Rearranged Clerodane Diterpenoids from the Stems of Tinospora baenzigeri
Four new rearranged clerodane-type diterpenoids (1–4), a new glucoside (5), and six known compounds (6–11) were obtained from the EtOAc crude extract of Tinospora baenzigeri stem. The structures of the new compounds were elucidated by interpreting their spectroscopic data, particularly 1D and 2D NMR. Single-crystal X-ray diffraction analysis was subsequently performed to confirm the structures and relative configurations of compounds 1–4. These compounds are rare examples of rearranged clerodanes, particularly compound 4, possessing a fully oxidized tetrahydrofuranyl ring. The isolated compounds were assayed for their protective effect against N-acetyl-p-aminophenol (APAP)-induced HepG2 cell damage. Compounds 8, 9, and 11 showed hepatoprotective activity at 10 μM with 17.0, 19.2, and 39.0% inhibition, respectively, whereas rearranged clerodanes (1–3 and 5) were weakly active.