Guaianolides from (Kovalevsk.) Kovalevsk.

In continuation of our phytochemical study on the aerial part of (Kovalevsk.) Kovalevsk, three new guaiane-type sesquiterpene lactones were isolated namely karatanolide (), 11,13-dihydrokaratanolide () and 11,13-dihydrokaratanolide (), in addition to the two known sesquiterpene lactones 1,10-dihydroxyarglabin () and artefin (). The structures of the isolated sesquiterpene lactones were elucidated by IR and NMR spectroscopy. Single crystal X-ray diffraction allowed to unambiguously confirm the structure of the isolated sesquiterpene lactones. Quantum chemical calculations of the guaianolides suggested the existence of two conformations for the seven-membered cycle. All sesquiterpene lactones from were isolated for the first time.