Stereochemistry of atropisomeric 9,10-dihydrophenanthrene dimers from Pholidota chinensis

Nine bis-9,10-dihydroplienanthrene and 9,10-dihydrophenanthrene/(dihydro)stilbene derivatives, including the new phochinenins G-L 1-6, were isolated from the whole plant of Pholidota chinensis. Their structures were elucidated on the basis of extensive spectroscopic investigations (1D, 2D NMR, and HR-EIMS). Owing to the sterically hindered rotation around the biaryl axis, some of these biaryl compounds can exist as a pair of enantiomers, but were isolated as racemates. Computed inversion barriers of selected atropisomeric derivatives suggested that phochinenins K 5, gymconpin C 7, and flavanthrin 9 have stable atropisomers. Their racemates were separated by HPLC on an optically active stationary phase, and were stereochemically characterized on-line by circular dichroism (CD) spectroscopy (LC-CD coupling), in conjunction with quantum-mechanics CD calculations. (C) 2008 Elsevier Ltd. All rights reserved.