Synthetic Study on Bioactive Oligomeric and Polymeric Flavonoids

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Natural polyphenols have traditional relation to human life as ingredients of wine, tea or herbal medicines, which has stimulated the search for new pharmaceutical candidates relevant to polyphenols. However, the biological studies of polyphenols are often hampered by the difficulty in isolating respective compounds in pure and structurally-defined forms, because the major availability has relied on natural sources, e. g., plant extracts, which are generally a mixture of closely related compounds, hardly tractable even by the aid of modern separation and analytical methods.This study focused on development of general synthetic method enabling access to various catechin derivatives, and exploited its selective oligomerization. Monomeric catechins are synthesized from 1, 3, 5-trifluorobenzene via intermolecular S_N2 reaction with the B-ring fragment followed by intramolecular S_NAr reaction. Oligomerizations of monomers are conducted based on the orthogonal synthetic strategy, giving higher oligomers ranging from 3-mer to 24-mer. The method was applicable to the synthesis of the hetero-trimer, which was isolated from Ziziphus jujuba, composing of epiafzelechin, epigallocatechin, and catechin units.