No abstract

Cyclic bisbibenzyls, like isoplagiochins C (1) and D (2), are stereochemically intriguing molecules: Although not equipped with any of the traditional stereogenic elements that render molecules conformationally stable per se, they are sometimes isolated in an optically active form and are thus chiral at room temperature. The paper describes quantum chemical calculations, in particular investigations of the conformational space and molecular dynamics simulations, showing that the helicity is a property of the entire molecule, whose ring strain makes the molecule configurationally stable overall, with (formally) three stereogenic elements (two biaryl axes and one helical stilbene unit). Only one of the biaryl axes (the 'upper' one, joining C-12' and C-14) has a stable configuration, leading to a population of four interconverting diastereomers, yet without racemization at room temperature. On the basis of these conformational and dynamic calculations, the circular dichroism spectrum of isoplagiochin C (1) was calculated, leading to the first assignment of the absolute configuration of a cyclic bisbibenzyl. Accordingly, 1 has the P-configuration at the stereochemically stable biaryl axis and constitutes a mixture of diastereomers with respect to the other biaryl axis and the helical stilbene unit. From the temperature dependence of the racemization rates, an enantiomerization barrier of 101.6 kJ/mol was determined. Likewise, for the first time for cyclic bisbibenzyls, the enantiomeric ratio of this natural product was determined, by chromatography on a chiral phase with CD-coupling. Accordingly, 1 from Plagiochila deflexa is not enantiomerically pure, but occurs in a 85:15 ratio in favor of the enantiomer that has the P-configuration at the stereochemically stable axis.

No abstract

Recently, a bisbibenzyl named plagiochin E showing remarkable antifungal and antitumor activities was isolated from Marchantia polymorpha, a liverwort. The total synthesis of the proposed structure for plagiochin E and of two structurally and biosynthetically related bisbibenzyls and comparison of the NMR data of the synthetic compounds with those of the isolated bisbibenzyls necessitates a structure revision for plagiochin E. Exemplarily for this metabolite, the stereostructure was investigated, by racemate resolution on a chiral Lux Cellulose-1 phase with HPLC-CD coupling and quantum chemical CD calculations, clearly assigning the Pconfiguration for the faster and the M-configuration to the slower enantiomer.