Neojaponicone A, a bioactive sesquiterpene lactone dimer with an unprecedented carbon skeleton from Inula japonica

Five new dimeric sesquiterpene lactones, neojaponicone A (1) and japonicones M-P (2-5) were isolated from the aerial part of Inula japonica. Neojaponicone A (1) has an unprecedented carbon skeleton with an unusual valerolactone system formed between a guaianolide and an eudesmane. The structure and absolute configuration of 1 were established by extensive analysis of spectroscopic data and computational approaches.