The role of chirality in a set of key intermediates of pharmaceutical interest, 3-aryl-substituted-γ-butyrolactones, evidenced by chiral HPLC separation and by chiroptical spectroscopies

Recent advances in chiral selectors immobilization and chiral mobile phase additives in liquid chromatographic enantio-separations: A review

For decades now, the separation of chiral enantiomers of drugs has been gaining the interest and attention of researchers. In 1991, the first guidelines for development of chiral drugs were firstly released by the US-FDA. Since then, the development in chromatographic enantioseparation tools has been fast and variable, aiming at creating a suitable environment where the physically and chemically identical enantiomers can be separated. Among those tools, the immobilization of chiral selectors (CS) on different stationary phases and the chiral mobile phase additives (CMPA) which have been progressed and studied extensively. This review article highlights the major advances in immobilization of CS together with their different recognition mechanisms as well as CMPA as a cheaper and successful alternative for chiral stationary phases. Moreover, the role of molecular modeling tool as a pre-step in the choice of CS for evaluating possible interactions with different ligands has been pointed up. Illustrations of reported methods and updates for immobilized CS and CMPA have been included.

Impact of conformation and intramolecular interactions on vibrational circular dichroism spectra identified with machine learning

Vibrational Circular Dichroism (VCD) spectra often differ strongly from one conformer to another, even within the same absolute configuration of a molecule. Simulated molecular VCD spectra typically require expensive quantum chemical calculations for all conformers to generate a Boltzmann averaged total spectrum. This paper reports whether machine learning (ML) can partly replace these quantum chemical calculations by capturing the intricate connection between a conformer geometry and its VCD spectrum. Three hypotheses concerning the added value of ML are tested. First, it is shown that for a single stereoisomer, ML can predict the VCD spectrum of a conformer from solely the conformer geometry. Second, it is found that the ML approach results in important time savings. Third, the ML model produced is unfortunately hardly transferable from one stereoisomer to another.

From Nature to Synthetic Compounds: Novel 1(N),2,3 Trisubstituted-5-oxopyrrolidines Targeting Multiple Myeloma Cells

The insurgence of drug resistance in treating Multiple Myeloma (MM) still represents a major hamper in finding effective treatments, although over the past decades new classes of drugs, such as proteasome inhibitors and immunomodulatory drugs, have been discovered. Recently, our research team, within a Nature-Aided Drug Discovery project, isolated from Hibiscus Sabdariffa L. calyces the secondary metabolite called Hib-ester which possesses antiproliferative properties against human multiple myeloma RPMI 8226 cells, reduces migration and cell invasion and inhibits proteasome without neurotoxic effects. In the present study, we explored the chemical spaces of the hit compound Hib-ester. We explored the structure-activity relationships (SAR), and we optimized the scaffold through sequentially modifying Hib-ester subunits. Compound screening was performed based on cytotoxicity against the RPMI 8226 cells to assess the potential efficacy toward human MM. The ability of the most effective molecules to inhibit the proteasome was evaluated and the binding mode of the most promising compounds in the proteasome chymotrypsin binding pocket was deciphered through molecular modeling simulations. Compounds 13 and 14 are more potent than Hib-ester, demonstrating that our strategy was suitable for the identification of a novel chemotype for developing possible drug candidates and hopefully widening the drug armamentarium against MM.

Spectroscopic investigation on 1,2-substituted ferrocenes with only planar chirality: How chiroptical data are related to absolute configuration and to substituents

Single enantiomers of three 1,2-substituted ferrocene derivatives, i.e. 1-methoxymethyl-2-hydroxymethylferrocene (1), 1-formyl-2-hydroxymethylferrocene (2) and 1-iodo-2-hydroxymethylferrocene (3), sharing the common hydroxymethyl substituent and the presence of planar chirality only, were investigated for their spectroscopic (IR and UV) and chiroptical (VCD and ECD) properties. Both enantiomers of 1 were obtained for the first time in optically pure form by lipase-catalyzed kinetic resolution of the corresponding racemate (±)-1 and separately converted into formyl derivatives (+)-2 and (-)-2. The experimental spectroscopic and chiroptical data were compared with DFT calculated spectra and excellent correspondence was found for all compounds, allowing one to confirm the previously assigned absolute configurations. The common features in the VCD spectra of a doublet between 940 and 965 cm^-1 and the short-wavelength (about 200 nm) doublet and the longest wavelength band in the ECD spectra were analyzed to test whether they may be taken as markers of the absolute configuration (AC). The predominance of conformers with intramolecular hydrogen bond for the first two investigated compounds is predicted by conformational analysis and also confirmed by NMR.

Chiral 2-phenyl-3-hydroxypropyl esters as PKC-alpha modulators: HPLC enantioseparation, NMR absolute configuration assignment, and molecular docking studies

Protein kinase C (PKC) isoforms play a pivotal role in the regulation of numerous cellular functions, making them extensively studied and highly attractive drug targets. In our previous work, we identified in racemate 1-2, based on the 2-benzyl-3-hydroxypropyl ester scaffold, two new potent and promising PKCα and PKCδ ligands, targeting the C1 domain of these two kinases. Herein, we report the resolution of the racemates by enantioselective semi-preparative HPLC. The attribution of the absolute configuration (AC) of homochirals 1 was performed by NMR, via methoxy-α-trifluoromethyl-α-phenylacetic acid derivatization (MTPA or Mosher's acid). Moreover, the match between the experimental and predicted electronic circular dichroism (ECD) spectra confirmed the assigned AC. These results proved that Mosher's esters can be properly exploited for the determination of the AC also for chiral primary alcohols. Lastly, homochiral 1 and 2 were assessed for binding affinity and functional activity against PKCα. No significative differences in the K_i of the enantiopure compounds was observed, thus suggesting that chirality does not seem to play a significant role in targeting PKC C1 domain. These results are in accordance with the molecular docking studies performed using a new homology model for the human PKCαC1B domain.

Vibrational Circular Dichroism study of chiral food additives: γ-valero- and γ-caprolactone

γ-valerolactone and γ-caprolactone are commonly used as flavor additives in the food industry. In the present work we fully explore the molecular structure and conformational distribution of enantiopure γ-valerolactone and γ-caprolactone in solution state by using Vibrational Circular Dichroism (VCD) spectroscopy assisted by quantum chemical calculations. In order to establish the most accurate DFT method for this type of samples a set of methods and basis sets have been implemented and their performances have been compared. Subsequently, we have performed a complete vibrational assignment, which allowed to detect certain key vibrational features related to specific solution-state conformational speciation. In spite of the rigidity of the samples being studied, our results point to the incidence of conformational mixture in CCl₄ solution in both samples.

Microwave-Assisted Extraction and HPLC-UV-CD Determination of (S)-usnic Acid in Cladonia foliacea

During the years, many usnic acid (UA) conjugates have been synthesized to obtain potent endowed with biological properties. Since (S)-UA is less abundant in nature than (R)-enantiomer, it is difficult to source, thus precluding a deeper investigation. Among the lichens producing UA, Cladonia foliacea is a valuable (S)-UA source. In the present work, we report on a rapid HPLC-UV/PAD-CD protocol suitable for the analysis and the identification of the main secondary metabolites present in C. foliacea extract. Best results were achieved using XBridge Phenyl column and acetonitrile and water, which were both added with formic acid as mobile phase in gradient elution. By combining analytical, spectroscopical, and chiroptical analysis, the most abundant analyte was unambiguously identified as (S)-UA. Accordingly, a versatile microwave-assisted extractive (MAE) protocol, assisted by a design of experiment (DoE), to quantitatively recover (S)-UA was set up. The best result in terms of UA extraction yield was obtained using ethanol and heating at 80 °C under microwave irradiation for 5 min. Starting from 100 g of dried C. foliacea, 420 mg of (S)-UA were achieved. Thus, our extraction method resulted in a suitable protocol to produce (S)-UA from C. foliacea for biological and pharmaceutical investigation or commercial purposes.

Enantiomeric Resolution and Absolute Configuration of a Chiral δ-Lactam, Useful Intermediate for the Synthesis of Bioactive Compounds

During the past several years, the frequency of discovery of new molecular entities based on γ- or δ-lactam scaffolds has increased continuously. Most of them are characterized by the presence of at least one chiral center. Herein, we present the preparation, isolation and the absolute configuration assignment of enantiomeric 2-(4-bromophenyl)-1-isobutyl-6-oxopiperidin-3-carboxylic acid (trans-1). For the preparation of racemic trans-1, the Castagnoli-Cushman reaction was employed. (Semi)-preparative enantioselective HPLC allowed to obtain enantiomerically pure trans-1 whose absolute configuration was assigned by X-ray diffractometry. Compound (+)-(2R,3R)-1 represents a reference compound for the configurational study of structurally related lactams.

New naphthyl derivatives from Aspergillus californicus

Two new naphthyl-products calinaphthyltriol A (1) and calinaphthalenone A (2) were isolated from Aspergillus californicus IBT 16748 together with one known compound ophiobolin X (3). Their structures were elucidated by extensive spectroscopic analyses. The absolute configuration of 2 was solved by comparing its optical rotation with data for the known compounds 4, 5, and 6 as well as theoretical calculations. The antibacterial and cytotoxic activities of 1 and 3 were evaluated. Both compounds did not show antibacterial activity (MIC > 96 µg·ml^-1) against a few selected clinically relevant Gram positive and Gram negative bacterial strains. However, they showed moderate cytotoxicity against HL-60 cell line with IC₅₀ values of 18 and 24 µg·ml^-1, respectively.

Solvent Effects and Aggregation Phenomena Studied by Vibrational Optical Activity and Molecular Dynamics: The Case of Pantolactone

Raman and Raman optical activity (ROA), IR, and vibrational circular dichroism (VCD) spectra of (R)- and (S)-pantolactone have been recorded in three solvents. ROA has been employed on water and DMSO solutions, VCD on DMSO and CCl₄ solutions. In the last solvent, monomer-dimer equilibrium is present. Due to the low conformational flexibility of the isolated molecule and to the possibility of aggregation, this compound has been used here to test different protocols for computation of the spectroscopic responses taking into account solvent effects. Molecular dynamics (MD) simulations have been carried out together with statistical clustering methods based on collective variables to extract the structures needed to calculate the spectra. Quantum mechanical DFT calculations based on PCM are compared with approaches based on different representations of the solvent shell (MM or QM level). Appropriate treatment of the solvent permits obtaining of good band-shapes, with the added advantage that the MD analysis allows one to take into account flexibility of dimeric structures justifying the broadness of observed bands and the absence of intense VCD couplets in the carbonyl and OH stretching regions.

BOPC1 Enantiomers Preparation and HuR Interaction Study. From Molecular Modeling to a Curious DEEP-STD NMR Application

The Hu family of RNA-binding proteins plays a crucial role in post-transcriptional processes; indeed, Hu-RNA complexes are involved in various dysfunctions (i.e., inflammation, neurodegeneration, and cancer) and have been recently proposed as promising therapeutic targets. Intrigued by this concept, our research efforts aim at identifying small molecules able to modulate HuR-RNA interactions, with a focus on subtype HuR, upregulated and dysregulated in several cancers. By applying structure-based design, we had already identified racemic trans-BOPC1 as promising HuR binder. In this Letter, we accomplished the enantio-resolution, the assignment of the absolute configuration, and the recognition study with HuR of enantiomerically pure trans-BOPC1. For the first time, we apply DEEP (differential epitope mapping)-STD NMR to study the interaction of BOPC1 with HuR and compare its enantiomers, gaining information on ligand orientation and amino acids involved in the interaction, and thus increasing focus on the in silico binding site model.

Insight into GEBR-32a: Chiral Resolution, Absolute Configuration and Enantiopreference in PDE4D Inhibition

Alzheimer's disease is the most common type of dementia, affecting millions of people worldwide. One of its main consequences is memory loss, which is related to downstream effectors of cyclic adenosine monophosphate (cAMP). A well-established strategy to avoid cAMP degradation is the inhibition of phosphodiesterase (PDE). In recent years, GEBR-32a has been shown to possess selective inhibitory properties against PDE type 4 family members, resulting in an improvement in spatial memory processes without the typical side effects that are usually correlated with this mechanism of action. In this work, we performed the HPLC chiral resolution and absolute configuration assignment of GEBR-32a. We developed an efficient analytical and semipreparative chromatographic method exploiting an amylose-based stationary phase, we studied the chiroptical properties of both enantiomers and we assigned their absolute configuration by 1H-NMR (nuclear magnetic resonance). Lastly, we measured the IC50 values of both enantiomers against both the PDE4D catalytic domain and the long PDE4D3 isoform. Results strongly support the notion that GEBR-32a inhibits the PDE4D enzyme by interacting with both the catalytic pocket and the regulatory domains.

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.

Separation and detection of cyproconazole enantiomers and its stereospecific recognition with chiral stationary phase by high-performance liquid chromatography

An optimal chiral analytical method of cyproconazole enantiomers was established based on BBD, and the stereospecific recognition mechanism was elucidated by docking.

Enantioseparation, Stereochemical Assignment and Chiral Recognition Mechanism of Sulfoxide-Containing Drugs

The distinct pharmacodynamic and pharmacokinetic properties of enantiopure sulfoxide drugs have stimulated us to systematically investigate their chiral separation, stereochemical assignment, and chiral recognition mechanism. Herein, four clinically widely-used sulfoxide drugs were chosen and optically resolved on various chiral stationary phases (CSPs). Theoretical simulations including electronic circular dichroism (ECD) calculation and molecular docking were adopted to assign the stereochemistry and reveal the underlying chiral recognition mechanism. Our results showed that the sequence of calculated mean binding energies between each pair of enantiomers and CSP matched exactly with experimentally observed enantiomeric elution order (EEO). It was also found that the length of hydrogen bond might contribute dominantly the interaction between two enantiomers and CSP. We hope our study could provide a fresh perspective to explore the stereochemistry and chiral recognition mechanism of chiral drugs.

Gamma irradiation-assisted degradation of rosmarinic acid and evaluation of structures and anti-adipogenic properties

Radiation is a promising technique for improving the safety and shelf-life of processed foods. In the present investigation, the degradation mechanism and bioactivity improvement of rosmarinic acid (RA) were studied in response to various gamma irradiation doses (10, 20, and 50 kGy). RA exposed to gamma irradiation at 50 kGy was completely degraded and showed an increased inhibitory effect against 3 T3-L1 preadipocyte compare to the parent compound. Structures of the newly generated compounds 2-4 from irradiated RA at 50 kGy were elucidated based on spectroscopic methods, including 1H nuclear magnetic resonance (NMR) and mass spectrometry (MS). Interestingly, compounds 2 and 5 exhibited significantly enhanced anti-adipogenic properties in 3 T3-L1 cells compared to the original compound. These results provide evidence that structural changes in RA induced by gamma irradiation might enhance biological efficacy.

Importance and Difficulties in the Use of Chiroptical Methods to Assign the Absolute Configuration of Natural Products: The Case of Phytotoxic Pyrones and Furanones Produced by Diplodia corticola

α-Pyrones and furanones are metabolites produced by Diplodia corticola, a pathogen of cork oak. Previously, the absolute configuration (AC) of diplopyrone was defined by chiroptical methods and Mosher's method. Using X-ray and chiroptical methods, the AC of sapinofuranone C was assigned, while that of the (4S,5S)-enantiomer of sapinofuranone B was established by enantioselective total synthesis. Diplofuranone A and diplobifuranylones A-C ACs are still unassigned. Here electronic and vibrational circular dichroism (ECD and VCD) and optical rotatory dispersion (ORD) spectra are reported and compared with density functional theory computations. The AC of the (4S,5S)-enantiomer of sapinofuranone B and sapinofuranone C is checked for completeness. The AC of diplobifuranylones A-C is assigned as (2S,2'S,5'S,6'S), (2S,2'R,5'S,6'R), and (2S,2'S,5'R,6'R), respectively, with the Mosher's method applied to define the absolute configuration of the carbinol stereogenic carbon. The AC assignment of sapinofuranones is problematic: while diplofuranone A is (4S,9R), sapinofuranones B and C are (4S,5S) according to ORD and VCD, but not to ECD. To eliminate these ambiguities, ECD and VCD spectra of a di-p-bromobenzoate derivative of sapinofuranone C are measured and calculated. For phytotoxicity studies, it is relevant that all six compounds share the S configuration for the stereogenic carbon atom of the lactone moiety.

(R)-(-)-Aloesaponol III 8-Methyl Ether from Eremurus persicus: A Novel Compound against Leishmaniosis

Leishmaniosis is a neglected tropical disease which affects several millions of people worldwide. The current drug therapies are expensive and often lack efficacy, mainly due to the development of parasite resistance. Hence, there is an urgent need for new drugs effective against Leishmania infections. As a part of our ongoing study on the phytochemical characterization and biological investigation of plants used in the traditional medicine of western and central Asia, in the present study, we focused on Eremurus persicus root extract in order to evaluate its potential in the treatment of leishmaniosis. As a result of our study, aloesaponol III 8-methyl ether (ASME) was isolated for the first time from Eremurus persicus root extract, its chemical structure elucidated by means of IR and NMR experiments and the (R) configuration assigned by optical activity measurements: chiroptical aspects were investigated with vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectroscopies and DFT (density functional theory) quantum mechanical calculations. Concerning biological investigations, our results clearly proved that (R)-ASME inhibits Leishmania infantum promastigotes viability (IC₅₀ 73 µg/mL), inducing morphological alterations and mitochondrial potential deregulation. Moreover, it is not toxic on macrophages at the concentration tested, thus representing a promising molecule against Leishmania infections.