Bioactive Phenanthrene and Bibenzyl Derivatives from the Stems of Dendrobium nobile

A new enantiomeric pair of spirodiketones, (+)- and (−)-denobilone A (1 and 2), three new phenanthrene derivatives (3–5), and three new biphenanthrenes (22–24), along with 11 known phenanthrene derivatives (6–16), five known bibenzyl derivatives (17–21), and four known biphenanthrenes (25–28), were isolated from Dendrobium nobile. The structures of 1–5 and 22–24 were elucidated using comprehensive spectroscopic methods. (+)-Denobilone and (−)-denobilone A (1 and 2) were isolated as a pair of enantiomers by chiral HPLC. The absolute configurations of (+)- and (−)-denobilone A (1 and 2) were determined by comparing their experimental and calculated electronic circular dichroism spectra. The absolute configuration of denobilone B (3) was determined by X-ray crystallographic analysis. The inhibitory activities of all compounds against nine phytopathogenic fungi and three cancer cell lines were evaluated.