Heliaquanoids A–E, Five Sesquiterpenoid Dimers from Inula helianthus-aquatica

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 NMR spectroscopy, a 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.