Koilodenoids A–G, immunosuppressive spiro dimers of diterpenoids from Koilodepas hainanense: structural elucidation and biomimetic transformation

Abstract

Seven undescribed dimeric diterpenoids, koilodenoids 1–7, were isolated from Koilodepas hainanense. Compound 1 possesses a new carbon skeleton featuring a unique oxaspiro[5.9]pentadecane fragment. Compounds 2–7 share a 6/6/6/6/6/6/6-fused heptacyclic framework with a rare oxaspiro[5.5]undecane motif. Their structures were characterized by a combination of diverse methods. Biomimetic transformation of 2 into 1 was achieved in high yield by photochemical oxidation under mild, as well as peroxide- and metal-free conditions. Compounds 1–7 showed moderate immunosuppressive activity.