Flavonoids and their glycosides, including anthocyanins

Flavanoids and chromanoids including flavanones, flavanols, chromanones, and chromanols are a large family of well-known natural products (Scheme 1). 2] These natural products have been shown to exhibit a wide range of biological activities including anticancer, antitumor, antibacterial, antimicrobial, antioxidant, estrogenic, and antiestrogenic properties. Additionally, these chiral flavanoids and chromanoids could readily be utilized in the synthesis of many benzopyran-containing natural products by established reaction pathways (Scheme 1). Although there are numerous reports on the synthesis of flavanones, flavanols, chromanones, and chromanols, only few stereoselective methods have been devised to access enantiomerically enriched products. In fact, to the best of our knowledge, so far no transition-metal-catalyzed method has been reported for the highly enantioselective synthesis of 2-substituted flavanols and chromanols despite their importance. Owing to minimal negative environmental impact, high atom economy, and operational simplicity, the transition-metal-catalyzed asymmetric hydrogenation has long held a respected position both in academia and in industry for the preparation of optically active compounds. However, even though the enantioselective hydrogenation of natural and commercial chromones and flavones seems to be one of the most straightforward ways to synthesize flavanones, flavanols, chromanones, and chromanols, the asymmetric catalytic hydrogenation of chromones and flavones remains largely unexplored. It would be highly desirable to develop a general, efficient catalytic system for the asymmetric hydrogenation of these heterocycles. During the course of our investigation into the asymmetric hydrogenation of (hetero)arenes, we found that the combination of ruthenium(II) and several NHCs (NHC = N-heterocyclic carbene) led to the formation of highly active and enantioselective catalysts for the hydrogenation of quinoxalines, benzofurans, and (benzo)thiophenes. 10] In light of the high level of reactivity and enantioselectivity of our Ru–NHC catalyst, we rationalized that chromones and flavones could be hydrogenated directly to flavanols and chromanols in a stereoselective manner. Further we could easily obtain the enantiomerically enriched flavanones and chromanones by a simple and selective subsequent oxidation. Herein we report the first asymmetric hydrogenation of chromones and flavones using a chiral NHC–Ru complex. This direct and general route is capable of accessing biologically active enantiomerically enriched flavanoids and chromanoids comprising all the above-mentioned structures: flavanones, flavanols, chromanones, and chromanols (Scheme 2). Scheme 1. Representative structures of biologically active flavanoid and chromanoid natural products.

Ruthenium-NHC-catalyzed asymmetric hydrogenation of flavones and chromones: general access to enantiomerically enriched flavanones, flavanols, chromanones, and chromanols.

Ferns are an important phytogenetic bridge between lower and higher plants. Historically they have been used in many ways by humans, including as ornamental plants, domestic utensils, foods, and in handicrafts. In addition, they have found uses as medicinal herbs. Ferns produce a wide array of secondary metabolites endowed with different bioactivities that could potentially be useful in the treatment of many diseases. However, there is currently relatively little information in the literature on the phytochemicals present in ferns and their pharmacological applications, and the most recent review of the literature on the occurrence, chemotaxonomy and physiological activity of fern secondary metabolites was published over 20 years ago, by Soeder (Bot Rev 51:442–536, 1985). Here, we provide an updated review of this field, covering recent findings concerning the bioactive phytochemicals and pharmacology of fern species.

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Phytochemicals from fern species: potential for medicine applications

Gastroprotective flavonoid constituents from Oroxylum indicum Vent.

Anti-hyperuricemic and nephroprotective effects of Smilax china L.

Four new flavan-4-ol glycosides, abacopterins A-D (1-4), were isolated from a methanol extract of the rhizomes of Abacopteris penangiana, together with two known compounds, triphyllin A (5) and 6,8-dimethyl-7-hydroxy-4'-methoxyanthocyanidin-5-O-beta-D-glucopyranoside (6). Their structures were elucidated by extensive spectroscopic analysis and chemical methods. The cytotoxic activity of 1-5 against HepG2 human hepatoma cells was investigated.

Flavan-4-ol Glycosides from the Rhizomes of Abacopteris penangiana.

Six new flavanone glycosides (1-6) were isolated from the methanol extract of the rhizomes of Cyclosorus acuminatus, together with the parent flavanone glycoside 2a. Their structures were established on the basis of spectroscopic and chemical methods. All compounds showed moderate activity against Streptococcus pneumoniae and Haemophilus influenzae.

Acetylated flavanone glycosides from the rhizomes of Cyclosorus acuminatus.

Objective To study the chemical constituents of the 95% ethanol extract from the rhizome of Smilax china. Method Seven compounds were isolated from ethyl acetate fraction of 95% ethanol extract by using repeated silica column chromatography, partition chromatography, preparing TLC, and other methods. Structure of three compounds of them was identified based on chemical methods and spectroscopic methods. Results The structure of them were established as dihydrokaempferol-5-O-beta-D-glucoside (I), beta-sitosterol (II), daucosterol (III). Conclusion Compound I was a novel compound which was first found from natural plants.

Jian Zou

Papers

Flavan-4-ol Glycosides from the Rhizomes of Abacopteris penangiana.

Acetylated flavanone glycosides from the rhizomes of Cyclosorus acuminatus.

Studies on chemical constituents of Smilax china