Copacamphane, picrotoxane and cyclocopacamphane sesquiterpenes from Dendrobium nobile

Dendronobilins K - N, four new sesquiterpenes with copacamphane-type (1), picrotoxane-type (2, 3) and cyclocopacamphane-type (4) skeletons, were isolated from the n-BuOH soluble fraction of the 60% ethanol extract of the stems of Dendrobium nobile. Their structures were established as 2beta,11,12-trihydroxycopacamphan-15-one (1), (2beta,3beta,4beta,5beta)-2,4,11,12-tetrahydroxypicrotoxan-3(15)-olactone (2), (2beta,3beta,5beta)-2,11,12,13-tetrahydroxypicrotoxan-3(15)-olactone (3), and (5beta,8beta)-cyclocopacamphane-5,8,12,15-tetrol (4) on the basis of spectroscopic analysis. Compounds 3 and 4 were inactive in both immunomodulatory and antioxidant bioassay in vitro.