8,14-seco-ent-Kaurane diterpenoids from Isodon glutinosus: enol–enol tautomerism and antitumor activity†

Glutinosasins A–D (1–4), four novel 8,14-seco-ent-kaurane diterpenoids featuring a β-carbonyl enol moiety, along with glutinosasin E (5), a new ent-kaurane diterpenoid, were isolated from the aerial parts of Isodon glutinosus. Their structures were elucidated through a combinatorial use of spectroscopic methods and single-crystal X-ray diffraction. Quantum chemical calculations further validated the unique structures of 1–4 and supported the presence of enol–enol tautomerism within these molecules. Notably, 4 demonstrated selective inhibitory activity against SW480 tumor cells, with an IC₅₀ value of 2.33 μM, and induced G2/M cell cycle arrest and apoptosis in a dose-dependent manner.