Cephalodiones A–D: Compound Characterization and Semisynthesis by 6+6 Cycloaddition

Abstract Cephalodiones A–D ( 1 – 4 ), the first example of C 19 ‐norditerpenoid dimers, were isolated and fully characterized from a Cephalotaxus plant. These new skeletal natural products shared a unique tricyclo6.4.1.1 2,7 tetradeca‐3,5,9,11‐tetraene‐13,14‐dione core that was capped in both ends with rigid multicyclic ring systems either C 2 ‐symmetrically or asymmetrically. Compounds 1 – 4 were proposed to be biosynthetically produced by the 6+6‐cycloaddition of two identical C 19 ‐norditerpenoid troponoids, which was validated by the semisyntheses of dimers 2 – 4 . Moreover, some compounds showed significant inhibition on Th17 cell differentiation.