Enantioselective Complexation of Chiral Oxirane Derivatives by an Enantiopure Cryptophane in Water

We show here that optically active cryptophane 1 bearing phenol functions can efficiently bind the two enantiomers of 2-(chloromethyl)oxirane, ethyloxirane, and 2,2-bioxirane derivatives in aqueous solution. The binding process is characterized by high-field H-1 NMR signals that are specific to the encapsulated species. In all cases, an enantioselective effect was observed and a relationship between the molecular volume of the guest and the binding constant can be established, even though the flexibility of the guest molecule also seems to play a role. In this series, the 2,2-bioxirane guest is a particularly interesting example, because this compound possesses two stereogenic centers. Thus, in solution, three different pairs of enantiomers can be obtained in the presence of the optically active host 1. Electronic circular dichroism (ECD) spectroscopy has also been used to characterize these complexes; specific ECD spectra recorded in the L-1(b) region were obtained for each diastereomer.