Cephalodiones A-D: Compound Characterization and Semisynthesis by [6+6] Cycloaddition.

Cephalodiones A-D (1-4), the first example of C -norditerpenoid dimers, were isolated and fully characterized from a Cephalotaxus plant. These new skeletal natural products shared a unique tricyclo[6.4.1.1 ]tetradeca-3,5,9,11-tetraene-13,14-dione core that was capped in both ends with rigid multicyclic ring systems either C -symmetrically or asymmetrically. Compounds 1-4 were proposed to be biosynthetically produced by the [6+6]-cycloaddition of two identical C -norditerpenoid troponoids, which was validated by the semisyntheses of dimers 2-4. Moreover, some compounds showed significant inhibition on Th17 cell differentiation.