Sesquiterpenoids from the Rhizome of Ligularia virgaurea

Two novel sesquiterpene dimers, compounds 1 and 2, were isolated from the rhizome of Ligularia virgaurea, together with the six known sesquiterpenoids 3–8. Their structures were established by physico‐chemical and spectroscopic methods, especially by means of 1D‐ and 2D‐NMR as well as HR‐MS analyses. A mechanism based on a classical Diels–Alder cyclization is proposed for the formation of the dimer 1 from the precursors 8 and the quinone form of 6 (Scheme).