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.
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