Due to their antioxidant potential, many naturally occurring phenolic compounds play an important role in the protection against various diseases. Their biological properties and mode of action may be affected by O-substituents, e.g. the presence of sugar residues. However, the O-glycosylation methods for low-nucleophilic or hindered phenolic compounds are not always satisfactory in term of yield as well as their regio- and stereoselectivity. In the present work, a variety of ribosylation approaches have been studied for two poliphenolic compounds, a biologically significant flavone, quercetin (1), and resveratrol (2), a compound occurring in the skin of grapes and red wine. Ribosylation of quercetin with 1,2,3,5-tetra-O-acetyl-β-d-ribofuranose in the presence of tin tetrachloride, i.e. under conditions which were successful in synthesis of genistein 4’-O-ribosides [1], failed completely. Similarly, the use of p-toluenesulfonic acid as a catalyst did not result in the formation of quercetin ribosides. Some promising results came from ribosylation experiments performed in the presence of boron trifluoride – diethyl etherate. The reactions yielded a mixture of α- and β-ribosides of quercetin, and the study on their structures, ratio and isolation are in progress. In the case of resveratrol, glycosylation with tetraacetylribose and tin tetrachloride gave a complicated mixture of compounds, from which an interesting product, 4-C-α,β-riboside could be isolated in a moderate yield. The application of boron trifluoride – diethyl etherate for ribosylation of resveratrol increases the yield of O-ribosides, as it has been judged from our preliminary experiments.

Acknowledgements: This work was supported by the network "Synthesis, structure and therapeutic properties of compounds and organic substances".

[1] J. Boryski, G. Grynkiewicz Synthesis-Stuttgart, 2170-2174 (2001).

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