Diterpenoids and Triterpenoids from the Aerial Parts of Isodon serra and Their Biological Activities

Five undescribed diterpenoids, serranins A-E (1-5), and four novel triterpenoids, serratic acids A-D (6-9), along with 32 known terpenoids (10-41) were isolated from the aerial parts of Isodon serra. The planar structures of 1-9 and their relative configurations were established on the basis of extensive spectroscopic analysis. Structurally, compounds 6-9 are the first examples of 18,19-seco-ursane p-coumaric or ferulic esters, while compounds 1-5 further enriched the plant's diterpene profile. Bioactivity evaluation revealed that four diterpenoids (2, 10, 12, and 13) exhibited potent cytotoxic activity against four cancer cell lines (B16-F10, A375, A549, and MDA-MB-231) with IC₅₀ values below 10 μM. Remarkably, 7 and 38 exhibited a comparable antimelanogenesis effect to that of positive control (kojic acid) in B16-F10 cells.