Selective Recognition of Alkyl Pyranosides in Protic and Aprotic Solvents

The design and synthesis of receptors capable of selective, noncovalent recognition of carbohydrates continues to be a signature challenge in bioorganic chemistry. We report a new generation of tripodal receptors incorporating three pyridine (compound 2) or quinoline (compound 3) rings around a central cyclohexane core for use in molecular recognition of monosaccharides in apolar and polar protic solvents. These tripodal receptors were investigated using 1H NMR, UV, and fluorescence titrations in order to determine their binding abilities toward a set of octyl glycosides. Receptor 2 displayed the highest binding affinity reported to date for noncovalent 1:1 binding of an α-glucopyranoside in chloroform (Ka = 212 000 ± 27 000 M−1) and an approximately 8-fold selectivity for the α anomer over the β anomer of the glucopyranoside. Most importantly, 2 retained its micromolar range of affinities toward monosaccharides in a polar and highly competitive solvent (methanol). The quinoline variant 3 also displayed micromolar binding affinities for selected monosaccharides in methanol (as measured by fluorescence) that were generally smaller than those of 2. Compound 3 was found to follow a selectivity pattern similar to that of 2, displaying higher affinities for glucopyranosides than for other monosaccharides. The binding stoichiometry was estimated to be 1:1 for the complexes formed by both 2 and 3 with glucopyranosides, as determined by Job plots. Nuclear Overhauser effect spectroscopy allowed for the derivation of a binding model consistent with the observed selectivities.