(<i>M</i>)- and (<i>P</i>)‑Bicelaphanol A, Dimeric Trinorditerpenes with Promising Neuroprotective Activity from <i>Celastrus orbiculatus</i>

(<i>M</i>)-Bicelaphanol A (<b>1</b>) and (<i>P</i>)-bicelaphanol A (<b>2</b>), two unprecedented dimeric trinorditerpenes existing as atropisomers, together with their monomer celaphanol A (<b>3</b>), were isolated from the root bark of <i>Celastrus orbiculatus</i>. The structures and absolute configurations of <b>1</b> and <b>2</b> were determined by spectroscopic and single-crystal X-ray diffraction analyses. Compound <b>1</b> exhibited a significant in vitro neuroprotective effect against a hydrogen peroxide-induced cell viability decrease in PC12 cells at 1 μM, while compounds <b>2</b> and <b>3</b> showed such effects at 10 μM.