Stereochemical Investigation of Macrocyclic Bisbibenzyls with a Stereogenic Center at One of the Ethylene Bridges.

7′-Hydroxyriccardin C ( 1) and marchantin E ( 2), two macrocyclic bisbibenzyls with a stereogenic center at the ethylene bridge from Chinese liverworts, were investigated stereochemically. Compound 1 was found to be optically active and occurred as a pair of dynamically interchangeable atropisomers at room temperature due to the low barrier of rotation around the biaryl bond, while 2 was a racemic mixture, which was successfully resolved by chiral HPLC into two enantiomers. Their absolute configurations were unequivocally assigned by analysis of temperature-dependent NMR spectra, X-ray crystallographic diffraction, and comparison of experimental and calculated ECD data. [ABSTRACT FROM AUTHOR]