Inherently Chiral Calix[5]arenes Incorporating an Axially Chiral Binaphthyl Moiety: Synthesis, Structures and Chiral Recognition

Inherent chirality was generated by regioselective monoalkylation of 1,3-substituted calix[5]crown-6 incorporating an axially chiral BINOL moiety, with ethyl bromoacetate. The resultant diastereomers 4a and 4b, which possess both axial chirality and inherent chirality, were readily separated by column chromatography. Respective hydrolysis afforded diastereomeric acids 5a and 5b. ¹H NMR spectra and X-ray crystallography established that esters 4a and 4b feature a cone-in conformation, which disappears in the corresponding acids 5a and 5b due to intramolecular hydrogen-bonding between the carboxyl group and a glycolic oxygen atom. Amino alcohol-induced fluorescence quenching was observed for 5a and 5b. The highest enantioselectivity, based on the ratio of Stern–Volmer constants K_SV(S)/K_SV(R), reached 1.90 in the case of 5a with 2-amino-1-phenylethanol (G1), and 5b with phenylalaninol (G2). The highest diastereoselectivity of 1.83, measured by K_SV(5a)/K_SV(5b), occurs in the case of 5a/5b with (R)-G2.