flavone 6,8-Di-C-pentosides…53.3 Bioassay of 6,8-Di-C-glycosylflavonoids………………………………56.4 Conclusion………………………………………………………………60hapter 3. Experimental Section…………………………………………………63.1 General Part………………………………………………………………63.2 Collection of Plant Material……………………………………………64.3. Extraction and Isolation Procedures……………………………………64.4. Anti-inflammatory Procedures…………………………………………66.5. Synthetic Procedures……………………………………………………66eferences…………………………………………………………………………113ppendix: 1H and 13C NMR Spectra…………………………………………13