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 N(G)-monomethyl-L-arginine (L-NMMA). Siphonols A-E (1-5) represent the first examples of isopimaranes oxygenated at C-20.