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A preparative method for the synthesis of optically pure N-(1-phenylethyl)amides of inherently chiral (cR)- and (cS)-dibenzoyloxy-calix[4]arene acetic acids has been developed. Their absolute stereochemical configuration was determined by X-ray diffraction analysis.

Abstract: This review summarized and systematized the currently known methods of substituting phenolic fragments of calix[4]arenes in the para-position, mainly focusing on the conditions of regioselective carrying out such processes. The reactions of the further transformation of the inserted functional groups, which allows the expansion of the family of para-substituted calixarenes, are also considered. These reactions can synthesize inherently chiral calix[4]arenes with asymmetric substitution on the upper rim.

Inherently chiral dialkyloxy-calix[4]arene acetic acids with asymmetric placement of substituents on the lower rim of the macrocycle were first studied as enantiodiscriminating additives to the mobile phase MeCN/H2 O/HCOOH (75/25/0.02 by volume) in the high-performance liquid chromatography (HPLC) separation of d,l-alanine and d,l-valine on the achiral stationary phase ZORBAX Original CN. The dependence of enantio-binding properties on the position of alkyl groups is demonstrated. The highest resolution (1.65) and enantioselectivity (1.80) were obtained for the 1,2-dipropyloxy-calix[4]arene acetic acid.

The enantiodiscrimination properties of stereomerically pure inherently chiral calix[4]arene acetic acids, as well as their esters and amides in relation to 1-phenylethylamine stereoisomers in liquid and gaseous phases were studied by means of proton magnetic resonance spectroscopy (1 H NMR) and quartz crystal microbalance (QCM) techniques. Series of QCM sensors with thin films of inherently chiral calix[4]arenes, immobilized on the quartz resonator surfaces by spin-coating and spreading drop methods have been fabricated. It was shown that sensors based on calix[4]arene acetic acids are able to recognize R- and S-forms of 1-phenylethylamine in gaseous phase under concentration level of 10-400 ppm.

bis-Methoxycarbonylmethoxy derivatives capable of reacting with the surface of aminopropylated silica gel with the formation of chemically and thermally stable amide bonds were obtained by regioselective distal dialkylation of hydroxyl groups of phosphorylated tetrahydroxycalix[4]arenes in high yields. It was found that phosphine oxide and phosphonate P=O groups localized on the upper rim cooperatively bind Eu3+ cations and effectively sorb them from nitric acid solutions.

All three possible inherently chiral (cS)-propyloxy-octyloxy-calix[4]arene acetic acids were synthesized and their absolute configuration was proved. Complexation of the calix[4]arene acetic acids with a number of chiral compounds have been studied by molecular modeling and 1H NMR spectroscopy and enantioselectivity of binding of R- and S-forms of 2-aminobutanol was found.

The Mitsunobu alkylation of 4-tert-butylcalix[4]arene with (S)-(2-diphenylthiophosphinoferrocenyl)methanol followed by desulfuration of the thiophosphine unit using tris(dimethylamino)phosphine afforded enantiomerically pure calixarene mono- and di(ferrocenylphosphine) ligands in high yields. The calixarene mono(ferrocenylphosphine) ligands exhibited good catalytic activity but low atropoenantioselectivity when used in the asymmetric Suzuki–Miyaura coupling reaction of 1-naphthaleneboronic acid and 1-bromo-2-methylnaphthalene. However, the di(ferrocenylphosphine) ligand displayed both good catalytic activity and enantioselectivity (ee values up to 86 %) when employed in the asymmetric Tsuji–Trost allylic alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate.

All six possible stereoisomers of calix[4]arenes with four different substituents on the lower rim of the macrocycle have been synthesized by stepwise alkylation of the hydroxyl groups of tert-butylcalix[4]arene using (R)- or (S)-1-phenylethylamide groups as chiral auxiliaries. They form three enantiomeric pairs, as confirmed by the values of their specific rotation.

The interaction of inherently chiral calix[4]arene carboxilic acids and their amides with amino acids in the organic phase has been studied using electron spectroscopy. It was found that the chiral calix[4]arenes are able of enantioselective recognition of L- and D-forms of amino acids. Stability constants of the calixarene – amino acid supramolecular complexes were determined and mechanism of the host-guest interaction was examined by molecular modeling method.

The preparative method for the synthesis of inherently chiral para-bromoalkoxycalix[4]arenes based on para-bromination, stepwise regioselective debenzoylation and the following alkylation of the readily available 25-propoxy-26,27-dibenzoyloxycalix[4]arene with propyl bromide or (R)-N-(1-phenylethyl)bromoacetamide has been developed. Three types of the inherently chiral calix[4]arenes in cone or partial cone conformations with asymmetrical (AHHHHBHH, AAHHHBHH, AHBHHCHH) substitution of both upper and lower rims have been obtained in racemic, diastereomerically pure or enantiomerically pure forms. Their structure and the absolute configuration have been determined by NMR and X-ray.

Both enantiomers of inherently chiral dipropoxy-p-tert-butylcalix[4]arene carboxylic acids with AABH substitution patterns have been prepared by the stereoselective reactions of 25,26-dipropoxy-p-tert-butylcalix[4]arene with enantiomerically pure (S)- or (R)-N-(1-phenylethyl)bromoacetamide, separation of the diastereomers by recrystallization, and removal of the chiral auxiliary groups. The absolute configurations of the obtained compounds have been established by X-ray analysis.

Both enantiomers of inherently chiral p-tert-butylcalix[4]arene carboxylic acids with ABHH substitution patterns have been prepared by stereoselective reaction of monopropoxy-p-tert-butylcalix[4]arene with an (R)-N-(1-phenylethyl)bromoacetamide, separation of the diastereomers by column chromatography, and removal of the chiral auxiliary groups. The absolute configuration of obtained compounds has been established by X-ray analysis.

By sulfonylation of tetra(p-tert-butyl)-27-propoxy-25-[N-(1-phenylethyl)carbamoylmethoxy]calix[4] arene diastereomeric inherently chiral calixarenes with the ABCH substitution at the lower rim were synthesized and separated by column chromatography. The alkylation of these compounds afforded the corresponding calixarenes with the ABCD substitution type. The absolute configuration of compounds was established by XRD analysis.

Both enantiomers of inherently chiral calixarene carboxylic acids with ABCD substitution patterns have been prepared by the benzoylation of 25-propoxy-27-(R)-N-(α-phenylethyl)amidomethyloxycalix[4]arene followed by resolution of the diastereomers formed, monobromination of them and removal of the benzoyl and α-phenylethylamide auxiliary groups. The absolute configuration of the calixarenes obtained has been established by X-ray analysis. Preliminary study of the chiral recognition properties of calixarene carboxylic acid was performed.

Proximal heteroalkylation of monoalkyl ethers of calix[4]arenes or p - tert -butylcalix[4]arenes in NaH/CH 3 CN or NaOH/DMSO, respectively, was applied for synthesis of inherently chiral calixarenes with ABHH substitution pattern. The introduction by the method of ( R )- or ( S )- N -(α-phenylethyl)acetamide chiral auxiliary group gives mixtures of diastereomeric derivatives of inherently chiral calixarenes, which were separated by column chromatography. The chiral calixarenes were thoroughly characterized by 1 H, 13 C NMR, and X-ray diffraction methods.