| Summary: | Heliaquanoid A (1), the first exo-2,4-linked Diels−Alder adduct between a pseudoguaianolide dienophile and a guaianolide diene, and heliaquanoids B−E (2−5), four new 2,4-linked Diels−Alder adducts between a xanthanolide dienophile and a guaianolide diene, were isolated from stems and leaves of Inula helianthus-aquatica. Their structures were determined by the NMR spectroscopy, modified Mosher′s method, electronic circular dichroism, and X-ray diffraction analysis. Compounds 2 and 3 exhibited moderate cytotoxic activities against HL-60 cells with IC50 values of 7.5 and 4.9 μM, respectively.
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