An Optically-Pure Hemicryptophane as NMR and ECD Responsive Probe for Chloroform

An optically pure hemicryptophane cavitand, in which the primary side of permethylated a-cyclodextrin (alpha-CD) is capped with a C-3-chiral cyclotribenzylene (CTB) derivative, responds to its solution environment, either by electronic circular dichroism or 1HNMR spectroscopy. In particular, the primary methoxy (6'OMe) protons of the alpha-CD component behave as solvent sensitive NMR probes. For example, they are more shielded in C2D2 Cl-4 than in CDCl3, and their chemical shifts show opposite temperature dependence. A similar contrasted behavior was observed in benzene vs. toluene, suggesting that the observed effects are related to the size of the solvent molecule. Molecular Dynamics simulations indicated that C2H2Cl4 and CHCl3 were included in the alpha-CD component of the hemicryptophane, but not in the CTB concavity, with binding energies of - 21.1 kJ x mol(-1) and - 32.7 kJ x mol(-1), respectively.