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

Organocatalytic Enantioselective Synthesis of Inherently Chiral Calix[4]arenes

Inherently chiral calix[4]arenes are an excellent structural scaffold for enantioselective synthesis, chiral recognition, sensing, and circularly polarized luminescence. However, their catalytic enantioselective synthesis remains challenging. Herein, we report an efficient synthesis of inherently chiral calix[4]arene derivatives via cascade enantioselective cyclization and oxidation reactions. The three-component reaction features a broad substrate scope (33 examples), high efficiency (up to 90 % yield), and excellent enantioselectivity (>95 % ee on average). The potential applications of calix[4]arene derivatives are highlighted by their synthetic transformation and a detailed investigation of their photophysical and chiroptical properties.

Simultaneous construction of inherent and axial chirality by cobalt-catalyzed enantioselective C-H activation of calix[4]arenes

The simultaneous construction of multiple stereogenic elements in a single step is highly appealing and desirable in the field of asymmetric synthesis. Furthermore, the catalytic enantioselective synthesis of inherently chiral calix[n]arenes with high enantiopurity has long been a challenging endeavor. Herein, we report an enantioselective cobalt-catalyzed C-H activation/annulation for the efficient construction of inherently chiral calix[4]arenes bearing multiple C-N axially chiral element. By employing the benzamide tethered calix[4]arene as the substrate, the C-H annulation with alkynes can be successfully accomplished, leading to the generation of multiple stereogenic elements. A wide range of calix[4]arenes and alkynes are found to be well compatible, and exhibit good yields, high enantioselectivity and excellent diastereoselectivity. Notably, the gram-scale reaction, catalytic application, synthetic transformations, and chiral recognition further showcase the potential applications of this protocol.

Inherently chiral calixarenes by a catalytic enantioselective desymmetrizing cross-dehydrogenative coupling

Under the catalysis of PdBr₂ and a chiral phosphoramidite ligand, the upper-rim mono (2-bromoaroyl)-substituted calix[4]arene derivatives underwent a facile enantioselective desymmetrization reaction to afford 9H-fluorene-embedded inherently chiral calixarenes in good yields with excellent enantioselectivities. The transannular dehydrogenative arene-arene coupling reaction proceeded most probably through an oxidative addition of the C_aryl-Br bond to a ligated palladium catalyst followed by a sequence of an enantioselective 1,5-palladium migration and an intramolecular C-H arylation sequence. This new family of inherently chiral calixarenes possesses unique chiroptical properties thanks to their highly rigid structure induced by the 9H-fluorene segment.

Direct meta substitution of calix[4]arenes

Calixarenes represent very popular building blocks in supramolecular chemistry. Compared to other macrocyclic families, they exhibit an almost infinite possibility of derivatization of the basic skeleton, which makes them ideal candidates for the design of new receptors or other functional systems. Although the chemistry of calixarenes is well established, there are still some substitution patterns that are unavailable or require a very lengthy synthetic approach. Among such synthetic challenges is the meta substitution of the aromatic skeleton (relative to phenolic oxygen), which, in conjunction with the 3D structure of calixarenes, leads to the inherent chirality and enables the synthesis of derivatives with a hitherto undescribed topology. This review deals with the current achievements in the meta substitution of calixarenes.

Recent applications of chiral calixarenes in asymmetric catalysis

The use of calixarenes in asymmetric catalysis is receiving increasing attention due to their tunable three-dimensional molecular platforms along with their easy syntheses and versatile modification at the upper and lower rims. This review summarizes the recent progress of synthesis and use of chiral calixarenes in asymmetric syntheses which emerged later than 2010.

meta-Bridged calix[4]arenes prepared by Friedel–Crafts alkylation

meta-Mercuration allowed the introduction of substituted hydroxymethyl groups that were intramolecularly cyclized to form a novel type of bridged calix[4]arenes.

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.

Use of residual dipolar couplings in conformational analysis of meta-disubstituted calix[4]arenes

Dimercuration of tetrapropoxy calix[4]arene followed by a reaction with isoamyl nitrite afforded dinitroso regioisomers with unique substitution patterns. The unusual conformational behaviour of these inherently chiral systems was revealed by the combination of dynamic NMR and residual dipolar coupling (RDC) techniques.

Characterization of cyclopentyllithium and cyclopentyllithium tetrahydrofuran complex

The solid-state structures of unsolvated, hexameric cyclopentyllithium and tetrameric cyclopentyllithium tetrahydrofuran solvate were determined by single-crystal X-ray diffraction. Cyclopentyllithium easily crystallized in hydrocarbon solvents. Solution-state structural analyses of cyclopentyllithium and cyclopentyllithium-tetrahydrofuran complexes in toluene-d8 were also carried out by diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques. The solution-state studies suggest that unsolvated cyclopentyllithium exists as hexamer and tetramer equilibrating with each other. Upon solvation with tetrahydrofuran, cyclopentyllithium exists mostly as a tetrahydrofuran tetrasolvated tetramer.

Inherently chiral phosphonatocavitands as artificial chemo- and enantio-selective receptors of natural ammoniums

Inherently chiral phosphonatocavitands with various bridging moieties at their wide rim were synthesized. Optical resolution by chiral HPLC was performed with cavitand 8 to afford enantiopure compounds (+)-8 and (-)-8. The molecular structures of hosts 8 and 12 were determined by X-ray diffraction. The host properties were investigated by (1)H and (31)P NMR spectroscopy. The phosphonatocavitands form inclusion complexes with chiral ammonium neurotransmitters, some presenting enantioselectivity towards the right or left-handed host enantiomers.

Synthesis of inherently chiral calix[4]arenes: stereocontrol through ligand choice

Employing a chiral oxazoline as an ortholithiation directing group allows the synthesis of inherently chiral calix[4]arenes suitable for elaboration into planar chiral molecules. An important finding has been that the diastereoselectivity of the reaction can be tuned by the choice of additive. These results have bearing on the elucidation of the general mechanism of oxazoline-directed ortholithiation.

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.