Influence of the chemical structure of water-soluble cryptophanes on their overall chiroptical and binding properties

The synthesis and the chiroptical properties of the two enantiomers of the hexacarboxylic acid cryptophane-A derivative, 1, are described in this article. The chiroptical and binding properties of 1 toward achiral and chiral guests have been investigated in water under basic conditions by polarimetry, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and (1)H NMR spectroscopy. These experiments reveal that the (1)H NMR spectra of 1 are very sensitive to the nature of the guest trapped in its cavity whereas ECD and VCD spectra remain unchanged. We also show that the two enantiomers of 1 are able to distinguish between the two enantiomers of a series of small chiral epoxides. The enantiodiscrimination increases with the size of the chiral guest whereas the corresponding binding constants decrease. In contrast to what was observed for other water-soluble cryptophanes, the molecular recognition process is found independent of the nature of the counterions surrounding host 1, shedding light on the importance of the chemical structure of cryptophanes on their binding and chiroptical properties.