Outstanding chiroptical properties: A signature of enantiomerically pure alleno-acetylenic macrocycles and monodisperse acyclic oligomers

A second series of shape-persistent alleno-acetylenic macrocycles and monodisperse acyclic oligomers with conformationally less flexible backbones were synthesized in enantiomerically pure form by short, high-yielding routes starting from optically active 1,3-diethynylallenes. All seven stereoisomers - two pairs of enantiomers and three achiral stereoisomers - in the macrocyclic series were separated and configurationally assigned. The electronic circular dichroism (ECD) spectra of the D₂-symmetric, (P,P,P,P)- and (M,M,M,M)-configured macrocycles display remarkably intense chiroptical responses. A strong amplification of chirality is observed in the acyclic oligomeric series. Their preference for helical secondary structures of one handedness was supported by X-ray analysis and computational studies. This new set of data provides proof that outstanding ECD responses are a hallmark of alleno-acetylenic macrocyclic and acyclic oligomeric chromophores. A series of shape-persistent alleno-acetylenic macrocycles and acyclic oligomers with more rigidified all-carbon backbones were prepared in enantiomerically pure form. X-ray analysis provides structural evidence for the postulated helical conformational preference of one handedness in the acyclic oligomers. Outstanding chiroptical properties are a hallmark of these alleno-acetylenic chromophores.