Synthesis and Resolution of a Multifunctional Inherently Chiral Calix[4]arene with an ABCD Substitution Pattern at the Wide Rim: The Effect of a Multifunctional Structure in the Organocatalyst on Enantioselectivity in Asymmetric Reactions

Electronic Tuning of Host-Guest Interactions within the Cavities of Fluorophore-Appended Calix[4]arenes

A series of fluorescent calix[4]arene scaffolds bearing electron-rich carbazole moiety conjugated at the lower rim have been prepared. Studies of the fluorescence quenching in the presence of the N-methyl pyridinium guest revealed that the electronic properties of the distal phenolic ring play a major role in the host–guest complexation. In particular, placing an electron-donating piperidine fragment at that ring significantly increased the host–guest interactions, while introducing the same fragment into the proximal phenolic ring weakened the fluorescence response. These results suggest that the dominant interactions between the guest and calixarene cavity involve the oxygen-depleted fluorophore-bearing aromatic ring and not the more electron-rich unsubstituted phenolic fragments.

Upper-Rim Monofunctionalisation in the Synthesis of Triazole- and Disulfide-Linked Multicalix[4]- and -[6]arenes

Covalently linked multiple calixarenes are valued in supramolecular chemistry. This work reports an easy and versatile synthetic route to covalently linked double and triple calix[4]arene and calix[6]arenes by a novel DMF-controlled selective alkylation of a convenient and readily available upper-rim dimethylaminomethyl-substituted tetrahydroxy and hexahydroxy calix[4]arene and -[6]arenes. Synthetic routes to upper-rim functionalised redox active disulfide-linked double-, tetra- and peptidohybrid-calixarenes employing either redox chemistry (CH₂ SH) or thiolates (CH₂ S^- ) are also opened up from the same key starting material.

Inherently chiral cone-calix[4]arenes via a subsequent upper rim ring-closing/opening methodology

Access to chiral calix[4]arenes can unlock novel supramolecular architectures for enantioselective catalysis and molecular recognition. However, accessibility to these structures has been significantly hindered so far. We report herein the synthesis and characterization of di- and trifunctionalized cone-calix[4]arenes featuring a lactone moiety spanning the distal positions at the upper rim. The lactones force the whole skeleton to assume pinched-cone conformations. The ring-closure is favored by the high conformational flexibility of the calixarene scaffold. The new lactones are remarkably stable in the solid state, while a quick hydrolysis to restore the parent carboxylic acids occurs in solution under acidic/basic conditions. Slow aminolyses of lactones 2-3 yield inherently chiral products featuring three different functionalities at the upper rim, at room temperature. The subsequent ring-closing/opening methodology presented here highlights the versatility of these lactones as powerful synthons for the preparation of a variety of threefold upper rim functionalized, inherently chiral calix[4]arenes fixed in the cone structure.

Solvent-Free Enantioselective Michael Reactions Catalyzed by a Calixarene-Based Primary Amine Thiourea

An upper-rim functionalized calix[4]arene-based thiourea installed onto the ( R, R)-1,2-cyclohexanediamine scaffold was synthesized with a view to investigate its catalytic ability in enantioselective Michael additions. The reactions were found to conveniently proceed under solvent-free conditions, observing good to high enantioselectivities. From this preliminary study, the calix[4]arene unit is likely to play a role in affecting the conversion and to a lesser extent to the stereochemical outcome of the reactions through van der Waals contacts and C-H···π interactions with the substrates.

A Bifunctional Thioether Linked Coumarin Appended Calix[4]arene Acquires Selectivity Toward Cu(2+) Sensing on Going from Solution to SAM on Gold

A bifunctional calix[4]arene molecule bearing coumarin moiety on the lower rim and thioether moiety on the upper rim (L1), has been synthesized and well characterized by (1)H, (13)C NMR and mass spectrometry. Suitably functionalized coumarin moieties are well suited for selective recognition of various cations and anions. Among the 10 different metal ions studied, only Cu(2+) and Fe(3+) exhibit appreciable changes in the absorption spectra owing to the availability of functional moieties present at both the lower as well as the upper rim of free L1 in acetonitrile solution. To bring better selectivity, we blocked one of these functional moieties by coating on to a surface so that only the other one is exposed to the environment for sensing. Such a study carried out in the present case using the self-assembled monolayer (SAM) of L1 on Au(111) resulted in selective sensing of Cu(2+) over several other metal ions as studied by surface plasmon resonance (SPR). The SAM of L1 on Au(111) was confirmed by different techniques, such as grazing incidence FT-IR, contact angle measurement, cyclic voltammetry and scanning tunneling microscopy. Thus, L1 is proven to be a suitable sensor for Cu(2+) when attached to gold surface.

Synthesis and conformational studies of chiral macrocyclic [1.1.1]metacyclophanes containing benzofuran rings

A novel hemisphere-shaped inherently chiral calixarene analogoue [1.1.1]metacyclophane containing both benzene and benzofuran rings was synthesized by fragment coupling approach.

Inherently chiral calix[4]arenes via oxazoline directed ortholithiation: synthesis and probe of chiral space

The diastereoselective oxazoline-directed lithiation of calix[4]arenes is reported with diastereoselective ratios of greater than 100:1 in some instances. Notably, it has been found that the opposite diastereomer can be accessed via this approach merely through the choice of an alkyllithium reagent. The inherently chiral oxazoline calix[4]arenes have also been preliminarily examined as ligands in the palladium-catalyzed Tsuji–Trost allylation reaction, returning results comparable to their planar chiral ferrocene counterparts pointing towards future application of these types of compounds.

Qualitative analysis of the helical electronic energy of inherently chiral calix[4]arenes: an approach to effectively assign their absolute configuration

For all microhelices on aromatic rings of inherently chiral calix[4]arene, an expression was derived from one approximation and one hypothesis on the basis of the electron-on-a-helix model of Tinoco and Woody as follows: , where µ = 1 for the right-handed microhelix and µ = −1 for the left-handed microhelix; and H and K are constant and greater than zero. The expression correlates microhelical electronic energy (E) with the atom polarizability difference (Δα) on both microhelix ends, which intuitively and clearly shows the impact of helical substituent polarizability on helical electronic energy. The case analysis almost entirely proves that the qualitative analysis of the helical electronic energy of inherently chiral Calix[4]arenes with the expression is scientific and can be used to effectively assign their absolute configuration.

Iron catalyzed enantioselective sulfa-Michael addition: a four-step synthesis of the anti-asthma agent Montelukast

Asymmetric conjugate addition of thiols to acyclic α,β-unsaturated ketones catalyzed by an iron(iii)–salen complex gives β-thioketones in high enantiomeric excess.

Inherently chiral calixarenes: synthesis, optical resolution, chiral recognition and asymmetric catalysis

Inherently chiral calixarenes, whose chirality is based on the absence of a planar symmetry or an inversion center in the molecules as a whole through the asymmetric array of several achiral groups upon the three-dimensional calix-skeletons, are challenging and attractive chiral molecules, because of their potential in supramolecular chemistry. The synthesis and optical resolution of all varieties of inherently chiral calixarenes are systematically discussed and classified, and their applications in chiral recognition and asymmetric catalysis are thoroughly illustrated in this review.

Inherently chiral concave molecules--from synthesis to applications

This tutorial review covers the recent development in the synthesis and application of molecules and finite assemblies that are chiral owing to their curvature. A modified definition of inherent chirality is provided. Various classes of chiral concave molecules are presented including salphen complexes, cyclic amides, derivatives of sumanene, trioxatricornan or subphthalocyanine, cyclotriveratrylenes, homooxacalix[3]arenes, calixarenes, resorcinarenes, phthalocyanines, corannulenes and cavitands. Some of these bowl shaped compounds exhibit high inversion barriers, comparable with the stability of classical carbon chirality centres, while the others (e.g. hydrogen bonded assemblies) can only be detected by NMR. This review is focused on practical aspects of synthesis, resolution and applications in chiral recognition and asymmetric synthesis.

Absolute configuration assignment of inherently chiral calix[4]arenes using DFT calculations of chiroptical properties

The racemate of an inherently chiral meta-substituted calix[4]arene derivative 3 has been resolved via enantioselective HPLC. Measured optical rotation dispersion and electronic circular dichroism have been compared with DFT theoretical predictions. The comparison indicates that the absolute configuration of the dextrorotatory enantiomer (+)-3 is cS. The procedure has been successfully tested against a literature precedent confirming the validity of the method.

An asymmetric ortholithiation approach to inherently chiral calix[4]arenes

A general asymmetric synthesis of inherently chiral calix[4]arenes is described: using a chiral oxazoline derived from L-valine, an ortholithiation strategy is employed to give inherently chiral calix[4]arenes with high (93%) enantiomeric excesses. A crystal structure of a phosphine oxide intermediate has been obtained, unambiguously assigning the major diastereomer in the reaction; a mechanism explaining this result is proposed.