Enantioseparation of planar chiral ferrocenes on cellulose-based chiral stationary phases: Benzoate versus carbamate pendant groups.

In this study, the enantioseparation of 14 planar chiral ferrocenes containing halogen atoms, and methyl, iodoethynyl, phenyl, and 2-naphthyl groups, as substituents, was explored with a cellulose tris(4-methylbenzoate) (CMB)-based chiral column under multimodal elution conditions. n-Hexane/2-propanol (2-PrOH) 95:5 v/v, pure methanol (MeOH), and MeOH/water 90:10 v/v were used as mobile phases (MPs). With CMB, baseline enantioseparations were achieved for nine analytes with separation factors (α) ranging from 1.24 to 1.77, whereas only three analytes could be enantioseparated with 1.14 ≤ α ≤ 1.51 on a cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC)-based column, used as a reference for comparison, under the same elution conditions. Pendant group-dependent reversal of the enantiomer elution order was observed in several cases by changing CMB to CDMPC. The impact of analyte and chiral stationary phase (CSP) structure, and MP polarity on the enantioseparation, was evaluated. The two cellulose-based CSPs featured by different pendant groups were also compared in terms of thermodynamics. For this purpose, enthalpy (ΔΔH°), entropy (ΔΔS°) and free energy (ΔΔG°) differences, isoenantioselective temperatures (Tiso ), and enthalpy/entropy ratios (Q), associated with the enantioseparations, were derived from van 't Hoff plots by using n-hexane/2-PrOH 95:5 v/v and methanol/water 90:10 v/v as MPs. With the aim to disclose the functions of the different substituents in mechanisms and noncovalent interactions underlying analyte-selector complex formation at molecular level, electrostatic potential (V) analysis and molecular dynamics simulations were used as computational techniques. On this basis, enantioseparations and related mechanisms were investigated by integrating theoretical and experimental data.

Identification of acute ST-elevation myocardial infarction via remote implantable loop recorder monitor.

The indications for the use of implantable loop recorders include the evaluation of unexplained palpitations and syncope, embolic stroke of undetermined source, dizziness and lightheadedness presumed to be due to arrhythmogenic etiology, and for atrial fibrillation to guide antiarrhythmic drug therapy or catheter ablation efficacy. Long-term monitoring is especially beneficial if symptoms occur sporadically or are asymptomatic in nature. This is the first case to our knowledge of an acute ST-elevation myocardial infarction which was identified from remote monitoring of an implantable loop recorder through a device clinic.

Durability of left bundle branch area pacing.

BACKGROUND: Left bundle branch area pacing (LBBAP) is a form of conduction system pacing. Long-term data on the safety and performance of LBBAP 1 year postdevice implantation has not been well described. METHODS AND RESULTS: Sixty-five patients (49% females) who received LBBAP for bradycardia indications using the SelectSecure 3830 lead (Medtronic) were retrospectively evaluated. Clinical variables were examined. Lead parameters were obtained at implant and during regular follow-up. Mean age of patients was 75.7 ± 10.1 years with left ventricular ejection fraction 59.8 ± 10.4%. Indications for pacing were atrioventricular block 55%, sinus node dysfunction 19%, tachy-brady syndrome 15%, atrioventricular node ablation 8%, and bail out cardiac resynchronization therapy 3%. Mean baseline QRS measured 120 ± 38 ms, paced QRS duration was 138 ± 22ms. Paced QRS narrowed by 24 ms in those with pre-existing left bundle branch block (BBB), increased by 1 ms in those with pre-existing right BBB, and increased by 42 ms in those with no BBB. LBBAP threshold at implant was 0.521 ± 0.153 V at 0.4 ms, and increased to 0.654 ± 0.186 V at 3 months (+26%), 0.707 ± 0.186 V at 6 months (+36%), and 0.772 ± 0.220 V at 12 months (+48%). Patients with left BBB showed the maximum benefit with QRS narrowing 24 ms. Pacing impedance remained unchanged with no procedure-related complications. CONCLUSION: LBBAP is a durable form of conduction system pacing with pacing thresholds remaining relatively stable over 12 months post device implantation. Patients with left BBB display the narrowest paced QRS.

Comparative enantioseparation of planar chiral ferrocenes on polysaccharide-based chiral stationary phases.

Planar chiral ferrocenes are well-known compounds that have attracted interest for application in synthesis, catalysis, material science, and medicinal chemistry for several decades. In spite of the fact that asymmetric synthesis procedures for obtaining enantiomerically enriched ferrocenes are available, sometimes, the accessible enantiomeric excess of the chiral products is unsatisfactory. In such cases and for resolution of racemic planar chiral ferrocenes, enantioselective high-performance liquid chromatography (HPLC) on polysaccharide-based chiral stationary phases (CSPs) has been used in quite a few literature articles. However, although moderate/high enantioselectivities have been obtained for planar chiral ferrocenes bearing polar substituents, the enantioseparation of derivatives containing halogens, or exclusively alkyl groups, remains rather challenging. In this study, the enantioseparation of ten planar chiral 1,2- and 1,3-disubstituted ferrocenes was explored by using five polysaccharide-based CSPs under multimodal elution conditions. Baseline enantioseparations were achieved for nine analytes with separation factors (α) ranging from 1.20 to 2.92. The presence of π-extended systems in the analyte structure was shown to impact affinity of the most retained enantiomer toward amylose-based selectors, observing retention times higher than 80 min with methanol-containing mobile phases (MPs). Electrostatic potential (V) analysis and molecular dynamics (MD) simulations were used in order to study interaction modes at the molecular level.

Enantioseparations of polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases and molecular dynamics simulations of selector-selectand interactions.

2'-(4-Pyridyl)- and 2'-(4-hydroxyphenyl)-TCIBPs (TCIBP = 3,3',5,5'-tetrachloro-2-iodo-4,4'-bipyridyl) are chiral compounds that showed interesting inhibition activity against transthyretin fibrillation in vitro. We became interested in their enantioseparation since we noticed that the M-stereoisomer is more effective than the P-enantiomer. Based thereon, we recently reported the enantioseparation of 2'-substituted TCIBP derivatives with amylose-based chiral columns. Following this study, herein we describe the comparative enantioseparation of both 2'-(4-pyridyl)- and 2'-(4-hydroxyphenyl)-TCIBPs on four cellulose phenylcarbamate-based chiral columns aiming to explore the effect of the polymer backbone, as well as the nature and position of substituents on the side groups on the enantioseparability of these compounds. In the frame of this project, the impact of subtle variations of analyte and polysaccharide structures, and mobile phase (MP) polarity on retention and selectivity was evaluated. The effect of temperature on retention and selectivity was also considered, and overall thermodynamic parameters associated with the analyte adsorption onto the CSP surface were derived from van 't Hoff plots. Interesting cases of enantiomer elution order (EEO) reversal were observed. In particular, the EEO was shown to be dependent on polysaccharide backbone, the elution sequence of the two analytes being P-M and M-P on cellulose and amylose tris(3,5-dimethylphenylcarbamate), respectively. In this regard, a theoretical investigation based on molecular dynamics (MD) simulations was performed by using amylose and cellulose tris(3,5-dimethylphenylcarbamate) nonamers as virtual models of the polysaccharide-based selectors. This exploration at the molecular level shed light on the origin of the enantiodiscrimination processes.

Enantioseparation of 5,5'-Dibromo-2,2'-Dichloro-3-Selanyl-4,4'-Bipyridines on Polysaccharide-Based Chiral Stationary Phases: Exploring Chalcogen Bonds in Liquid-Phase Chromatography.

The chalcogen bond (ChB) is a noncovalent interaction based on electrophilic features of regions of electron charge density depletion (σ-holes) located on bound atoms of group VI. The σ-holes of sulfur and heavy chalcogen atoms (Se, Te) (donors) can interact through their positive electrostatic potential (V) with nucleophilic partners such as lone pairs, π-clouds, and anions (acceptors). In the last few years, promising applications of ChBs in catalysis, crystal engineering, molecular biology, and supramolecular chemistry have been reported. Recently, we explored the high-performance liquid chromatography (HPLC) enantioseparation of fluorinated 3-arylthio-4,4'-bipyridines containing sulfur atoms as ChB donors. Following this study, herein we describe the comparative enantioseparation of three 5,5'-dibromo-2,2'-dichloro-3-selanyl-4,4'-bipyridines on polysaccharide-based chiral stationary phases (CSPs) aiming to understand function and potentialities of selenium σ-holes in the enantiodiscrimination process. The impact of the chalcogen substituent on enantioseparation was explored by using sulfur and non-chalcogen derivatives as reference substances for comparison. Our investigation also focused on the function of the perfluorinated aromatic ring as a π-hole donor recognition site. Thermodynamic quantities associated with the enantioseparation were derived from van't Hoff plots and local electron charge density of specific molecular regions of the interacting partners were inspected in terms of calculated V. On this basis, by correlating theoretical data and experimental results, the participation of ChBs and π-hole bonds in the enantiodiscrimination process was reasonably confirmed.

Histologic subtyping affecting outcome of triple negative breast cancer: a large Sardinian population-based analysis.

BACKGROUND: Triple Negative breast cancer (TNBC) includes a heterogeneous group of tumors with different clinico-pathological features, molecular alterations and treatment responsivity. Our aim was to evaluate the clinico-pathological heterogeneity and prognostic significance of TNBC histologic variants, comparing "special types" to high-grade invasive breast carcinomas of no special type (IBC-NST). METHODS: This study was performed on data obtained from TNBC Database, including pathological features and clinical records of 1009 TNBCs patients diagnosed between 1994 and 2015 in the four most important Oncology Units located in different hospitals in Sardinia, Italy. Kaplan-Meier analysis, log-rank test and multivariate Cox proportional-hazards regression were applied for overall survival (OS) and disease free survival (DFS) according to TNBC histologic types. RESULTS: TNBC "special types" showed significant differences for several clinico-pathological features when compared to IBC-NST. We observed that in apocrine carcinomas as tumor size increased, the number of metastatic lymph nodes manifestly increased. Adenoid cystic carcinoma showed the smallest tumor size relative to IBC-NST. At five-year follow-up, OS was 92.1, 100.0, and 94.5% for patients with apocrine, adenoid cystic and medullary carcinoma, respectively; patients with lobular and metaplastic carcinoma showed the worst OS, with 79.7 and 84.3%, respectively. At ten-years, patients with adenoid cystic (100.0%) and medullary (94.5%) carcinoma showed a favourable prognosis, whereas patients with lobular carcinoma showed the worst prognosis (73.8%). TNBC medullary type was an independent prognostic factor for DFS compared to IBC-NST. CONCLUSIONS: Our study confirms that an accurate and reliable histopathologic definition of TNBC subtypes has a significant clinical utility and is effective in the therapeutic decision-making process, with the aim to develop innovative and personalized treatments.

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 (VS) and by comparing and dissecting in atomic/atomic group contributions the VS maxima (VS,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.

Recent studies of docking and molecular dynamics simulation for liquid-phase enantioseparations.

Liquid-phase enantioseparations have been fruitfully applied in several fields of science. Various applications along with technical and theoretical advancements contributed to increase significantly the knowledge in this area. Nowadays, chromatographic techniques, in particular HPLC on chiral stationary phase, are considered as mature technologies. In the last thirty years, CE has been also recognized as one of the most versatile technique for analytical scale separation of enantiomers. Despite the huge number of papers published in these fields, understanding mechanistic details of the stereoselective interaction between selector and selectand is still an open issue, in particular for high-molecular weight chiral selectors like polysaccharide derivatives. With the ever growing improvement of computer facilities, hardware and software, computational techniques have become a basic tool in enantioseparation science. In this field, molecular docking and dynamics simulations proved to be extremely adaptable to model and visualize at molecular level the spatial proximity of interacting molecules in order to predict retention, selectivity, enantiomer elution order, and profile noncovalent interaction patterns underlying the recognition process. On this basis, topics and trends in using docking and molecular dynamics as theoretical complement of experimental LC and CE chiral separations are described herein. The basic concepts of these computational strategies and seminal studies performed over time are presented, with a specific focus on literature published between 2015 and November 2018. A systematic compilation of all published literature has not been attempted.

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.

Clinical and pathological factors influencing survival in a large cohort of triple-negative breast cancer patients.

BACKGROUND: To provide further information on the clinical and pathological prognostic factors in triple-negative breast cancer (TNBC), for which limited and inconsistent data are available. METHODS: Pathological characteristics and clinical records of 841 TNBCs diagnosed between 1994 and 2015 in four major oncologic centers from Sardinia, Italy, were reviewed. Multivariate hazard ratios (HRs) for mortality and recurrence according to various clinicopathological factors were estimated using Cox proportional hazards models. RESULTS: After a mean follow-up of 4.3 years, 275 (33.3%) TNBC patients had a progression of the disease and 170 (20.2%) died. After allowance for study center, age at diagnosis, and various clinicopathological factors, all components of the TNM staging system were identified as significant independent prognostic factors for TNBC mortality. The HRs were 3.13, 9.65, and 29.0, for stage II, III and IV, respectively, vs stage I. Necrosis and Ki-67 > 16% were also associated with increased mortality (HR: 1.61 and 1.99, respectively). Patients with tumor histotypes other than ductal invasive/lobular carcinomas had a more favorable prognosis (HR: 0.40 vs ductal invasive carcinoma). No significant associations with mortality were found for histologic grade, tumor infiltrating lymphocytes, and lymphovascular invasion. Among lymph node positive TNBCs, lymph node ratio appeared to be a stronger predictor of mortality than pathological lymph nodes stage (HR: 0.80 for pN3 vs pN1, and 3.05 for >0.65 vs <0.21 lymph node ratio), respectively. Consistent results were observed for cancer recurrence, except for Ki-67 and necrosis that were not found to be significant predictors for recurrence. CONCLUSIONS: This uniquely large study of TNBC patients provides further evidence that, besides tumor stage at diagnosis, lymph node ratio among lymph node positive tumors is an additional relevant predictor of survival and tumor recurrence, while Ki-67 seems to be predictive of mortality, but not of recurrence.

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.

Recent trends and applications in liquid-phase chromatography enantioseparation of atropisomers.

The term Atropisomerism defines a type of enantiomerism that arises from hindered rotation around an axis that makes two rotational isomers enantiomers. Atropisomeric compounds are present in nature, being important building blocks of many biologically active compounds. Atropisomers have been extensively studied in environmental and toxicological chemistry and, in particular, separating them has become a need in specific fields as organic synthesis and pharmaceutical research. High-performance LC has been fruitfully applied in this context, and despite HPLC separation on chiral stationary phase is considered as mature technology nowadays, in the last years several advancements and applications contributed to study compounds where axial chirality is of utmost importance. Moreover, dynamic chromatography has been exploited as a versatile tool for kinetic studies of systems with slow internal motion gaining essential information on their stereodynamic behavior. On this basis, current topics and trends in liquid-phase chromatography enantioseparation of atropisomers are presented herein with specific focus on new representative applications with HPLC, covering years 2010-2016. Some examples concerning supercritical fluid chromatography applications are also reported. This review aims to provide the reader with a modern overview of the field to highlight the renewed interest toward the chemistry of molecules with atropisomeric properties. A systematic compilation of all published literature has not been attempted.

Liquid chromatography enantioseparations of halogenated compounds on polysaccharide-based chiral stationary phases: role of halogen substituents in molecular recognition.

Halogenated chiral molecules have become important in several fields of science, industry, and society as drugs, natural compounds, agrochemicals, environmental pollutants, synthetic products, and chiral supports. Meanwhile, the perception of the halogen moiety in organic compounds and its role in recognition processes changed. Indeed, the recognition of the halogen bond as an intermolecular interaction occurring when the halogen acts as a Lewis acid had a strong impact, particularly in crystal engineering and medicinal chemistry. Due to this renewed interest in the potentialities of chiral organohalogens, here we focus on selected recent applications dealing with enantioseparations of halogenated compounds on polysaccharide-based chiral stationary phases (CSPs), widely used in liquid chromatography (LC). In particular, recently the first case of halogen bonding-driven high-performance LC (HPLC) enantioseparation was reported on a cellulose-based CSP. Along with enantioseparations performed under conventional HPLC, representative applications using supercritical fluid chromatography (SFC) are reported.

High-performance liquid chromatography enantioseparation of polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases under multimodal elution.

An investigation on the high-performance liquid chromatography enantioseparation of 12 polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases is described. The overall study was directed toward the generation of efficient separations in order to obtain pure atropisomers that will serve as ligands for building homochiral metal organic frameworks. Four coated columns--namely, Lux Cellulose-1, Lux Cellulose-2, Lux Cellulose-4, and Lux Amylose-2--and two immobilized columns--namely, Chiralpak IC and IA--were used under normal, polar organic, and reversed-phase elution modes. Moreover, Chiralcel OJ was considered under normal-phase and polar organic conditions. The effect of the chiral selector and mobile phase composition on the enantioseparation, the enantiomer elution order and the beneficial effect of nonstandard solvents were studied. The effect of water in the mobile phase on the enantioselectivity and retention was investigated and retention profiles typical of hydrophilic interaction liquid chromatography were observed. Interesting phenomena of solvent-induced enantiomer elution order reversal occurred under normal-phase mode. All the considered 4,4'-bipyridines were enantioseparated at the multimilligram level.

Unravelling dispersion forces in liquid-phase enantioseparation. Part II: Planar chiral 1-(iodoethynyl)-3-arylferrocenes.

BACKGROUND: In the first part of our study on possible contribution of dispersion forces in liquid-phase enantioseparations, the enantioseparation of the axially chiral 3,3'-dibromo-5,5'-bis-ferrocenylethynyl-4,4'-bipyridine with an amylose tris(3,5-dimethylphenylcarbamate)-based chiral column appeared reasonably consistent with a picture of the enantioselective recognition based on the interplay between hydrogen bond (HB), π-π stacking and dispersion interactions. RESULTS: In the second part of this study, we evaluated the impact of analyte and chiral stationary phase (CSP) structure, mobile phase and temperature on the enantioseparations of planar chiral 1-(iodoethynyl)-3-arylferrocenes (3-aryl = phenyl, 2-naphthyl, 4-methylphenyl, 4-t-butylphenyl) with polysaccharide-based chiral columns. The main aim of the present study was to understand the molecular bases of the high affinity observed for the second eluted (Rp)-enantiomer of some of these analytes toward amylose phenylcarbamate-based selectors when methanol-containing mixtures were used as mobile phases. Significantly, higher affinity of the second eluted (Rp)-enantiomer toward the selector could be also observed for the sterically hindered 1-(iodoethynyl)-3-(4-t-butylphenyl)ferrocene (k2 = 6.21) compared to the smaller 1-(iodoethynyl)-3-(4-methylphenyl)ferrocenes (k2 = 4.07) as 2.5% methanol was added to the n-hexane-based mobile phase. SIGNIFICANCE: This study reasonably showed that the contribution of dispersion forces may explain the unusually large retention of the second eluted enantiomers observed for the enantioseparation of some planar chiral 1-(iodoethynyl)-3-arylferrocenes with amylose-based selectors. Based on the obtained results, we can conclude that in liquid-phase enantioseparation steric repulsion can be turned into attraction depending on the features of analyte, selector, and mobile phase.

Lithiation of prochiral 2,2'-dichloro-5,5'-dibromo-4,4'-bipyridine as a tool for the synthesis of chiral polyhalogenated 4,4'-bipyridines.

Lithiation of the achiral tetrahalogenated 4,4'-bipyridine 1 with alkyllithiums was investigated. n-BuLi was found to induce either the chlorine-directed deprotolithiation reaction alone or with a concomitant halogen-lithium exchange furnishing after iodine trapping chiral 4,4'-bipyridines 2 and 6, respectively. The role of n-BuLi in the deprotolithiation process of 1 was elucidated on the basis of isolated secondary derivatives. After deprotolithiation, the lithiated species could be trapped by different electrophiles such as MeI, TMSCl, MeSSMe, R3SnCl (R = Me or n-Bu), and PPh2Cl. Moreover, 4,4'-bipyridine 2 was submitted to cross-coupling reactions (Suzuki and Sonogashira) which occurred selectively at the carbon-iodine bond. All compounds of this new family of atropisomeric 4,4'-bipyridines were separated by chiral HPLC (high-performance liquid chromatography), and the absolute configurations of obtained enantiomers were mainly assigned by XRD (X-ray diffraction) using anomalous dispersion.

Comparative HPLC enantioseparation of thirty-six aromatic compounds on four columns of the Lux® series: impact of substituents, shapes and electronic properties.

With the aim to define a combined computational/chromatographic empirical approach useful for the high-performance liquid chromatography (HPLC) method development of new chiral compounds, 36 racemic aromatic compounds with different chemical structures were used as test probes on four polysaccharide-based chiral stationary phases (CSPs) of the Lux series, namely Lux Cellulose-1, Lux Cellulose-2, Lux Cellulose-4, and Lux Amylose-2, using classical n-hexane/2-propanol mixtures as mobile phase. Electrostatic potential surfaces (EPSs) determined using Density Functional Theory (DFT) calculations were used to derive size, shape, and electronic properties of each analyte. Then a comparative HPLC screening was carried out in order to evaluate the impact of substituents, shapes, and electronic properties of the analytes on the chromatographic behavior as the column changes. The four CSPs showed good complementary recognition ability. The elution sequence was determined in 30 cases out of 36. The success rate to afford baseline separations (R(s) ≥ 1.5) was estimated: 29 compounds out of 36 showed baseline enantioseparation on at least one of the four selected CSPs. The combined computational-chromatographic screening furnished useful collective structure-chromatographic behavior relationships and a map of the chiral discrimination abilities of the considered CSPs towards the analytes. On this basis, the chromatographic behavior of new analytes on a set of polysaccharide-based CSPs can be mapped through the qualitative correlation of chromatographic parameters (k, α, R(s)) to computed molecular properties of the analytes.

Optimization of the HPLC enantioseparation of 3,3'-dibromo-5,5'-disubstituted-4,4'-bipyridines using immobilized polysaccharide-based chiral stationary phases.

The HPLC enantioseparation of nine atropisomeric 3,3',5,5'-tetrasubstituted-4,4'-bipyridines was performed in normal and polar organic (PO) phase modes using two immobilized polysaccharide-based chiral columns, namely, Chiralpak IA and Chiralpak IC. The separation of all racemic analytes, the effect of the chiral selector, and mobile phase (MP) composition on enantioseparation and the enantiomer elution order (EEO) were studied. The beneficial effect of nonstandard solvents, such as tetrahydrofuran (THF), dichloromethane (DCM), and methyl t-butyl ether on enantioseparation was investigated. All selected 4,4'-bipyridines were successfully enantioseparated on Chiralpak IA under normal or PO MPs with separation factors from 1.14 to 1.70 and resolutions from 1.3 to 6.5. Two bipyridines were enantioseparated at the multimilligram level on Chiralpak IA. Differently, Chiralpak IC was less versatile toward the considered class of compounds and only five bipyridines out of nine could be efficiently separated. In particular, on these columns, the ternary mixture n-heptane/THF/DCM (90:5:5) as MP had a positive effect on enantioseparation. An interesting phenomenon of reversal of the EEO depending on the composition of the MP for the 3,3'-dibromo-5,5'-bis-(E)-phenylethenyl-4,4'-bipyridine along with an exceptional enantioseparation for the 3,3'-dibromo-5,5'-bis-ferrocenylethynyl-4,4'-bipyridine (α = 8.33, R(s) = 30.6) were observed on Chiralpak IC.