Sesquiterpene Lactones with Astrodaucane Skeleton from Astrodaucus orientalis.

Four sesquiterpene lactones, astrodaucanolide A-D (1-4) with unique structures, toghether with two known phenylpropanoid esters (5 and 6) were isolated from a flower extract of Astrodaucus orientalis. The structures were established by 1D and 2D NMR spectroscopy and high-resolution mass spectrometry (HRESIMS), and the absolute configuration of 1-4 was determined by electronic circular dichroism (ECD). Compounds 1-4 novel architecture represents a new class of sesquiterpenes with new skeleton. A putative biosynthetic pathway for their scaffold is proposed with a germacryl cation as the precursor. The suggested biosynthesis pathway is similar to that of eudesmane sesquiterpenes with a different direction of protonation which then leads to the new skeleton, named astrodaucane by the 1,2-methyl migration.