Britanlins A-D, four novel sesquiterpenoids from Inula britannica

A phytochemical investigation on the flowers of Inula britannica has led to the isolation of four novel sesquiterpenoids, britanlins A-D. Britanlins A-C feature the rare presilphiperfolane-type frameworks, which play an important role in biogenesis of triquinane sesquiterpenoids. Britanlin D possessed a rearranged pseudoguaiane skeleton, which was first isolated from nature. The structures of britanlins A to D were elucidated on the basis of extensive spectroscopic methods. The structures of britanlins A and D were further confirmed by single crystal X-ray diffraction.