Chiral Hexahalogenated 4,4′-Bipyridines

Evaluation of various polysaccharide-based stationary phases for enantioseparation of chloro-containing derivatives in normal phase liquid chromatography

Six polysaccharide-based chiral stationary phases were screened to separate the enantiomers of six chloro-containing derivatives and one derivative bearing electron donating mesomeric substituents, chosen for comparison. These compounds are expected to be P2X7 receptor antagonists with potential anti-inflammatory activity. The study was carried out with four different mobile phases composed of n-heptane and ethanol or isopropanol. Thus, a total of 168 experiments were implemented to find the best conditions aimed at scaling-up the separation of these anti-inflammatory compounds. Chiralpak AD-H separated half of them, i.e., 1, 2, and 6; Chiralpak AS separated also three out of the six compounds, i.e., 1, 2, and 3; Lux Cellulose-5 separated 2, 4, and 6; Lux Cellulose-2 separated 1, 2, and 4; Chiralcel OD-H separated compounds 2 and 5; and finally Chiralcel OJ separated only 3, thus having the lowest rate of success. Additionally, the influence of (i) the stationary and mobile phases and (ii) the chemical structure of the analytes on retention and resolution was investigated.

Iodinated 4,4'-Bipyridines with Antiproliferative Activity Against Melanoma Cell Lines

In the last decade, biological processes involving halogen bond (HaB) as a leading interaction attracted great interest. However, although bound iodine atoms are considered powerful HaB donors, few iodinated new drugs were reported so far. Recently, iodinated 4,4'-bipyridines showed interesting properties as HaB donors in solution and in the solid state. In this paper, a study on the inhibition activity of seven halogenated 4,4'-bipyridines against malignant melanoma (MM) cell proliferation is described. Explorative dose/response proliferation assays were first performed with three 4,4'-bipyridines by using four MM cell lines and the normal BJ fibroblast cell line as control. Among them, the A375 MM cell line was the most sensitive, as determined by MTT assays, which was selected to evaluate the antiproliferative activity of all 4,4'-bipyridines. Significantly, the presence of an electrophilic iodine impacted the biological activity of the corresponding compounds. The 3,3',5,5'-tetrachloro-2-iodo-4,4'-bipyridine showed significant antiproliferation activity against the A375 cell line, and lower toxicity on BJ fibroblasts. Through in silico studies, the stereoelectronic features of possible sites determining the bioactivity were explored. These results pave the way for the utilization of iodinated 4,4'-bipyridines as templates to design new promising HaB-enabled inhibitors of MM cell proliferation.

Total Synthesis of Floyocidin B: 4,5-Regioselective Functionalization of 2-Chloropyridines

The recently discovered natural product (NP) (+)-floyocidin B with antimicrobial activity against Mycobacterium tuberculosis displays a hitherto unknown dihydroisoquinolinone scaffold in the class of the epoxyquinone NPs. The 4,5-regioselective functionalization of 2-chloropyridines was identified as a suitable strategy leading to the total syntheses of (+)-floyocidin B and analogs. In this paper, we present the long and winding evolution process to the final synthetic pathway, including model systems for route scouting and elucidation of side products, which enabled us to understand the unique reactivity of this unprecedented scaffold. A special focus was laid on method studies with different 2-chloropyridines, disclosing an unexpected effect of the 2-chloro substituent on the regioselectivity compared to 2-unsubstituted or carbon-substituted pyridines. Finally, a head-to-head comparison with the previously described synthesis of all four stereoisomers of the NP (−)-avicennone C revealed significant differences in the reactivity of these structurally closely related scaffolds.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Stereoselective Processes Based on σ-Hole Interactions

The σ-hole interaction represents a noncovalent interaction between atoms with σ-hole(s) on their surface (such as halogens and chalcogens) and negative sites. Over the last decade, significant developments have emerged in applications where the σ-hole interaction was demonstrated to play a key role in the control over chirality. The aim of this review is to give a comprehensive overview of the current advancements in the use of σ-hole interactions in stereoselective processes, such as formation of chiral supramolecular assemblies, separation of enantiomers, enantioselective complexation and asymmetric catalysis.

Characterising Supramolecular Architectures in Crystals Featuring I⋯Br Halogen Bonding: Persistence of X⋯X’ Secondary-Bonding in Their Congeners

The Cambridge Structural Database was surveyed for crystals featuring I⋯Br secondary-bonding in their supramolecular assemblies occurring independently of other obvious supramolecular synthons and devoid of other halogen bonding interactions. In all, 41 crystals satisfied these criteria, with nine examples of zero-dimensional aggregation (uniformly two-molecule aggregates) and 30 one-dimensional chains of varying topology (linear, zigzag and helical). There is one example each of two- and three-dimensional patterns. Type-I, type-II and intermediate bonding situations are apparent; for type-II bonding, the ratio of iodide:bromide functioning as the electrophile is 2:1. Most molecules participated, on average, in one I⋯Br contact, although smaller numbers of half (zero-dimensional) or two contacts (two- and three-dimensional) were observed. The propensity of the formation of related halogen bonding interactions in congeners of the 41 investigated crystals was also studied. Congeners were apparent for 11 crystals, with seven of these exhibiting isostructural relationships, in terms of space-group symmetry and unit-cell parameters. Isostructural relationships do not ensure the formation of analogous aggregation patterns, particularly and in accord with expectation, for the lighter halides. When formed, often distinct aggregation patterns are observed despite the isostructural relationships. Hetero-atomic halogen bonding offers surprises and opportunities in crystal engineering endeavours.

Factors Impacting σ- and π-Hole Regions as Revealed by the Electrostatic Potential and Its Source Function Reconstruction: The Case of 4,4'-Bipyridine Derivatives

Positive electrostatic potential (V) values are often associated with σ- and π-holes, regions of lower electron density which can interact with electron-rich sites to form noncovalent interactions. Factors impacting σ- and π-holes may thus be monitored in terms of the shape and values of the resulting V. Further precious insights into such factors are obtained through a rigorous decomposition of the V values in atomic or atomic group contributions, a task here achieved by extending the Bader-Gatti source function (SF) for the electron density to V. In this article, this general methodology is applied to a series of 4,4'-bipyridine derivatives containing atoms from Groups VI (S, Se) and VII (Cl, Br), and the pentafluorophenyl group acting as a π-hole. As these molecules are characterized by a certain degree of conformational freedom due to the possibility of rotation around the two C-Ch bonds, from two to four conformational motifs could be identified for each structure through conformational search. On this basis, the impact of chemical and conformational features on σ- and π-hole regions could be systematically evaluated by computing the V values on electron density isosurfaces (V_S) and by comparing and dissecting in atomic/atomic group contributions the V_S maxima (V_S,max) values calculated for different molecular patterns. The results of this study confirm that both chemical and conformational features may seriously impact σ- and π-hole regions and provide a clear analysis and a rationale of why and how this influence is realized. Hence, the proposed methodology might offer precious clues for designing changes in the σ- and π-hole regions, aimed at affecting their potential involvement in noncovalent interactions in a desired way.

Rational Design, Synthesis, Characterization and Evaluation of Iodinated 4,4'-Bipyridines as New Transthyretin Fibrillogenesis Inhibitors

The 3,3',5,5'-tetrachloro-2-iodo-4,4'-bipyridine structure is proposed as a novel chemical scaffold for the design of new transthyretin (TTR) fibrillogenesis inhibitors. In the frame of a proof-of-principle exploration, four chiral 3,3',5,5'-tetrachloro-2-iodo-2'-substituted-4,4'- bipyridines were rationally designed and prepared from a simple trihalopyridine in three steps, including a Cu-catalysed Finkelstein reaction to introduce iodine atoms on the heteroaromatic scaffold, and a Pd-catalysed coupling reaction to install the 2'-substituent. The corresponding racemates, along with other five chiral 4,4'-bipyridines containing halogens as substituents, were enantioseparated by high-performance liquid chromatography in order to obtain pure enantiomer pairs. All stereoisomers were tested against the amyloid fibril formation (FF) of wild type (WT)-TTR and two mutant variants, V30M and Y78F, in acid mediated aggregation experiments. Among the 4,4'-bipyridine derivatives, interesting inhibition activity was obtained for both enantiomers of the 3,3',5,5'-tetrachloro-2'-(4-hydroxyphenyl)-2-iodo-4,4'-bipyridine. In silico docking studies were carried out in order to explore possible binding modes of the 4,4'-bipyridine derivatives into the TTR. The gained results point out the importance of the right combination of H-bond sites and the presence of iodine as halogen-bond donor. Both experimental and theoretical evidences pave the way for the utilization of the iodinated 4,4'-bipyridine core as template to design new promising inhibitors of TTR amyloidogenesis.

Halogen bond in separation science: A critical analysis across experimental and theoretical results

The halogen bond (XB) is a noncovalent interaction involving a halogen acting as electrophile and a Lewis base. In the last decades XB has found practical application in several fields. Nevertheless, despite the pivotal role of noncovalent interactions in separation science, investigations of XB in this field are still in their infancy, and so far a limited number of studies focusing on solid phase extraction, liquid-liquid microextraction, liquid-phase chromatography, and gas chromatography separation have been published. In addition, in the last few years, our groups have been systematically studying the potentiality of XB for HPLC enantioseparations. On this basis, in the present paper up-to-date results emerging from focused experiments and theoretical analyses performed by our laboratories are integrated with a descriptive presentation of XB features and the few studies published until now in separation science. Then, the aim of this article is to provide a comprehensive and critical discussion of the topic, and account for some still open issues in the application of XB to separate chemical mixtures.

Chiral Chalcogen Bond Donors Based on the 4,4'-Bipyridine Scaffold

Organocatalysis through chalcogen bonding (ChB) is in its infancy, as its proof-of-principle was only reported in 2016. Herein, we report the design and synthesis of new chiral ChB donors, as well as the catalytic activity evaluation of the 5,5′-dibromo-2,2′-dichloro-3-((perfluorophenyl)selanyl)-4,4′-bipyridine as organocatalyst. The latter is based on the use of two electron-withdrawing groups, a pentafluorophenyl ring and a tetrahalo-4,4′-bipyridine skeleton, as substituents at the selenium center. Atropisomery of the tetrahalo-4,4′-bipyridine motif provides a chiral environment to these new ChB donors. Their synthesis was achieved through either selective lithium exchange and trapping or a site-selective copper-mediated reaction. Pure enantiomers of the 3-selanyl-4,4′-bipyridine were obtained by high performance liquid chromatography enantioseparation on specific chiral stationary phase, and their absolute configuration was assigned by comparison of the measured and calculated electronic circular dichroism spectra. The capability of the selenium compound to participate in σ-hole-based interactions in solution was studied by ¹⁹F NMR. Even if no asymmetric induction has been observed so far, the new selenium motif proved to be catalytically active in the reduction of 2-phenylquinoline by Hantzsch ester.

Halogen bond in high-performance liquid chromatography enantioseparations: Description, features and modelling

Halogen bond (XB)-driven enantioseparations involve halogen-centred regions of electronic charge depletion (σ-hole) as electrophilic recognition sites. The knowledge in this field is still in its infancy. Indeed, although the influence of halogens on enantioseparation have been often considered, only recently the function of electrophilic halogens (Cl, Br, I) as enantioseparations 'drivers' has been demonstrated by our groups. Further to these studies, in this paper we focus on some unexplored issues. First, as XB-driven chiral recognition mechanisms are at an early stage of comprehension, a theoretical investigation based on a series of 32 molecular dynamic (MD) simulations was performed by using polyhalogenated 4,4'-bipyridines and polysaccharide-based polymers as ligands and receptors, respectively. Enantiomer elution orders (EEOs) were derived from calculations and the theoretical model accounted for some analyte- and chiral stationary phase (CSP)-dependent experimental EEO inversions. Then, the function of halogen-centred σ-holes in competitive systems, presenting also hydrogen bond (HB) centres as recognition sites, was considered. In this regard, Pirkle's enantioseparations of halogenated compounds performed on Whelk-O1 were theoretically re-examined and electrostatic potentials (EPs) associated with both σ-holes on halogens and HB centres were computed and compared. Then, the enantioseparation of halogenated 2-nitro-1-arylethanols was performed on cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) and the influence of halogen substituents on the chromatographic results was evaluated by correlating theoretical and experimental data.

Polysaccharide-based chiral stationary phases as halogen bond acceptors: A novel strategy for detection of stereoselective σ-hole bonds in solution

In the last few years, halogen bonds have been exploited in a variety of research areas both in the solid state and in solution. Nevertheless, several factors make formation and detection of halogen bonds in solution challenging. Moreover, to date, few chiral molecules containing electrophilic halogens as recognition sites have been reported. Recently, we described the first series of halogen-bond-driven enantioseparations performed on cellulose tris(3,5-dimethylphenylcarbamate) by high-performance liquid chromatography. Herein the performances of amylose tris(3,5-dimethylphenylcarbamate) as halogen bond acceptor were also investigated and compared with respect to cellulose tris(3,5-dimethylphenylcarbamate). With the aim to explore the effect of polysaccharide backbone on the enantioseparations, the thermodynamic parameters governing the halogen-dependent enantioseparations on both cellulose and amylose polymers were determined by a study at variable temperature and compared. Molecular dynamics were performed to model the halogen bond in polysaccharide-analyte complexes. Chiral halogenated 4,4'-bipyridines were used as test compounds (halogen bond donors). On this basis, a practical method for detection of stereoselective halogen bonds in solution was developed, which is based on the unprecedented use of high-performance liquid chromatography as technical tool with polysaccharide polymers as molecular probes (halogen bond acceptors). The analytical strategy showed higher sensitivity for the detection of weak halogen bonds.

Silver(i) coordination polymers with 3,3′,5,5′-tetrasubstituted 4,4′-bipyridine ligands: towards new porous chiral materials

The first homochiral metal–organic framework (MOF) based on an atropisomeric 4,4′-bipyridine ligand is described.

Insights into halogen bond-driven enantioseparations

Although the halogen bond (XB) has been so far mainly studied in silico and in the solid state, its potential impact in solution is yet to be fully understood. In this study, we describe the first systematic investigation on the halogen bond in solvated environment by high-performance liquid chromatography (HPLC). Thirty three atropisomeric polyhalogenated-4,4'-bipyridines (HBipys), containing Cl, Br and I as substituents, were selected and used as potential XB donors (XBDs) on two cellulose-based chiral stationary phases (CSPs) containing potential XB acceptors (XBAs). The impact of the halogens on the enantiodiscrimination mechanism was investigated and iodine showed a pivotal role on the enantioseparation in non-polar medium. Electrostatic potentials (EPs) were computed to understand the electrostatic component of CSP-analyte interaction. Moreover, van't Hoff studies for ten HBipys were performed and the thermodynamic parameters governing the halogen-dependent enantioseparations are discussed. Finally, a molecular dynamic (MD) simulation is proposed to model halogen bond in polysaccharide-analyte complexes by inclusion of a charged extra point to represent the positive 'σ-hole' on the halogen atom. On the basis of both experimental results and theoretical data, we have profiled the halogen bond as a chemo-, regio-, site- and stereoselective interaction which can work in HPLC environment besides other known interactions based on the complementarity between selector and selectand.