Isolation, Structure Elucidition, and Immunosuppressive Activity of Diterpenoids from Ligularia fischeri

Six new (1-3 and 6-8) and seven known diterpenoids were isolated from the whole plant of Ligularia fischeri. Compound 1 is a new 15,16-dinorerythroxylane-type diterpenoid possessing a C₁₈ skeleton, and 2 is the first example of a 6/6/6/6/5/5-fused hexacyclic ent-kaurane diterpenoid with 19,20-olide and 11,16-epoxy moieties. The structures of the new compounds were elucidated by spectroscopic analysis and chemical methods. The absolute configurations of 1 and 7 were determined by single-crystal X-ray diffraction. Compounds 1-13 were evaluated for their immunosuppressive activity, and 4, 7, and 13 showed moderate inhibitory activities against human B lymphoblast HMy2.CIR cells with IC₅₀ values of 56.3 ± 2.2, 13.3 ± 0.8, and 31.4 ± 0.9 μM, respectively.