Enantioselective Catalytic Synthesis of Inherently Chiral Calixarenes

Since the introduction of the concept of inherent chirality by Böhmer, an important part of research focused on the asymmetric synthesis of calixarene macrocycles. However, long synthetic procedures and tedious separation strategies hampered the application of this technology in many topics of organic chemistry, including enantioselective molecular recognition and catalysis. Very recently, a new generation of enantioselective catalytic methodologies has been reported, able to provide highly functionalized, inherently chiral calixarenes in a straightforward manner. In this review, we will discuss these new catalytic methods and the versatile properties of such macrocycles that call for potential applications in many areas of science.

Polyaromatic Calixarene Hosts: Calix[4]pyrenes

Calixpyrenes, calix[4]arenes incorporating one or two pyrene moieties as a part of their hydrophobic cavities, have been prepared and fully characterized. Distally di-O-propoxy diether of the calix dipyrene, which exists in the pinched cone conformation with nearly parallel pyrene moieties, demonstrates strongly enhanced binding of an organic cation (N-methylpyridinium) compared with the analogous diethers of the parent calix[4]arene.

De novo synthesis of inherently chiral heteracalix[4]aromatics from enantioselective macrocyclization enabled by chiral phosphoric acid-catalyzed intramolecular SNAr reaction

We report herein the synthesis of highly enantiopure inherently chiral N3,O-calix[2]arene[2]triazines from enantioselective macrocyclization enabled by chiral phosphoric acid-catalyzed intramolecular nucleophilic aromatic substitution reaction. In contrast to documented examples, the inherent chirality of the acquired compounds arises from one heteroatom difference in the linking positions of heteracalix[4](het)arenes.

The Formation of Inherently Chiral Calix[4]quinolines by Doebner-Miller Reaction of Aldehydes and Aminocalixarenes

The formation of inherently chiral calix[4]arenes by the intramolecular cyclization approach suffers from a limited number of suitable substrates for these reactions. Here, we report an easy way to prepare one class of such compounds: calixquinolines, which can be obtained by the reaction of aldehydes with easily accessible aminocalix[4]arenes in acidic conditions (Doebner-Miller reaction). The synthetic procedure represents a very straightforward approach to the inherently chiral macrocyclic systems. The complexation studies revealed the ability of these compounds to complex quaternary ammonium salts with different stoichiometries depending on the guest molecules. At the same time, the ability of enantioselective complexation of chiral N-methylammonium salts was demonstrated.

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.

The atropisomeric dynamic behaviour of symmetrical N-acylated amine-bridged calix[4]arenes

N-acylated amine-bridged calix[4]arenes exhibit unusual dynamic behaviour which can be attributed to the restricted rotation around the amide C–N bond.

Facile synthesis of a C4-symmetrical inherently chiral calix[4]arene

Inherently chiral calix[4]arenes with C₄-symmetry are extremely rare and difficult to synthesise, severely hampering any effort to expand on their potential as chiral supramolecular catalysts and building blocks. Herein we report a reaction of a tetracarbamate calix[4]arene with NBS which results in a high yield of an inherently chiral calix[4]arenes with C₄-symmetry. Furthermore, employing a chiral N-Boc proline moiety allows for separation of the diastereomers formed, thus obtaining the pure enantiomers after hydrolysis. The enantiomers could be assigned based on the CD spectra and DFT calculated values.