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

Abstract

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