Synthesis and chiral recognition of novel chiral fluorescence receptors bearing 9-anthryl moieties

Two chiral fluorescence receptors 1 and 2 have been synthesized, and their structures characterized by IR, ¹H NMR, MS spectra and elemental analysis. The chiral recognition of the receptors was studied by ¹H NMR and fluorescence spectra. The results demonstrate that the receptors and tetrabutylammonium mandelate formed a 1:1 complex. Two receptors exhibit good chiral recognition abilities towards the enantiomers of tetrabutylammonium mandelate.     

Synthesis and enantioselective recognition of an (S)-BINOL-based colorimetric chemosensor for mandelate anions

New chiral receptors 1 and 2 based on (S)-BINOL and thiourea units were synthesized. The chiral recognition of receptors for chiral anions were studied by fluorescence, UV–vis, and ¹H NMR spectra. The results of the non-linear curve fitting indicated that the receptors and guest anions formed a 1:1 stoichiometric complex. The obvious color change of receptor 2 can be observed by the naked eye when the enantiomers of mandelate anions were added, which demonstrates that receptor 2 may be used as a colorimetric sensor for mandelate anions.     

Enantioselective recognition by optically active chiral fluorescence sensors bearing amino acid units

Chiral fluorescence receptors 1 and 2 were synthesized and their structures characterized by IR, ¹H NMR, ¹³C NMR, MS spectra, and elemental analysis. The chiral recognition abilities of 1 and 2 were studied by ¹H NMR and fluorescence spectra. The results demonstrate that receptors 1 and 2 with bis(tetrabutylammonium) dibenzoyl tartrate formed a 1:1 complex. Receptor 2 exhibits an excellent enantioselective recognition ability toward the enantiomers of bis(tetrabutylammonium) dibenzoyl tartrate.     

Enantioselective fluorescent sensors for amino acid derivatives based on BINOL bearing benzoyl unit

The derivatives of BINOL, (S)-1 and (R)-1, and their analogues have been prepared and the structures of these compounds have been characterized by IR, MS, ¹H, and ¹³C NMR spectroscopy and elemental analysis. The enantioselective recognition of these receptors has been studied by fluorescence titration and ¹H NMR spectroscopy. The receptors exhibited different chiral recognition abilities toward some enantiomers of chiral materials and formed 1:1 complexes between host and guest. Receptor (S)-1 or (R)-1 exhibits excellent enantioselective fluorescent recognition abilities toward amino acid derivatives.     

Fluorescent sensors for amino acid anions based on calix[4]arenes bearing two dansyl groups

Two novel types of chiral calix[4]arenes containing hydrazide and dansyl groups were synthesized and examined for their enantioselective recognition abilities by the fluorescence and ¹H NMR spectra in CHCl₃. The results indicate that both 4a and 4b have excellent enantioselectivities to the N-protected alanine or phenylalanine anions.     

Phenanthroline-based fluorescence sensors for Eu3+ ion and subsequent enantioselective discriminating of malate

A pair of novel chiral fluorescent and colorimetric sensors L1 and D1 had been designed and synthesized, which were based on phenanthroline as the fluorophore and mandelohydrazide as the binding site. They showed selective recognition toward Eu³⁺ ion as ‘turn-off’ mode in DMSO-H₂O solution (v/v = 1:5, 10 mM HEPES, pH = 7.4). The fluorescence discrimination towards chiral carboxylate anions had been studied for L1-Eu³⁺ and D1-Eu³⁺ complex and they showed good enantioselective recognition ability towards malate as ‘turn-on’ mode. The interaction of sensor L1, D1 with Eu³⁺ and L1-Eu³⁺, D1-Eu³⁺ towards malate caused different color changes in the naked eye.     

Fluorescent enantioselective receptor for S-mandelate anion based on cholic acid

A chiral fluorescent receptor 1 based on cholic acid was designed and synthesized. The enantioselective recognition ability of 1 to mandelate anion was studied by the fluorescence in CH₃CN and a ¹H NMR spectroscopic method. The results indicate that 1 exhibited a good enantioselectivity to the enantiomer of the mandelate anion in CH₃CN.     

Synthesis and amines enantiomeric recognition ability of binaphthyl-appended 22-crown-6 ethers derived from rosin acid

Novel chiral 22-crown-6 ethers (5a–b) bearing methoxycarbonyl side groups derived from rosin acid and 2,2′-dihydroxy-1,1′-binaphthyl (BINOL) were prepared in optically pure forms, and their enantiodiscriminating abilities toward protonated primary amines and amino acid methyl ester salts were examined by the UV–vis titration method. These receptors exhibit good chiral recognition towards the isomers (up to K_L/K_D?=?5.23, ΔΔG₀?=?4.10?kJ?mol^?1) in CHCl₃:MeOH?=?2:1 at 25?°C.     

Synthesis and Enantioselective Discrimination of Chiral Fluorescence Receptors Bearing Amino Acid Units

Two chiral fluorescence receptors （1, 2） were synthesized, and their structures were characterized by IR, ^1H NMR, ^13C NMR, mass spectra and elemental analysis. The chiral recognition of receptors was studied by ^1H NMR and fluorescence spectra. The results demonstrate that receptors and dibenzoyl tartrate anion formed a 1 ： 1 complex. The receptor 1 exhibited a good enantioselective recognition ability toward the enantiomers of dibenzoyl tartrate anion.     

Synthesis of new chiral calix[4]azacrowns for enantiomeric recognition of carboxylic acids

Two novel chiral calix[4]azacrown ethers 4 and 5 bearing a furfuryl group on the nitrogen atom were developed by the reaction of dibromo- or ditosyl derivatives of p-tert-butylcalix[4]arenes 2 and 3 with a chiral diol, 1. The enantioselective recognition of these receptors towards the enantiomers of racemic carboxylic acids has been studied by ¹H NMR spectroscopy. The molar ratio and the association constants of the chiral compounds 4 and 5 with each of the enantiomers of guest molecules were determined by using Job plots and a nonlinear least-squares fitting method, respectively. The Job plots indicate that both of the hosts form 1:1 instantaneous complexes with (R)- or (S)-mandelic acid and (l)- or (d)-dibenzoyltartaric acid. The receptors exhibited different chiral recognition abilities towards the enantiomers of racemic guests.     

Preparation of a metal–ligand fluorescent chemosensor and enantioselective recognition of carboxylate anions in aqueous solution

Two chiral fluorescent chemosensors 1 and 2 were synthesized, and the structure characterized by IR, ¹H NMR, ¹³C NMR, MS spectra and elemental analysis. Their recognition ability was studied in aqueous solution (Tris–HCl buffer pH 7.4, MeOH/H₂O = 1:1) through fluorescence spectra. Receptors 1 and 2 showed a good binding ability to the copper ion. The host 1-Cu²⁺ complex showed a chiral recognition ability to mandelate anions with a preferable binding to l-mandelate than d-mandelate anions. The host 1-Cu²⁺ complex and l- or d-mandelate anions formed 1:1 stoichiometric complex. The binding constant for l-mandelate is 576 M⁻¹, whereas that for d-mandelate is only 38 M⁻¹, which can be distinguished by the different change of fluorescence intensity.     

Synthesis and enantiomeric recognition ability of 22-crown-6 ethers derived from rosin acid and BINOL

Four novel chiral 22-crown-6 ethers 6a-b, 7a-b bearing hydroxyl side groups derived from rosin acid and BINOL were prepared in optically pure forms, and their enantiodiscriminating abilities towards protonated primary amines and amino acid methyl ester salts were examined by UV-vis titration methods. These receptors exhibited good chiral recognition towards the isomers (up to K_D/K_L = 6.02, ΔΔG₀ = 4.45 kJ mol⁻¹) and showed different complementarity to various chiral guests.     

Chiral calix[4]azacrowns for enantiomeric recognition of amino acid derivatives

In this study the synthesis of novel chiral calix[4]azacrown derivatives has been reported. The enantioselectivity of chiral receptors was investigated by using UV-vis spectroscopy. All the chiral calix[4]arene derivatives exhibited certain chiral recognition toward the enantiomers of phenylalanine (Phe-OMe·HCl) and alanine methyl ester hydrochlorides (Ala-OMe·HCl). As a chiral receptor, the furfuryl-armed calix[4]azacrown ether 7 has the best enantiomeric discriminating ability for α-amino acid ester hydrochlorides (up to K_L/K_D=2.08, ΔΔG₀=−1.82 kJ mol⁻¹) in CHCl₃. The enantiomeric recognition abilities for guests are also discussed from a thermodynamic point of view.     

Fluorescent PET chemosensors for lithium

The synthesis of the chiral diaza-9-crown-3 derivatives 1 (S,S) and 2 (R,R) is described. These sensors were designed as luminescent chemosensors for lithium where the fluorescence emission from the naphthalene moieties was ‘switched on’ upon Li⁺ recognition by the crown ether moiety in organic solvents, showing excellent selectivity over other group I and group II cations. Even though the recognition of Li⁺ was not achieved in water (pH 7.4) or aqueous alcohol solution, the fluorescence (which was switched on at pH 7.4) was substantially modulated by spherical anions, where the fluorescence emission was quenched in the presence of Br⁻ and I⁻ but less by Cl⁻ and not by acetate. This indicates that the emission was quenched by heavy-atom affect. The recognition of Li⁺ was also investigated by ¹H NMR in CD₃CN and by observing the changes in the circular dicromism spectra. For the former, the resonances for the crown ether moiety and α-methyl protons adjacent to the ring were sifted upfield and broadened, whereas for 1 the intensity of the CD signal for the π-π transition was substantially modulated upon Li⁺ recognition.     

Naphthyridine-based receptors for flurometric detection of urea and biotin

Naphthyridine-based receptors 1–4 have been designed and synthesized for the recognition of urea in CHCl₃ containing 1% CH₃CN. Receptor 1 also binds biotin and its methyl ester with moderate binding constant values. In comparison, receptor 2 is less efficient in recognising biotin and its methyl ester analogue. Receptor 1 binds urea and biotin with binding constant values of 1.02 × 10⁴ and 1.08 × 10⁴ M^?1, respectively, in CHCl₃ containing 1% CH₃CN and shows significant change in emission of naphthyridine upon complexation. Characterization and sensing properties of the receptors were evaluated by ¹H NMR, UV–vis and fluorescence spectroscopic methods.     

Metal ion based chiral fluorescence sensor selective for dihydrogenphosphate

A Cu(II) based conformationally restricted chiral fluorescence sensor (receptor 2) has been designed and synthesized for selective sensing of anions. The anion recognition property of the Cu²⁺-complex has been studied in acetonitrile by fluorescence methods which show remarkable sensitivity toward dihydrogen phosphate via fluorescence modulation of the Cu²⁺-complex over the other anions examined.     

Asymmetric synthesis of new chiral long chain alcohols

Sixteen new chiral alcohols with alkyl (C₁₁–C₁₉) and aryl, substituted aryl, hetero aryl and biaryl groups 2a–2t were synthesized by three different asymmetric reduction methods from their corresponding ketones 1a–1t. Chiral NaBH₄ (method A), chiral BH₃ (method B) and chiral AIP (method C) were used as asymmetric reduction catalysts. Chiral NaBH₄ was modified by four different ligands 3a–3d, chiral BH₃ and chiral AIP by four different ligands 4a–4d. Ligand 4c was synthesized for the first time in this work. Chiral NaBH₄ generated chiral alcohols of (R)-configuration and chiral BH₃ and chiral AIP of (S)-configuration with high enantiomeric excesses, were analysed by chiral HPLC. In order to determine the ee values by chiral HPLC, sixteen corresponding racemic alcohols, synthesized by reducing their corresponding ketones via NaBH₄, were used for chiral resolution on a Daicel OD HPLC column. The sixteen starting ketones were synthesized in this study by Friedel–Craft acylation. The new chiral alcohols were characterized by IR, NMR, (¹H and ¹³C), MS, elemental analyses and specific rotation. The reduction methods A, B and C were applied to these ketones for the first time in this study and were compared with each other. The relationship between the structure of the ketone and the yield and the enantiomeric excess was discussed.     

Novel C1-symmetric chiral crown ethers bearing rosin acids groups: synthesis and enantiomeric recognition for ammonium salts

Four types of novel C₁-symmetric chiral crown ethers including 28-crown-8, 20-crown-6, 17-crown-5 and 14-crown-3 (9a–m) were synthesized and their enantiodiscriminating abilities with protonated primary amines (10–14) were examined by ¹H NMR spectroscopy. 20-crown-6 crown ethers exhibited good chiral recognition properties toward these guests and showed different complementarity to some chiral guests, indicating that 20-crown-6 crown ethers could be used as a chiral NMR solvating agent to determine the enantiopurity of these guests. In addition, the binding model and binding site between the hosts and guests were also studied by the computational modeling and experimental calculation.     

Anion recognition by 2,2′-binaphthalene derivatives bearing urea and thiourea groups at 8- and 8′-positions by UV-vis and fluorescence spectroscopies

Synthesis and anion recognition properties of 2,2′-binaphthalene derivatives bearing two thiourea (1) and urea (2) groups at 8- and 8′-positions were studied. The structure of receptor 1 was determined by X-ray crystallography. UV-vis spectra of the receptors showed characteristic changes around 300-400 nm through isosbestic points upon the addition of biologically relevant anions such as acetate, dihydrogenphosphate, and chloride in MeCN and DMSO due to restriction of the rotation around the single bond connecting two naphthyl moieties by cooperative guest binding of two recognition sites. Job’s plots showed 1:1 complexation for guest anions. The fluorescence quantum yields of free form of 1 and 2 in MeCN were determined to be 0.021 and 0.57, respectively. The fluorescence intensities of the receptors diminished upon the addition of anions in MeCN. The association constants of receptors 1 and 2 were one or two orders of magnitude greater than the corresponding monothiourea and urea receptors 3 and 4 indicating cooperative hydrogen bonding with guest anions. The selectivity trends of association of anions were F⁻>AcO⁻>H₂PO₄⁻>Cl⁻>>HSO₄⁻≈NO₃⁻≈Br⁻≈I⁻ for 1, and F⁻>AcO⁻≈Cl⁻>H₂PO₄⁻>Br⁻>HSO₄⁻>I⁻≈NO₃⁻ for 2. Receptor 2 showed remarkable Cl⁻ selectivity presumably owing to suitable orientation for effective hydrogen bond formation with Cl⁻.     

Novel C2-symmetric chiral 18-crown-6 derivatives with two aromatic sidearms as chiral NMR discriminating agents

Novel C₂-symmetric chiral 18-crown-6 derivatives with two aromatic sidearms 2-4 were prepared, and their enantiomeric recognition abilities as chiral NMR discriminating agents towards primary ammonium salts were examined. Among these chiral crown ethers, the most effective enantiomeric discrimination of racemic ammonium salts in the ¹H NMR spectra was attained by the derivative with two pyrenylmethyl sidearms.