Metathesis‐Based Stapling of a Pyridine–Acetylene–Phenol Oligomer Having Alkenyl Side Chains after Intermolecular Templation by Native Saccharides

Towards Biomimetic Recognition of Glycans by Synthetic Receptors

Carbohydrates are abundant in Nature, where they are mostly assembled within glycans as free polysaccharides or conjugated to a variety of biological molecules such as proteins and lipids. Glycans exert several functions, including protein folding, stability, solubility, resistance to proteolysis, intracellular traffic, antigenicity, and recognition by carbohydrate-binding proteins. Interestingly, misregulation of their biosynthesis that leads to changes in glycan structures is frequently recognized as a mark of a disease state. Because of glycan ubiquity, carbohydrate binding agents (CBAs) targeting glycans can lead to a deeper understanding of their function and to the development of new diagnostic and prognostic strategies. Synthetic receptors selectively recognizing specific carbohydrates of biological interest have been developed over the past three decades. In addition to the success obtained in the effective recognition of monosaccharides, synthetic receptors recognizing more complex guests have also been developed, including di- and oligosaccharide fragments of glycans, shedding light on the structural and functional requirements necessary for an effective receptor. In this review, the most relevant achievements in molecular recognition of glycans and their fragments will be summarized, highlighting potentials and future perspectives of glycan-targeting synthetic receptors.

Subtle Modification of Imine-linked Helical Receptors to Significantly Alter their Binding Affinities and Selectivities for Chiral Guests

Aromatic helical receptors P-1 and P-2 were slightly modified by aerobic oxidation to afford new receptors P-7 and P-8 with right-handed helical cavities. This subtle modification induced significant changes in the binding properties for chiral guests. Specifically, P-1 was reported to bind d-tartaric acid (K_a =35500 M^-1 ), used as a template, much strongly than l-tartaric acid (326 M^-1 ). In contrast, its modified receptor P-7 exhibited significantly reduced affinities for d-tartaric acid (3600 M^-1 ) and l-tartaric acid (125 M^-1 ). More dramatic changes in the affinities and selectivities were observed for P-2 and P-8 upon binding of polyol guests. P-2 was determined to selectively bind d-sorbitol (52000 M^-1 ) over analogous guests, but P-8 showed no binding selectivity: d-sorbitol (1890 M^-1 ), l-sorbitol (3330 M^-1 ), d-arabitol (959 M^-1 ), l-arabitol (4970 M^-1 ) and xylitol (4960 M^-1 ) in 5% (v/v) DMSO/CH₂ Cl₂ at 25±1 °C. These results clearly demonstrate that even subtle post-modifications of synthetic receptors may significantly alter their binding affinities and selectivities, in particular for guests of long and flexible chains.

A Pyridine-Acetylene-Aniline Oligomer: Saccharide Recognition and Influence of this Recognition Array on the Activity as Acylation Catalyst

In order to create new functions of foldamer-type hosts, various kinds of recognition arrays are expected to be developed. Here, a pyridine-acetylene-aniline unit is presented as a new class of a saccharide recognition array. The conformational stabilities of this array were analyzed by DFT calculation, and suggested that a pyridine-acetylene-aniline oligomer tends to form a helical structure. An oligomer of this array was synthesized, and its association for octyl β-D-glucopyranoside was confirmed by ¹ H NMR measurements. UV/Vis, circular dichroism, and fluorescence titration experiments revealed its high affinity for octyl glycosides in apolar solvents (K_a =10⁴ to 10⁵  M^-1 ). This oligomer was relatively stable under basic conditions, and therefore this array was expected to be applied to the derivatization of saccharides. A 4-(dialkylamino)pyridine attached pyridine-acetylene-aniline oligomer proved to catalyze the acylation of the octyl glucoside.