Spiroalanfurantones A–D, Four Eudesmanolide–Furan Sesquiterpene Adducts with a Pentacyclic 6/6/5/5/5 Skeleton from Inula helenium

Spiroalanfurantones A–D (1–4), four eudesmanolide–furan sesquiterpene adducts with an unprecedented pentacyclic 6/6/5/5/5 skeleton, were isolated from the roots of Inula helenium. Their structures were elucidated by spectroscopic data analysis and single-crystal X-ray diffraction analysis. The plausible biosynthetic pathway for 1–4 is presented. Bioassay showed that compounds 1 and 2 significantly inhibited nitric oxide production in lipopolysaccharide-induced RAW264.7 macrophages with IC50 values of 17.3 and 9.5 μM, respectively.