New sesquiterpenoids from the stems of Dendrobium nobile and their neuroprotective activities

Three new sesquiterpenoids, (+)-(1R,2S,3R,4S,5R,6S,9R)-3,11,12-trihydroxypicrotoxane-2(15)-lactone (1), (−)-(1S,2R,3S,4R,5S,6R,9S,12R)-3,11,13-trihydroxypicrotoxane-2(15)-lactone (2), and (+)-(1R,5R,6S,8R,9R)-8,12-dihydroxy-copacamphan-3-en-2-one (3), together with five known compounds, were isolated from the n-butanol soluble fraction of a 95% EtOH extract of the stems of Dendrobium nobile. Their structures were determined by extensive spectroscopic analysis. Particularly, to solve difficult stereochemical problems, electronic circular dichroism calculations, NMR data calculations, and a single-crystal X-ray diffraction were performed. Interestingly, compounds 1 and 2 were picrotoxinin-type sesquiterpenoids with an unusual C15,2-lactone ring. All new sesquiterpenoids (1–3) showed a significant neuroprotective activity against H2O2-induced oxidative damage in PC12 cells. Notably, at 25 and 50 μM, compounds 1 and 2 showed the best protective effects, even better than the positive control (vitamin E).