Endo-Complexation of Alkylammonium Ions by Calix[4]arene Cavity: Facilitating Cation−π Interactions through the Weakly Coordinating Anion Approach

Hexahexyloxycalix[6]arene, a Conformationally Adaptive Host for the Complexation of Linear and Branched Alkylammonium Guests

Hexahexyloxycalix[6]arene 2b leads to the endo-cavity complexation of linear and branched alkylammonium guests showing a conformational adaptive behavior in CDCl₃ solution. Linear n-pentylammonium guest 6a⁺ induces the cone conformation of 2b at the expense of the 1,2,3-alternate, which is the most abundant conformer of 2b in the absence of a guest. In a different way, branched alkylammonium guests, such as tert-butylammonium 6b⁺ and isopropylammonium 6c⁺, select the 1,2,3-alternate as the favored 2b conformation (6b⁺/6c⁺⊂2b^1,2,3-alt), but other complexes in which 2b adopts different conformations, namely, 6b⁺/6c⁺⊂2b^cone, 6b⁺/6c⁺⊂2b^paco, and 6b⁺/6c⁺⊂2b^1,2-alt, have also been revealed. Binding constant values determined via NMR experiments indicated that the 1,2,3-alternate was the best-fitting 2b conformation for the complexation of branched alkylammonium guests, followed by cone > paco > 1,2-alt. Our NCI and NBO calculations suggest that the H-bonding interactions (⁺N-H···O) between the ammonium group of the guest and the oxygen atoms of calixarene 2b are the main determinants of the stability order of the four complexes. These interactions are weakened by increasing the guest steric encumbrance, thus leading to a lower binding affinity. Two stabilizing H-bonds are possible with the 1,2,3-alt- and cone-2b conformations, whereas only one H-bond is possible with the other paco- and 1,2-alt-2b stereoisomers.

Unusual Calixarenes Incorporating Chromene and Benzofuran Moieties Obtained via Propargyl Claisen Rearrangement

Monopropargyloxy-tripropoxy-calix[4]arene 1 was subjected to a propargyl Claisen rearrangement to give unusual calix[3]arene[1]chromene and homocalix[3]arene[1]benzofuran macrocycles. Quantum mechanical density functional theory calculations indicated that an initial [3,3] sigmatropic reaction affords a highly reactive allene intermediate, stabilized by two main diradical pathways leading to six- and five-membered oxygenated rings. In the presence of a n-butylammonium guest, calix[3]arene[1]chromane 6 forms two stereoisomeric complexes stabilized by ⁺N–H···O and cation···π interactions.

Synthesis and characterization of new Na+ complexes of N-benzyl cyclic peptoids and their role in the ring opening polymerization of l-lactide

Na⁺ complexes of cyclic peptoid were employed for the first time as catalyst for the l-lactide polymerization.

(Thia)calixarenephosphonic Acids as Potent Inhibitors of the Nucleic Acid Chaperone Activity of the HIV-1 Nucleocapsid Protein with a New Binding Mode and Multitarget Antiviral Activity

The nucleocapsid protein (NC) is a highly conserved protein that plays key roles in HIV-1 replication through its nucleic acid chaperone properties mediated by its two zinc fingers and basic residues. NC is a promising target for antiviral therapy, particularly to control viral strains resistant to currently available drugs. Since calixarenes with antiviral properties have been described, we explored the ability of calixarene hydroxymethylphosphonic or sulfonic acids to inhibit NC chaperone properties and exhibit antiviral activity. By using fluorescence-based assays, we selected four calixarenes inhibiting NC chaperone activity with submicromolar IC₅₀ values. These compounds were further shown by mass spectrometry, isothermal titration calorimetry, and fluorescence anisotropy to bind NC with no zinc ejection and to compete with nucleic acids for the binding to NC. Molecular dynamic simulations further indicated that these compounds interact via their phosphonate or sulfonate groups with the basic surface of NC but not with the hydrophobic plateau at the top of the folded fingers. Cellular studies showed that the most soluble compound CIP201 inhibited the infectivity of wild-type and drug-resistant HIV-1 strains at low micromolar concentrations, primarily targeting the early steps of HIV-1 replication. Moreover, CIP201 was also found to inhibit the flipping and polymerization activity of reverse transcriptase. Calixarenes thus form a class of noncovalent NC inhibitors, endowed with a new binding mode and multitarget antiviral activity.

Self-sorting assembly of a calixarene/crown ether polypseudorotaxane gated by ion-pairing

A polypseudorotaxane composed of a calix[5]arene-diammonium supramolecular polymer and dibenzo-24-crown-8 wheels self-assembles only in the presence of superweak counterions

Tuning the biomimetic performances of 4-hydroxyproline-containing cyclic peptoids

Five new cyclic peptoids containing (2S,4R)-4-hydroxyproline (Hyp) residues have been designed and synthesized using a mixed “submonomer/monomer” approach.

Threading of an Inherently Directional Calixarene Wheel with Oriented Ammonium Axles

The threading of monostoppered alkylbenzylammonium axles 7⁺ and 8⁺ with the calix[6]-wheel 3 can occur by both routes of entering the macrocycle 3 in the cone conformation: passage through the upper rim and the through the lower rim. Thus, under thermodynamic conditions, with both the axles 7⁺ and 8⁺, the two possible orientations of calix[2]pseudorotaxane, namely, endo-benzyl and endo-alkyl, are formed by a stereoselectivity controlled by the endo-alkyl rule. Interestingly, by ¹H NMR monitoring of the threading process between 8⁺ and 3, we revealed two calix[2]pseudorotaxane isomers in which the calix-wheel adopts 1,2,3-alternate and cone conformations, which represent the kinetic and thermodynamic species, respectively. Finally, the synthesis of ammonium-based oriented calix[2]rotaxane is here described.

Cyclic Peptoids as Mycotoxin Mimics: An Exploration of Their Structural and Biological Properties

Cyclic peptoids have recently emerged as important examples of peptidomimetics for their interesting complexing properties and innate ability to permeate biological barriers. In the present contribution, experimental and theoretical data evidence the intricate conformational and stereochemical properties of five novel hexameric peptoids decorated with N-isopropyl, N-isobutyl, and N-benzyl substituents. Complexation studies by NMR, in the presence of sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), theoretical calculations, and single-crystal X-ray analyses indicate that the conformationally stable host/guest metal adducts display architectural ordering comparable to that of the enniatins and beauvericin mycotoxins. Similarly to the natural depsipeptides, the synthetic oligolactam analogues show a correlation between ion transport abilities in artificial liposomes and cytotoxic activity on human cancer cell lines. The reported results demonstrate that the versatile cyclic peptoid scaffold, for its remarkable conformational and complexing properties, can morphologically mimic related natural products and elicit powerful biological activities.

Calix[5]arene Through-the-Annulus Threading of Dialkylammonium Guests Weakly Paired to the TFPB Anion

Through-the-annulus threading of calix[5]arene penta-O-ethers by dialkylammonium cations coupled to the loosely coordinating superweak TFPB^- anion has been achieved. ¹H NMR titration data show that preorganization of the calix[5]arene scaffold leads to great thermodynamic stability of the pseudorotaxane complexes as well as to a favorable threading kinetic. Thus, calix[5]arene 1c, bearing tert-butyl groups at the wide rim, was threaded by all of the cations under study (with the exception of the dibenzylammonium 2b⁺) more tightly than the other derivatives under investigation (K_a's up to 2.02 ± 0.2 × 10⁵ M^-1) because of its preorganized cone conformation. According to DFT calculations, van der Waals interactions between the tert-butyl groups of 1c and the alkyl chain of the cationic axle are likely responsible for the remarkable stability observed. The threading of the calix[5]arene wheels with the asymmetric pentylbenzylammonium axle 2c⁺ led to the toposelective formation of the endo-pentylpseudorotaxane stereoisomer in agreement with the known "endo-alkyl rule". Owing to the steric hindrance of the axle phenyl group, the threading of the guest was seen to occur in a unidirectional fashion through the calixarene narrow rim.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

A supramolecular assembly of methyl-substituted cucurbit[5]uril and its potential applications in selective absorption

A supramolecular assembly is formed in the presence of the structure-directing agent, hydroquinone (Hyq). The structure-directing effect of Hyq is useful in the construction of a novel assembly which could be a Q[n]-based porous material for absorption and separation.