Design of a novel inherently chiral calix[4]arene for chiral molecular recognition

A newly designed inherently chiral calix[4]arene was synthesized and resolved to an optically pure form. Enantiomeric recognition ability of the chiral calix[4]arene was examined using 1H NMR experiments with mandelic acid. In addition, the chiral calix[4]arene was applied to asymmetric reactions, as an organocatalyst.     

Synthesis of upper rim calix[4]arene carcerands

The synthesis of novel tetra-olefinic calix[4]arene carcerands is described using a synthetic strategy involving palladium catalysed Heck coupling followed by imine formation.     

Monosubstituted lower rim thiacalix[4]arene derivatives

A clean and partially green route to monoalkyled thiacalix[4]arenes has been demonstrated. We have discovered that tetraalkylammonium halide can cleave selectively one of the two aryl alkyl ethers of dialkylated para-tert-butylthiacalix[4]arenes. Thus, p-tert-butyl thiacalix[4]arenes differently monoalkyled at the lower rim with acetyl, propyl, and benzyl group were synthesized in good yield.     

En route to inherently chiral tetraoxacalix[2]arene[2]triazines

This paper describes our attempts to synthesize inherently chiral heteroatom-bridged calixaromatics. Based on a stepwise fragment-coupling approach using a chiral 3,5-dihydroxybenzamide or benzoate, 2,4-dihydroxyacetophenone, and cyanuric chloride as reactants, chiral tetraoxacalix[2]arene[2]triazine derivatives 16 and 17 were synthesized in good yields. Subsequent macrocyclic condensation with a diamine 6 furnished efficiently the pairs of diastereomers of inherently chiral tetraoxacalix[2]arene[2]triazine azacrowns 18 and 19.     

Synthesis and characterization of an inherently chiral calix[6]arene in the 1,4-alternate conformation

An inherently chiral calix[6]arene possessing a C₂-symmetric A-B-H substitution pattern was synthesized via a two step process starting from the parent hexa-t-butylcalix[6]arene. The racemic, inherently chiral compound exists as a single isomer with the 1,4-alternate conformation. The inherent chirality was confirmed by treatment of the racemic compound with Pirkle’s reagent to form diastereomeric complexes in solution.     

Preparation of both antipodes of enantiopure inherently chiral calix[4]crowns

Both antipodes of enantiopure inherently chiral calix[4]crown derivatives were successfully obtained through separation of their BINOL diastereomeric derivatives of corresponding racemates using preparative TLC instead of conventional HPLC methods. Such a result provides a basis for future study of chiral recognition and asymmetric catalysis with inherently chiral calixrene derivatives.     

Preparation of enantiopure inherently chiral calix[5]arenes

Enantiopure inherently chiral calix[5]arenes were successfully prepared by separation of their (R)-BINOL diastereomeric derivatives of the corresponding racemates by column chromatography instead of HPLC. A new type of inherently chiral calix[5]arene was obtained.     

New lower rim looped calix[4]arene for ratiometric and chromogenic recognition of Cu2+

A new calix[4]arene derivative in its cone conformation and bearing Schiff base loop at the lower rim has been synthesized and evaluated as a specific molecular probe for copper ions. The new molecular receptor 4 shows a selective visible change in color from colorless to yellow only in the presence of Cu²⁺ ions which was confirmed by a significant bathochromic shift (?λ_max = 76 nm) in its absorption spectrum. The stoichiometry of the copper complex was calculated to be 1:1. These results may help to design more efficient chemical sensors for determining copper in biological systems.     

Thiacalix[4]arene derivatives with proximally bridged lower rim

New types of lower rim proximally bridged thiacalix[4]arenes have been prepared by direct aminolysis of starting tetraacetate derivative in the cone conformation using aliphatic α,ω-diamines. X-ray crystallography revealed the highly preorganized array of -C(O)NH- bonds resulting in strong intramolecular hydrogen bonding between amide groups of both bridges. The length of the corresponding diamine was found to have an essential influence on the yield of these bridged molecules.     

Lower rim substituted p-tert-butyl calix[4]arene; Part 14. Synthesis, structures and binding studies of calix[4]arene thioamides

A number of p-tert-butylcalix[4]arene thioamides were synthesized and characterized by ¹H-NMR and elemental analysis. Compounds 1–5 are O-substituted derivatives with –CH₂–C(=S)–N–X groups, where NX = morpholidyl, NEt₂, NHC₂H₄Ph, NHCH₂Ph and NHEt, respectively. The X-ray structures of the ligands 1, 3, 5 and of the complex 3·Pb(ClO₄)₂, (compound 6), are presented and their slightly distorted cone conformation is established. The influence of the nature of the thioamide functions (secondary or tertiary) on the extractability of some selected metal cations was investigated. Whereas all these calixarenes show the highest extraction level for Ag⁺, tertiary thioamides are more efficient extractants for Pb²⁺ than secondary thioamides.     

Synthesis and cationic selectivity studies of novel calix[4]arene derivatives containing heteroatom at the lower rim

A series of calixarene derivatives 2―5 containing heteroatom at the lower rim have been synthesized. 1H NMR studies and crystallographic structures demonstrated that the calix[4]arene derivatives adopted cone conformations. Their cationic binding abilities and selectivities towards heavy and tran- sition metal ions have been evaluated by solvent extraction of aqueous metal picrates. The obtained results indicated that the introduction of nitrogen, sulfur, and/or phosphor atoms to the calix[4]arene framework could effectively enhance their binding ability and selectivity for heavy and transition metal ions, such as Pb2 or Ag .     

Synthesis of calix[4]arene library substituted with peptides at the upper rim

A fluorescence-labeled calix[4]arene library substituted with peptides at the upper rim was synthesized. Screening of the library for binding a dye-labeled oligopeptide indicated that some peptidocalix[4]arenes selectively bind the oligopeptide. The chemosensitivity of the library members for a target peptide was also investigated.     

Novel calix[4]resorcinols with sulfamide fragments in the lower rim

Russian Journal of General Chemistry - Novel calix[4]resorcinols modified by sulfamide fragments were synthesized via acid-catalyzed opening of the pyrrolidine ring in...     

Synthesis and cation binding properties of fluorescent calix[4]arene derivatives bearing tryptophan units at the lower rim

The fluorescent peptidocalixarenes, 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(O-methyl-l-tryptophanylcarbonylmethoxy)calix[4]arene (1) and 5,11,17,23-tetra-tert-butyl-25,27-di(O-methyl)-26,28-bis(O-methyl-l-tryptophanylcarbonylmethoxy)calix[4]arene (2), were prepared by introducing tryptophan subunits at a lower calixarene rim. Coordination abilities of 1 and 2 towards Eu(III) and alkali metal cations were studied by spectrophotometric, spectrofluorimetric, conductometric and potentiometric titrations in acetonitrile at 25°C. Rather strong complexation was observed for smaller alkali metal cations Li⁺ and Na⁺ (log K _Li1 >6, log K _Li2 >6, log K _Na1  = 8.25, log K _Na2  = 6.94), and moderate for K⁺ (log K _K1  = 5.09, log K _K2  = 4.09). Larger Rb⁺ and Cs⁺ cations did not fit in the ion binding site of 1 so no complexation was detected, whereas the more flexible ligand 2 accommodated Rb⁺ cation (log K _Rb2  = 3.44). The fluorescence of 1 (λ_ex = 280 nm, λ_em = 340 nm) was remarkably quenched by Eu(III). Stability constant of 1:1 (Eu³⁺:1) complex determined spectrofluorimetrically amounted to log K _Eu1  = 6.16.     

An inherently chiral calix[4]crown carboxylic acid in the 1,2-alternate conformation

For better understanding of the structure/property relationship of inherently chiral calixarenes in the 1,2-alternate conformation, we designed and synthesized an inherently chiral calix[4]crown-4 carboxylic acid 1,2-alternate conformer. Resolution of the racemates was effected by condensation with (S)-BINOL as a chiral auxiliary and separation of the resultant diastereomers via preparative TLC plates, followed by hydrolysis of the isolated diastereomers to afford enantiopure antipodes of the title compound. Preliminary property study revealed that the title compound has the ability to enantioselectively discriminate 2-phenylglycinol by ¹H NMR spectroscopy.     

Photo-physical behavior as chemosensor properties of anthracene-anchored 1,3-di-derivatives of lower rim calix[4]arene towards divalent transition metal ions

Anthracene anchored 1,3-di-derivatives of lower rim p-tert-butyl-calix[4]arene were synthesized and characterized. These derivatives were subjected to the binding studies with the divalent metal ions, viz., Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ using fluorescence and absorption spectra. The imine moiety that is in conjugation with the anthryl unit is responsible for quenching the fluorescence in the absence of metal ion, however, in the presence of Fe²⁺ and Cu²⁺, the spectra showed very high enhancement in fluorescence intensity indicating that the lone pair present on the imine-N is involved in the metal ion binding and as a result the photo-induced electron transfer is prevented. Based on the photo-physical studies, it has been found that the anthracene derivative that is coupled with the calix[4]arene unit through an imine bond acts as a chemosensor for Fe²⁺ and Cu²⁺. The fluorescence studies are further augmented by the absorption spectra.     

An inherently chiral calix[4]crown carboxylic acid in the partial cone conformation

We have designed and synthesized an inherently chiral calix[4]arene partial cone conformer, in which a crown-4 ether moiety, a carboxyl group, and a propyl group are convergent. Chemical resolution was achieved by use of (S)-BINOL as a chiral auxiliary. ¹H NMR titration data revealed that the title compound does not have the ability to differentiate both enantiomers of chiral amine and aminoalcohol guests. The approximately symmetric calix[4]arene skeleton of the partial cone conformer is presumed to be responsible for its undesirable chiral recognition ability.     

Functionalisation of the upper rim of calix[4]arene via alcoholysis and hydrosilylation reactions

Metalation of 5,17-dibromo-25,26,27,28-tetra propoxy calix[4]arene (1) with n-BuLi in THF at −78 °C gave organolithium reagent, which reacted with Me₂HSiCl to give 5,17-bis(dimethylsilyl)-25,26,27,28-tetra propoxy calix[4]arene (2). The Si-H groups of calixarene 2 were treated with methanol, ethanol, propanol, butanol, pentanol, hexanol, 2-propanol and 2-methyl propanol in the presence of Karstedt catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyl disiloxane complex, solution in xylene) to give the corresponding 5,17-bis(alkoxydimethylsilyl)-25,26,27,28-tetra propoxy calix[4]arene (3). Moreover, calixarene 2 was easily functionalized with a variety of alkenes using Karstedt catalyst to give the corresponding organosilylated calix[4]arene (4).