Effective Enantioselective Recognition by Chiral Amino‐Phosphonium Salts**

Modulating Helix-Preference of an Axially-Twisted Molecular Scaffold Through Diastereomeric Salt Formation with Tartaric Acid Stereoisomers

Herein, we designed a chiral, axially-twisted molecular scaffold (ATMS) using pyridine-2,6-dicarboxamide (PDC) unit as pivot, chiral trans-cyclohexanediamine (CHDA) residues as linkers, and pyrene residues as fluorescent reporters. R,R-ATMS exclusively adopted M-helicity and produced differential response in UV-vis, fluorescence, and NMR upon addition of tartaric acid (TA) stereoisomers allowing naked-eye detection and enantiomeric content determination. Circular dichroism (CD) profile of R,R-ATMS underwent unique changes during titration with TA stereoisomers - while loss of CD signal at 345 nm was observed with equimolar D-TA and meso-TA, inversion was seen with equimolar L-TA. Temperature increase weakened these interactions to partially recover the original CD signature of R,R-ATMS. 2D NMR studies also indicated the significant structural changes in R,R-ATMS in the solution state upon addition of L-TA. Single crystal X-ray diffraction (SCXRD) studies on the crystals of the R,R-ATMS⊃D-TA salt revealed the interacting partners stacked in arrays and ATMS molecules stabilized by π-π stacking between its PDC and pyrene residues. Contrastingly, tightly-packed supramolecular cages comprised of four molecules each of R,R-ATMS and L-TA were seen in R,R-ATMS⊃L-TA salt, and the ATMS molecules contorted to achieve CH-π interactions between its pyrene residues. These results may have implications in modulating the helicity of topologically-similar larger biomolecules.

Construction of a pillar[5]arene-based supramolecular chiral polymer linked to aminophosphine salt for chiral recognition of enantiomers of mandelic acid

In recent years, supramolecular chirality has been greatly developed in asymmetric synthesis, chiral sensing and other research fields, but its application in molecular chiral recognition has not been extensively studied. In this paper, L-Boc-tyrosine methoxyester and phosphorus chloride salts were introduced into the framework of pillar[n]arene, and a pillar[5]arene-based supramolecular chiral polymer L-TPP-P was constructed. The supramolecular polymer had stable supramolecular chiral properties and could be used as a chiral solvation reagent for chiral recognition of mandelic acid MA. The molar ratio method and Scatchard plot showed that the complexation ratio of L-TPP-P (pillar[5]arene monomer as the reference object) and MA was 1 : 1, and the complexation constants of L-TPP-P with R-MA and S-MA were 4.51 × 105 M-1 and 6.5 × 104 M-1, respectively. The significant affinity difference of L-TPP-P for different enantiomers of MA showed the excellent chiral recognition and stereoselectivity of pillar[5]arene-based supramolecular polymers for MA. This study provides a new idea for a novel supramolecular polymer chiral recognition reagent or chiral recognition method.

Amino Acid-Based Molecular and Membranous Chiral Tools for Enantiomeric Recognition

Given the need, both academic and industrial, for new approaches and technologies for chiral discrimination of enantiomers, the present work demonstrates the development through rational design and integration of two new chiral platforms (molecular and membranous) for enantioselective recognition through visual as well as microscopic observation. The molecular platform (TPT) is based on the tryptophan derivative developed through the condensation of two tryptophan units with terepthaloyl chloride. While TPT based on l-tryptophan recognizes R-mandelic acid over the S-isomer, the host with reverse chirality (TPDT) recognizes S-mandelic acid over R-isomer. The role of chemical functionality in this sensitive recognition process was established experimentally by developing an analogue of TPT and by judiciously using different chiral analytes. Importantly, a detailed theoretical study at the molecular level revealed the U-shaped conformation of TPT, creating a cavity for accommodating a chiral guest with selective functional interaction resulting in the discrimination of enantiomers. Finally, a chiral polymeric mat of poly(methyl methacrylate) (PMMA)/polyacrylonitrile (PAN) (2:3) impregnated with TPT was developed via electrospinning. The resulting fibrous mat was successfully utilized for chiral recognition through microscopic and architectural observation. Hence, the present work reports simple chiral tools for enantiomeric recognition.

Squarate-Tethered Enantiomeric Imido-Pd(II) Cages for Recognition and Separation of Chiral Organic Molecules

Chiral coordination cages have emerged as an efficient platform for enantioselective processes via host-guest interactions. Here, we report an enantiomeric pair of tetrahedral cages of formula [(Pd3[PO(N(*CH(CH3)Ph)3])4(C4O4)6] supported by chiral tris(imido)phosphate trianions and squarate (C4O4)2- linkers. These cages exhibit unusual coordination isomerism for Pd(II)-linker bonds compared with the other Pd(II) cages of this family. Further, they were employed for the recognition and separation of small chiral molecules containing various functionalities. High enantioselectivities of 67 and 41 were found in the case of R-4-hydroxydihydrofuran-2(3H)-one and S-epichlorohydrin, recognized by the R-isomer of the cage. Chiral separation studies showed remarkable enantiomeric excess values of 93 and 85% for S-epichlorohydrin and R-4-benzyl-2-oxazolidinone, respectively, from their racemic mixtures. These studies showcase the potential of coordination cages for enantioselective applications.

Effective enantioselective recognition by steady-state fluorescence spectroscopy: Towards a paradigm shift to optical sensors with unusual chemical architecture

A series of amino acid-derived 1,2,3-triazoles presenting the amino acid residue and the benzazole fluorophore connected by a triazole-4-carboxylate spacer was studied for enantioselective recognition using only steady-state fluorescence spectroscopy in solution. In this investigation, the optical sensing was performed with D-(-) and L-(+)-Arabinose and (R)-(-) and (S)-(+)-Mandelic acid as chiral analytes. The optical sensors showed specific interactions with each pair of enantiomers, allowing photophysical responses, which were used for their enantioselective recognition. DFT calculations confirm the specific interaction between the fluorophores and the analytes corroborating the observed high enantioselectivity of these compounds with the studied enantiomers. Finally, this study investigated nontrivial sensors for chiral molecules by a mechanism different than turn-on fluorescence and has the potential to broad chiral compounds with fluorophoric units as optical sensors for enantioselective sensing.

Chiral discrimination of cyclodecapeptide to anti-COVID-19 clinical candidates: a theoretical study

Various undesirable side effects are frequently associated with isomers of chiral clinical agents. The separation of chiral medicines remains a challenging issue in the medicines research. In this work, we employed cyclic decapeptide as the host molecule and the M06-2X theoretical computational method for chiral recognition of four clinical candidate guests and their isomers, including bucillamine, molnupiravir, azvudine, and VV116, which are relevant for the treatment of COVID-19. The obtained results indicated that bucillamine and molnupiravir and their respective isomers may be distinguished by cyclic decapeptide and that some of the isomers of Azvudine and VV116 may be discriminated by cyclic decapeptide. The inclusion conformation, deformation analysis, and electrostatic potential analysis also visualized the binding modes and binding sites between cyclic peptides and medicine candidates. A series of weak interaction analyses suggest that hydrogen bonding and dispersion interactions may be the primary factors for the recognition and separation of the clinical candidates by cyclic decapeptides. Visualized analyses of noncovalent interaction, hydrogen bond interaction, and NBO, AIM topological demonstrated that the difference of dispersion interaction is not obvious between the complexes, while the type and number of hydrogen bonds are very different, hinting that hydrogen bonds might be crucial for the differentiation of molnupiravir and its isomers. These findings might provide a theoretical reference for the identification and separation of chiral compounds in host-guest interaction.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11224-023-02149-5.

A chiral phosphazane reagent strategy for the determination of enantiomeric excess of amines

Methods for measuring enantiomeric excess (ee) of organic molecules by NMR spectroscopy provide rapid analysis using a standard technique that is readily available. Commonly this is accomplished by chiral derivatisation of the detector molecule (producing a chiral derivatisation agent, CDA), which is reacted with the mixture of enantiomers under investigation. However, these CDAs have almost exclusively been based on carbon frameworks, which are generally costly and/or difficult to prepare. In this work, a methodology based on the readily prepared inorganic cyclodiphosph(iii)azane CDA ClP(μ-N^tBu)₂POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be highly effective in the measurement of ee’s of chiral amines, involving in situ reaction of the chiral amines (R*NH₂) with the P–Cl bond of the CDA followed by quaternization of the phosphorus framework with methyl iodide. This results in sharp ³¹P NMR signals with distinct chemical shift differences between the diastereomers that are formed, which can be used to obtain the ee directly by integration. Spectroscopic, X-ray structural and DFT studies suggest that the NMR chemical shift differences between diastereomers is steric in origin, with the sharpness of these signals resulting from conformational locking of the bornyl group relative to the P₂N₂ ring induced by the presence of the P(v)-bonded amino group (R*NH). This study showcases cheap inorganic phosphazane CDAs as simple alternatives to organic variants for the rapid determination of ee.

The simple inorganic cyclodiphosph(iii)azane chiral derivatisation agent ClP(μ-^tBuN)₂POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be effective in the measurement of ee’s of chiral amines using ³¹P NMR spectroscopy.

Enantioselective Recognition and Separation of C2 Symmetric Substances via Chiral Metal-Organic Frameworks

A promising route toward the enantioselective recognition and separation of racemic molecules is the design of chiral metal-organic frameworks (CMOFs) with high enantioselectivity and stability. Herein, we report porous CMOFs Δ- and Λ-RuEu-MOFs constructed from the D₃-symmetry helical chiral Ru(phen)₃-derived tricarboxylate ligand and Eu₂ units, which can be utilized as adsorbents for the enantioselective recognition and separation of 1,1'-bi-2-naphthol (BINOL) derivatives. Investigation of the circular dichroism enantiodifferentiation between the host and guest suggested that Δ- and Λ-RuEu-MOFs can be employed as chiral sensors to discriminate axial enantiomers due to their diastereomeric host-guest relationship. Density functional theory calculations reveal that chiral recognition is attributed to the distinguishing binding affinities stemming from N···H-O hydrogen bonds and π-π stacking between the host and guest. Moreover, the reticulate structure of Δ- and Λ-RuEu-MOFs can be readily recycled and reused for the successive enantioselective separation of BINOL up to 80% ee.

Effective enantiomeric identification of aromatic amines by tyrosine-modified pillar[5]arenes as chiral NMR solvating agents

Two novel tyrosine-modified pillar[5]arenes have been synthesized and applied as chiral NMR solvating agents to establish an efficient ¹H NMR method for enantioselective recognition and configuration assignment towards α-aromatic ethylamines.