Configurational Assignment of Cyclic Bisbibenzyls by HPLC‐CD and Quantum‐Chemical CD Calculations

Macrocyclic bisbibenzyls are intriguing natural products that are obtained from liverworts. Recently, they have received growing interest due to the discovery of novel representatives and because of their antitumoral and antifungal activities. Depending on the individual coupling type, these structures can possess one or two biaryl axes, which, with larger ortho substituents or increased ring strain within the macrocycle, leads to the occurrence of configurationally stable atropisomers. Within this work, representatives of the plagiochin and riccardin types, both containing one biaryl unit and one biaryl ether moiety, have been investigated for their stereochemical stabilities and electronic circular dichroism (ECD) by means of HPLC-UV and HPLC-CD in combination with quantum-chemical DFT and ab initio RICC2 calculations. We demonstrate that the computation of the chiroptical properties of such macrocycles is a nontrivial task, because of the presence of charge-transfer (CT) excitations especially in the riccardin-type structures, which leads to a failure of DFT methods in the excited-states calculations, thus necessitating the use of the coupled-cluster approach for a reliable attribution of the absolute configurations of the enantiomers. In the course of this study, the previous assignments of the absolute configurations of the atropisomers of plagiochin E and riccardin D are revised.