Enantioselective Recognition and Separation of Racemic 1-Phenylethanol by a Pair of 2D Chiral Coordination Polymers

A comprehensive review on the enantiomeric separation of chiral drugs using metal-organic frameworks

Chiral drugs play an important role in modern medicine, but obtaining pure enantiomers from racemic mixtures can pose challenges. When a drug is chiral, only one enantiomer (eutomer) typically exhibits the desired pharmacological activity, while the other (distomer) may be biologically inactive or even toxic. Racemic drug formulations introduce additional health risks, as the body must still process the inactive or detrimental enantiomer. Some distomers have also been linked to teratogenic effects and unwanted side effects. Therefore, developing efficient and scalable methods for separating chiral drugs into their pure enantiomers is critically important for improving patient safety and outcomes. Metal-organic frameworks (MOFs) show promise as novel materials for chiral separation due to their highly tunable structures and interactions. This review summarizes recent advancements in using MOFs for chromatographic and spectroscopic resolution of drug enantiomers. Both the opportunities and limitations of MOF-based separation techniques are discussed. A thorough understanding of these methods could aid the continued development of pure enantiomer formulations and help reduce health risks posed by racemic drug mixtures.

Exploring enantioselective recognition of dTMP-Co-bpe coordination polymer for natural amino acids using molecular simulations and circular dichroism

The 1D homochiral coordination polymer (CP-1) {[Co(dTMP)(bpe)2(H2O)3]·9H2O}n was constructed by using 2'-deoxy thymidine 5'-monophosphate disodium salt (dTMP·2Na), and auxiliary ligand bpe (1,2-bis(4-pyridyl)ethene) and characterized by single-crystal XRD, PXRD, IR, UV-visible, CD and TGA analyses. Molecular simulations revealed the selective chiral behaviour of CP-1 towards phenylalanine and histidine, as indicated by their higher binding free energies compared to other amino acids. Theoretical parameters were also compared with experimental UV-visible verdicts. Notably, the D-enantiomers of phenylalanine and histidine demonstrated strong bonding abilities and optimal configurations for probing and distinguishing them from their L-counterparts. These findings led to propositions suggesting that the dissimilarities between these D and L amino acid forms and their binding orientations with CP-1 may contribute to alterations in the CD signal. CP-1 exhibited a robust inherent circular dichroism (CD) signal in aqueous solutions, modulated by the presence of specific amino acids, namely D/L phenylalanine and D/L histidine. Leveraging the measurement of CD signal intensity, a sensor capable of detecting unmodified amino acids has been developed. Unlike previously reported approaches that relied on complex chemical reactions between initially CD-silent molecules and probed amino acids, this new method offers a more straightforward means of amplifying the CD signal. Consequently, this change facilitates a more accurate differentiation between the enantiomers of these specific amino acids compared to others.

Chiral Assembly and Recognition of Seven Copper (II) Coordination Polymers from Tartaric Acid Derivative Ligands

A pair of enantiomeric ligands, (2R,3R)-dibenzyl-2,3-bis(isonicotinoyloxy)succinate ((R,R)-L) and (2S,3S)-dibenzyl-2,3-bis(isonicotinoyloxy)succinate ((S,S)-L), are designed and synthesized. Seven copper (II) coordination polymers {[Cu((R,R)-L)Br 2 (THF)]·CH 3 CN} n ( 1a ) and {[Cu((S,S)-L)Br 2 (THF)]·CH 3 CN} n ( 1b ), {[Cu((R,R)-L)Cl 2 (THF)]·CH 3 CN} n ( 2a ) and {[Cu((S,S)-L)Cl 2 (THF)]·CH 3 CN} n ( 2b ), {[Cu((R,R)-L)(NO 3 ) 2 (CH 3 CN)]} n ( 3a ) and {[Cu((S,S)-L)(NO 3 ) 2 (CH 3 CN)]} n ( 3b ), {[Cu((R,R)-L) 2 (CH 3 CN) 2 ](ClO 4 ) 2 ·3CH 3 CN} n ( 4 ) were obtained through the assemblies with CuBr 2 , CuCl 2 ·2H 2 O, Cu(NO 3 ) 2 ·3H 2 O, Cu(ClO 4 ) 2 ·6H 2 O, respectively. Single-crystal X-ray diffraction and circular dichroism analysis demonstrate that 1a-3a , 1b-3b have a mono chiral one-dimensional (1D) chain-like spiral structure, while 4 have 1D chain-like structure whose metal centers have chiral propeller coordination environment. Ligand structure, anions and solvent systems have a regulatory effect on the formation of chiral helical structure. Further investigation has proved that 1a can be used as circular dichroism spectrum probe for monitoring L-/D-cysteine and L-/D-penicillamine configuration and concentration in aqueous media based on ligand interchange mechanism.

Camphor nitroimine: a key building block in unusual transformations and its applications in the synthesis of bioactive compounds

The development of new drugs requires a lot of time and high financial investments. It involves a research network in which there is the participation of several researchers from different areas. For a new drug to reach the market, thousands of substances must be evaluated. There are several tools for this and the use of suitable building blocks can facilitate the process by allowing a lead compound to have suitable parameters. These compounds are key structures containing special functional groups that also permit adequate synthetic transformations, leading to several structures of interest in a short period of time. In this review, the use of camphor nitroimine as a potential key building block is explored. Derived from camphor, an abundant natural product present in various plant species, this nitroimine has proved to be quite versatile, allowing the access to substances with miscellaneous biological activities, ligands to asymmetric catalysis, asymmetric oxidants, O-N transfer agents and other applications. Its easy conversion to camphecene and other derivatives is described, as well as their applications in medicinal chemistry. Druglikeness analyses were performed on these studied agents as well as on their bioactive derivatives in order to assess their use in the development of potential drugs.

Enantioselective chiral sorption of 1-phenylethanol by homochiral 1D coordination polymers

Enantiomeric selectivity is shown within the pores of a 1D coordination polymer, dependent on the nature of the pore space.

A pair of 2D chiral Ag(i) enantiomers with dual chiral elements: syntheses, structures, and photoluminescent and chiroptical properties

In this work, two new enantiopure bis-monodentate N-donor chiral ligands, namely (-)/(+)-2-(4'-pyridyl)-4,5-pinene-pyridine (L R /L S ), have been designed and synthesized. Using L R and L S as bridging ligands to react with AgClO4, a pair of novel 2D chiral Ag(i) enantiomers formulated as [Ag2(L R )2(ClO4)2] n (R-1) and [Ag2(L S )2(ClO4)2] n (S-1) were isolated and characterized. In R-1 and S-1, each Ag(i) ion is bonded by two N atoms from two different chiral L R or L S ligands, leading to the formation of 1D right- or left-handed -L-Ag(i)-L- helical chains. Moreover, two adjacent helical chains are further doubly linked by two monodentate ClO4 - anions through weak Ag-O contacts to form 2D network structures, in which dual chiral elements, i.e., center chirality and helical chirality coexist. Interestingly, each free ligand L R /L S and R-1/S-1 enantiomers show very different ECD spectra in the solid state and in solution, which are correlated to the intermolecular interactions and molecular structures in each state, respectively. Notably, as a representative, R-1 exhibits intense room temperature photoluminescence both in the solid state and in solution with different emission features and mechanisms, while it also shows more intense emission than that of free ligand L R . In particular, R-1 and S-1 represent the first examples of 2D Ag(i) chiral coordination polymers (CCPs) supported by ClO4 - anions, possessing dual chiral elements.

Template effect of innocent and coordinating anions on the formation of interpenetrated 2D and 3D networks: methyl orange and iodine sorption studies

A pyridyl based Schiff base resulted in a 2D staircase network (CP1) with Cd(ii) when ClO₄⁻ is the counter anion while the use of benzene-1,3-disulphonate counter anion resulted in a novel 3D Cd(ii) coordination polymer with threefold interpenetration (CP2).

Three pairs of luminescent coordination polymers based on CoII and CdII clusters for the detection of antibiotics, pesticides and chiral nitro aromatic compounds

Three couples of coordination polymers (CPs), namely, [Co((R/S)-Hcna)₂]_n (1-D/L), [Cd₆((1R,2R/1S,2S)-cpba)₄(phen)₆(H₂O)₃]_n (2-D/L) and [Cd₂((1R,2R/1S,2S)-Hcpba)₂(phen)₂]_n (3-D/L) {(R/S)-H₂cna = (R/S)-6-(1-carboxyethoxy)-2-naphthoic acid, (1R,2R/1S,2S)-H₃cpba = (1R,2R/1S,2S)-2,2′-((5-carboxy-1,3-phenylene)bis(oxy))dipropionic acid, phen = 1,10-phenanthroline} are successfully synthesized under hydrothermal conditions. Structural analysis shows that CP 1 has a 3D 3,6-c net structure with a point symbol of (4·6²)₂(4²·6¹⁰·8³). CPs 2 and 3 are obtained under very similar reaction conditions except using different solvent ratios. The presence of the planar chelating ligand phen in CPs 2 and 3 limited the spatial growth of the structure, resulting in the formation of different 1D structures. All CPs crystallized in the chiral space group P2₁, CPs 1–3 are all SHG active. Their luminescence sensing activities for organics such as antibiotics, pesticides and nitro aromatics are also investigated. The results showed that CP 1 can effectively identify trace amounts of nitrofurans (NFs) and CP 3 has obvious recognition ability toward nitrofurans (NFs) and nitroimidazoles (NMs). Both CPs 1 and 3 could selectively detect 2,6-dichloro-4-nitroaniline (DCN). The luminescence of CPs 1 and 3 can also be quenched by (D/L)-4-nitrophenylalanine ((D/L)-NPA) and (1R,2R/1S,2S)-2-amino-1-(4-nitrophenyl)propane-1,3-diol ((1R,2R/1S,2S)-ANPO).

Three pairs of luminescent coordination polymers based on (R/S)-H₂cna and (1R,2R/1S,2S)-H₃cpba for the detection of antibiotics, pesticides and chiral nitro aromatic compounds.

Enantioselective Separation over a Chiral Biphenol-Based Metal-Organic Framework

A chiral porous 3D metal-organic framework (MOF) is constructed from an enantiopure carboxylate ligand of 1,1'-biphenol, which can be utilized as adsorbent for the separation of aromatic alcohols and sulfoxides with enantioselectivity of up to 99.4%. Single-crystal X-ray diffraction analysis reveals the binding sites and host-guest interactions clearly, providing microscopic insight into the origin of the enantiosorption in the framework.

Homochiral coordination polymers constructed from V-shaped oxybisbenzoyl-based amino acid derivatives: structures, magnetic and photoluminescence properties

Five novel amino acid-modified HCPs have been prepared using the V-shaped semi-rigid 4,4′-oxybis(benzoic acid) instead of usual rigid polycarboxylate units.

Synthesis of Helical Phenolic Resin Bundles through a Sol-Gel Transcription Method

Chiral and helical polymers possess special helical structures and optical property, and may find applications in chiral catalysis and optical devices. This work presents the preparation and formation process of helical phenolic resins through a sol-gel transcription method. A pair of bola-type chiral low-molecular-weight gelators (LMWGs) derived from valine are used as templates, while 2,4-dihydroxybenzoic acid and formaldehyde are used as precursors. The electron microscopy images show that the phenolic resins are single-handed helical bundles comprised of helical ultrafine nanofibers. The diffused reflection circular dichroism spectra indicate that the helical phenolic resins exhibit optical activity. A possible formation mechanism is proposed, which shows the co-assembly of the LMWGs and the precursors.

Homochiral and heterochiral Mn(II) coordination frameworks: spontaneous resolution dependent on dipyridyl ligands

An unsymmetric tetracarboxylic ligand collaborates with 4,4'-bipyridine to induce spontaneous resolution to form homochiral 3D coordination frameworks, while the use of longer dipyridyl ligands instead of 4,4'-bipyridine leads to isoreticular but heterochiral frameworks.

Synthesis and Biological Activities of Camphor Hydrazone and Imine Derivatives

Both sonochemical and classical methodologies have been employed to convert camphor, 1,7,7-trimethylbicyclo[2.2.1]heptan-2-one, C₉H₁₆C=O, into a number of derivatives including hydrazones, C₉H₁₆C=N-NHAr 3, imines, C₉H₁₆C=N-R 7, and the key intermediate nitroimine, C₉H₁₆C=N-NO₂ 6. Reactions of nitroamine 6 with nucleophiles by classical methods provided the desired compounds in a range of yields. In evaluations of activity against Mycobacterium tuberculosis, compound 7j exhibited the best activity (minimal inhibitory concentration (MIC) = 3.12 µg/mL), comparable to that of the antitubercular drug ethambutol. The other derivatives displayed modest antimycobacterial activities at 25-50 µg/mL. In in vitro tests against cancer cell lines, none of the synthesized camphor compounds exhibited cytotoxic activities.

Emerging enantiomeric resolution materials with homochiral nano-fabrications

The major scientific challenge of enantiomeric separation is to develop simple, rapid, and sensitive routine analytical methods. Generally, enantio-resolution is still based on "three-point interaction" theory, which indicates that homochiral sites are needed for enantio-selective interaction. However, in recent years, advanced materials with precise homochiral fabrication at the nanoscale have been synthesized, and have shown great potential in development of high-throughput enantio-resolution methods. This tutorial review summarizes fabrication and applications of homochiral materials for enantio-selective recognition and separation. These materials, which include intrinsic and restructured chiral metal surfaces, plasmonic nanostructures, coordination polymers, organic polymer sensors, and molecularly imprinted polymers, have been applied as sensors or chiral stationary phases (CSPs) for efficient enantio-resolution.

Bulky metallocavitands with a chiral cavity constructed by aluminum and magnesium atrane-likes: enantioselective recognition and separation of racemic alcohols

Novel tetranuclear metallocavitands with a chiral cavity were synthesized via self-assembly of aluminum/magnesium atrane-likes and were successfully applied to the chiral resolution of 2-butanol.

Optical recognition of alkyl nitrile by a homochiral iron(ii) spin crossover host

In this work, a chiral mononuclear spin crossover host1·MeCN was found to have the ability to optically recognize lactonitrile (LN) and methylglutaronitrile (MGN).

Applications of homochiral metal-organic frameworks in enantioselective adsorption and chromatography separation

Chiral separation is of great importance for drug development, pharmacology, and biology. Chiral metal-organic frameworks (MOFs) is a new class of porous solid materials with high surface area, large pore size, high chemical stability, uniformly structured cavities, and the availability of modification. The excellent properties of MOFs have attracted intense interest to explore their performance and mechanism in chiral separation. This review summarizes three synthetic strategies of chiral MOFs and their applications in enantioselective adsorption and chromatographic separation. All the experimental and molecular simulation results demonstrated that high enantioselectivity was strongly correlated with a close match between the size of the pore and chiral molecules.