Chlotrichenes A and B, Two Lindenane Sesquiterpene Dimers with Highly Fused Carbon Skeletons from Chloranthus holostegius

Two unprecedented lindenane sesquiterpene dimers, chlotrichenes A (1) and B (2), were obtained from Chloranthus holostegius var. trichoneurus. Their structures were characterized by NMR, ECD, and X-ray diffraction analysis. They possess a new type of spirocarboncyclic dimeric framework formed by endo-Diels–Alder reaction, and the structure of 1 features a unique 3/5/6/6/6/6/5/3-fused octacyclic skeleton by subsequently plausible epoxidation–cyclization reactions of 2. Compound 2 showed synergetic cytotoxicity with DOX on U2 OS cells (CI: 0.94 ± 0.03).