A borderline case between meso and stable C1: an axially chiral, yet configurationally semi-stable biphenyl with two oppositely configured [10]paracyclophane portions

The synthesis of the bi[10]paracyclophanes and from the meso-configured bioxepine is described. These compounds are stereochemically remarkable: the biaryl axis that connects the constitutionally identical, but oppositely configured planar-chiral paracyclophane portions, is configurationally semi-stable. Thus, is an unprecedented borderline case of a (planar-chiral)–(axially chiral)–(planar-chiral) molecule that is right in between a meso-compound (as a macroscopical result of the—albeit slow—rotation about the central C,C-bond) and C1-symmetry (with respect to the existence of separable—even though configurationally unstable—discrete atropo-enantiomers). Despite their restricted configurational stability, these atropo-enantiomers were resolved on a chiral phase at 5 °C and were stereochemically assigned by LC–CD coupling, in combination with quantum chemical CD calculations.