Recognition of Amides by New Rigid Calix[4]arene-Based Cavitands

Abstract

The synthesis of new hosts specifically designed for the recognition of amides, characterized by two binding regions: a rigid calix[4]arene cavity and a sidearm, inserted at its rim, able to form strong hydrogen bonds, is described. The binding abilities of the new receptors toward amides of general structure R(1)CONR(2)R(3) have been investigated in CDCl(3) solution by (1)H NMR spectroscopy. When the additional binding site is the N-phenylureido group spaced by a methylene unit from the apolar cavity, binding constants up to 756 M(-)(1) were measured. Neither the two separate potential binding sites, nor the model host, where the calix[4]arene skeleton is flexible show detectable binding ability toward the series of guests examined. The rigidity of the calix[4]arene apolar cavity is the key control element in determining the efficiency of these molecular recognition processes. The presence of NH groups in the guest controls the efficiency and selectivity of binding.

Monotopic and heteroditopic calix[4]arene receptors as hosts for pyridinium and viologen ion pairs: a solution and solid-state study

“…Melting points are uncorrected. Compounds 1, 20 2, 20 3, 24 4, 25 and N-methylpyridinium triflimide 34 (NMPTf 2 N) were synthesised according to reported procedures. All other reagents were of reagent-grade quality as obtained from commercial suppliers and were used without further purification.…”

Multipoint Molecular Recognition of Amino Acids and Biogenic Amines by Ureidocalix[5]arene Receptors

“…On the other hand, in the -Ahx⊂1a complex, formation of hydrogen bonding(s) between the carboxylate group of the guest and the NH hydrogen(s) of 1a (CO 2 -‚‚‚HsN), produces a similar downfield shift on the resonances of the latter. 15a, 16 A scrutiny of the K a values in Table 1 reveals that the ureido secondary binding site of 1 displays a higher affinity for carboxylate rather than ammonium moieties, and that the presence of a bulky R-amino ester moiety (Lys-OMe‚2HCl) lowers the association constant by about 1 order of magnitude. Remarkably, in the absence of CD 3 OD complexation of -Ahx by 1a increases from 82 to nearly 100% (see Figure 2, trace c).…”

“…We have also developed calix[5]arene-based n -butylammonium selective liquid-membrane electrodes (insensitive to commonly interfering inorganic cations) and disclosed preliminary data on the potential of these macrocycles as receptors for carboxyl-protected bioactive substrates. 12a, Structural modifications to improve such properties were then concentrated on the replacement of a p-tert -butyl with a ureido group. The latter has been successfully used for the complexation of carboxylates and amides in the calix[4]arene series . We now report that calix[5]arenes 1 in a fixed cone conformation bearing a urea unit at the upper rim show a strong affinity for biogenic amines, ω-amino acids, and lysine derivatives.…”

“…12a,14 Structural modifications to improve such properties were then concentrated on the replacement of a p-tertbutyl with a ureido group. The latter has been successfully used for the complexation of carboxylates 15 and amides 16 in the calix[4]arene series. 17 We now report that calix[5]arenes 1 in a fixed cone conformation bearing a urea unit at the upper rim show a strong affinity for biogenic amines, ω-amino acids, and lysine derivatives.…”

“…On the other hand, in the ε-Ahx⊂ 1a complex, formation of hydrogen bonding(s) between the carboxylate group of the guest and the NH hydrogen(s) of 1a (CO 2 - ···HN), produces a similar downfield shift on the resonances of the latter. 15a, …”