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Stilbenes in the roots of <i>Carex acuta</i> and <i>Carex lepidocarpa</i> were studied. Root samples were extracted with 100% methanol and analyzed by HPLC and LC-MS. In this way, <i>trans</i>-resveratrol dimers (<i>m</i>/<i>z</i> 455 Da [M + H]⁺), trimers (<i>m</i>/<i>z</i> 681 Da [M + H]⁺) and tetramers (<i>m</i>/<i>z</i> 907 Da [M + H]⁺) were identified in the extracts. Using LC-NMR in stop-flow mode, pallidol and <i>trans</i>-ε-viniferin as dimers were identified. After the separation of individual peaks and their measurement by ¹H NMR, <i>cis</i> and <i>trans</i>-miyabenol A as a tetramer and <i>cis</i>-miyabenol C as a trimer were identified. In the case of miyabenol A, it is a chromatographically inseparable mixture of <i>cis</i> and <i>trans</i> isomers in the ratio of 2:3 according to ¹H NMR measurement. In the case of <i>cis</i>-miyabenol C, the <i>Z-trans-trans</i>-miyabenol C configuration was confirmed. The remaining unidentified peak with a practically identical UV-VIS spectrum to that of <i>cis</i>-miyabenol C is most likely another isomer of miyabenol C.