Siphonols A–E: Novel nitric oxide inhibitors from Orthosiphon stamineus of Indonesia

From the methanolic extract of Orthosiphon stamineus, four novel highly oxygenated isopimarane-type diterpenes named siphonols A–D (1–4) and a novel biogenetically interesting norisopimarane-type diterpene named siphonol E (5) were isolated. The new compounds 1–3 and 5 showed more potent inhibitory effects on the nitric oxide (NO) production in lipopolysaccharide (LPS)-activated macrophage-like J774.1 cells than a positive control NG-monomethyl- -arginine ( -NMMA). Siphonols A–E (1–5) represent the first examples of isopimaranes oxygenated at C-20.