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Neutral imidazole/aminopyridine- and indole/aminopyridine-based receptors, 1 and 2, have been established as highly effective and selective carbohydrate receptors. These receptors effectively recognise neutral carbohydrates through multiple interactions, including neutral hydrogen bonds and CH...pi interactions between the sugar CH groups and the aromatic rings of the receptors. The design of these receptors was inspired by the binding motifs observed in the crystal structures of protein-carbohydrate complexes. The formation of very strong complexes with beta-glucopyranoside 5, beta-maltoside 8, and alpha-maltoside 9 in organic media has been characterised by 1H NMR spectroscopy and confirmed by a second, independent technique, namely fluorescence spectroscopy. The syntheses, molecular-modelling studies, binding properties of the receptors 1 and 2 toward selected mono- and disaccharides as well as comparative binding studies with receptors 3 and 4 are described.

Compounds 5b-8b and 10b, and 11b, containing a triethylbenzene scaffold substituted with both neutral and cationic recognition sites, were shown to be effective receptors for N-acetylneuraminic acid (NeuAc), the most common occurring sialic acid, in highly competitive solvents. These compounds were established to be more powerful receptors for NeuAc than the symmetrical pyridinium- and quinolinium-based compounds 9b and 12b in aqueous media. As in natural protein-sialic acid complexes, the combination of neutral/charge-reinforced hydrogen bonds, ion pairs, CH-π, and van der Waals interactions seems to be responsible for the effective binding of this naturally widespread anionic carbohydrate in aqueous media.