Synthesis and study of calix[6]cryptamides: A new class of heteroditopic receptors that display versatile host-guest properties toward neutral species and organic associated ion-pair salts

Molecular recognition of organic ammonium ions in solution using synthetic receptors

Ammonium ions are ubiquitous in chemistry and molecular biology. Considerable efforts have been undertaken to develop synthetic receptors for their selective molecular recognition. The type of host compounds for organic ammonium ion binding span a wide range from crown ethers to calixarenes to metal complexes. Typical intermolecular interactions are hydrogen bonds, electrostatic and cation-π interactions, hydrophobic interactions or reversible covalent bond formation. In this review we discuss the different classes of synthetic receptors for organic ammonium ion recognition and illustrate the scope and limitations of each class with selected examples from the recent literature. The molecular recognition of ammonium ions in amino acids is included and the enantioselective binding of chiral ammonium ions by synthetic receptors is also covered. In our conclusion we compare the strengths and weaknesses of the different types of ammonium ion receptors which may help to select the best approach for specific applications.

Calix[4]arene-based rotaxane host systems for anion recognition

The synthesis, structure and anion binding properties of the first calix[4]arene-based [2]rotaxane anion host systems are described. Rotaxanes 9.Cl and 12.Cl, consisting of a calix[4]arene functionalised macrocycle wheel and different pyridinium axle components, are prepared via adaption of an anion templated synthetic strategy to investigate the effect of preorganisation of the interlocked host's binding cavity on anion binding. Rotaxane 12.Cl contains a conformationally flexible pyridinium axle, whereas rotaxane 9.Cl incorporates a more preorganised pyridinium axle component. The X-ray crystal structure of 9.Cl and solution phase (1)H NMR spectroscopy demonstrate the successful interlocking of the calix[4]arene macrocycle and pyridinium axle components in the rotaxane structures. Following removal of the chloride anion template, anion binding studies on the resulting rotaxanes 9.PF(6) and 12.PF(6) reveal the importance of preorganisation of the host binding cavity on anion binding. The more preorganised rotaxane 9.PF(6) is the superior anion host system. The interlocked host cavity is selective for chloride in 1:1 CDCl(3)/CD(3)OD and remains selective for chloride and bromide in 10 % aqueous media over the more basic oxoanions. Rotaxane 12.PF(6) with a relatively conformationally flexible binding cavity is a less effective and discriminating anion host system although the rotaxane still binds halide anions in preference to oxoanions.

A ditopic ion-pair receptor based on stacked nucleobase quartets

Pass the salt, please! State-of-the-art computations indicate that the stacking complex of a guanine quartet and an adenine quartet (G(4)A(4)) can function as a potent ditopic receptor for NaCl in aqueous solution (see picture; Na(+), Cl(-) yellow, O red, N blue, C black, H white).

Synthesis of the first calix[6]crypturea via a versatile tris-azide precursor

Various nitrogenous calix[6]arene based receptors have been synthesized in one step from a new C3v symmetrical calix[6]arene intermediate decorated with azido groups. Hence, the first calix[6]crypturea has been obtained in high yield through a unique one-pot process consisting of a domino Staudinger/aza-Wittig reaction followed by a [1 + 1] macrocyclization reaction with a tripodal amine. The conformational properties and some of the host-guest properties of the new calix[6]arene derivatives have been studied by NMR spectroscopy.

Second generation of calix[6]aza-Cryptands: synthesis of heteroditopic receptors for organic ion pairs

The efficient syntheses of calix[6]azacryptands decorated with anion-binding groups on the narrow rim have been achieved from an 1,3,5-tris-protected calix[6]hexa-amine. These heteroditopic receptors can bind ammonium ions or organic ion pair salts with a positive cooperativity. In regard to their functionalization at the 1,3,5-phenolic positions, these compounds constitute the first examples of a second generation of C3 v symmetrical calix[6]azacryptands.