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.