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C-glucosidic ellagitannins from Lythri herba (ph. eur.) and their microbial metabolites
Lythri herba, a pharmacopoeial plant material (Ph. Eur.), is obtained from the flowering parts of purple loosestrife (Lythrum salicaria L.). Although extracts from this plant material were proven to possess some interesting biological activities and its pharmacopoeial standardization is based on total tannin content determination, the phytochemical characterization of this main group of compounds has not been yet fully conducted. Five main C-glucosidic ellagitannins – monomeric: vescalagin and castalagin together with new dimeric structures salicarinins A-C composed of: vescalagin and stachyurin, vescalagin and casuarinin, castalagin and casuarinin units connected via formation of valoneoyl group – were isolated using column chromatography and preparative HPLC. Structures were determined according to 1H and 13C NMR (1D and 2D), ESI-TOF, MSn and circular dichroism spectra, together with acidic hydrolysis products analysis. Formation of potentially bioactive dibenzo[b,d]pyran-6-one derivatives – urolithins A, B and C – by the human intestinal flora was established for Lythri herba extract and vescalagin.