Chiral Purity Research Articles

Molecular Self-Assembly as a Trigger of Life Origin and Development.

The origin and reason for the homochirality of living cells go with the problem of a relatively narrow spectrum of the actual biological monomers compared to the whole theoretically possible spectrum of amino acids or carbohydrates. A limited number of bio-monomers implies some special feature differing from all other similar molecules that are not present in the living cell. Here we propose one of the candidates for such a peculiarity: the ability to form highly elongated helical supramolecular structures(strings) when precipitating from homochiral solutions. The strings' forming can be accompanied by spontaneous splitting and/or chiral purification of the initially racemic mixture. Our previous theoretical reasoning was based mainly on the biomimetic systems, while now we describe the strings forming in homochiral amino acid solutions.

Quality by design-assisted development of a capillary electrophoresis method for the enantiomeric purity determination of tenofovir.

A CE method was developed and validated for the assessment of the chiral purity of the drug tenofovir applying a quality by design approach. Following selection of a quaternary ammonium β-CD as chiral selector, a fractional factorial resolution V+ design was employed for identification of the critical process parameters, while a central composite design served for method optimization. The final method used a 40/50.2cm, 50μm id fused-silica capillary, a BGE composed of a 100mM sodium phosphate buffer, pH 6.4, containing 45mg/mL quaternary ammonium β-CD, an applied voltage of 18kV, and a capillary temperature of 22°C. Robustness was assessed by a Plackett-Burman design. The method was validated according to guideline Q2(R1) of the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use and enabled the determination of the (S)-enantiomer of tenofovir at the 0.1% level.

Organocatalytic Chiral Synthesis of Centrally Acting Muscle Relaxant (S)-Mephenesin

Chiral synthesis of centrally acting muscle relaxant (S)-mephenesin was achieved using L-proline catalyzed a-aminoxylation of 3-(2-methyl-phenxoy)propanal as chirality induction step. The chiral synthesis started with commercially available 2-cresol and was accomplished in four steps with overall yield 56%. The enantiomeric excess of final product (S)-mephenesin is >98%. The chiral purity was determined by chiral-HPLC using Chiralcel-OD column. The synthesis involves oxidation of primary alcohol to aldehyde with iodoxybenzoic acid (IBX) as one of the steps.

Improved Scalable Synthesis of Clinical Candidate KZR‐616, a Selective Immunoproteasome Inhibitor

AbstractKZR‐616 is a selective immunoproteasome inhibitor that is under clinical evaluation for the treatment of Systemic Lupus Erythematosus (SLE). KZR‐616 represents a first‐in‐class drug candidate for the treatment of autoimmune diseases targeting the immunoproteasome. Laboratory replication of the published synthetic route of KZR‐616 resulted in an inseparable mixture of enantiomers and diastereomers. Herein, we describe a stereoselective and scalable synthetic pathway for KZR‐616 that avoids laborious separation of enantiomeric mixtures of amino acids. Moreover, the new route provides KZR‐616 in higher overall yield (42.8 %), did not require a chiral chromatographic purification step, and the final product was obtained in high purity. The synthesized compound was evaluated for inhibition of human immunoproteasome and constitutive proteasome and found to target the β1 and β5 subunits of the immunoproteasome with higher potency than the equivalent subunits of the constitutive proteasome. Therefore, we have confirmed that KZR‐616, synthesized using this new method, is an immunoproteasome selective inhibitor.

Synthesis and Characterization of Optically Pure Gamma‐PNA Backbones by SIBX‐Mediated Reductive Amination

Chiral peptide nucleic acid (PNA) is a derivative of regular PNA by introducing a chiral center to its backbone, and is known to bind more strongly to DNA or RNA than regular PNA. In particular, in the case of a γ‐backbone, the L isomer stabilizes the PNA/DNA duplex, and the D‐isomer has the opposite effect. Therefore, the synthesis of an optically pure γ‐backbone is very important. Here, we report a novel synthetic strategy for the suppression of epimerization during the synthesis of the γ‐PNA backbone. A stabilized form of 2‐iodoxybenzoic acid (SIBX) was used as an oxidative reagent in the key intermediate of the N‐Boc‐amino acetaldehyde synthesis. This paper reports (1) the synthesis and comparison of three different γ‐PNA backbones (lysine, alanine, and glutamate) by three different synthetic routes (SIBX, lithium aluminum hydride, and Red‐Al) and (2) the determination of chiral purity from their derivative compounds. The enantiomeric excess purity of SIBX‐mediated γ‐PNA backbones was determined to be more than 99.4%, as ascertained by the high‐performance liquid chromatography (HPLC) chromatogram on a standard RP‐C18 column. It is comparatively higher than that of the other methods examined in this work.

The radiation from ultrafast point dipoles, moving uniformly near chiral media

The article discusses the features of the radiation of ultrafast point clusters of charged particles moving uniformly near a gyrotropic medium interface. It is shown that some types of electromagnetic radiation – transient and/or Cherenkov radiation – have the characteristics of superchiral fields. Therefore, they can be effectively used to study chiral structures (for example, to detect circular dichroism, the frequency characteristics of refractive indices), various materials, including biomaterials. Ultrafast (relativistic) particles can serve as a «tool» not only for studying the structure of various materials. They can also be used as «generators» of quasiparticles that determine the «dynamic» properties of the materials under study, as well as the features of their interaction with radiation of various nature and response to external influences. In this paper, some types of circularly polarized EM waves propagating in optically active (magnetoactive, naturally active, gyrotropic, and chiral) media are considered. Using the generalized reciprocity theorem for media characterized by the Hermitian permittivity tensor, we consider the transient and Cherenkov radiation excited by a uniformly moving bunch of charged particles when it crosses (or moves along) the interface of media, one of which is an optically active gyrotropic medium. It is shown that the superchiral electromagnetic fields of the transition and Cherenkov radiation of dipoles can serve as a source of chiral collective excitations in magnetoactive and naturally active media. The investigated mechanisms of interaction of electromagnetic radiation with chiral materials (structures and media) are one of the possible physical approaches to solving the problem of the chiral purity of the biosphere and to elucidate the factor of deracemization of the organic primeval environment. A new hypothesis is presented, suggesting that ultra-high-speed clumps of charged particles of cosmic origin can cause deracemization of the prebiosphere.

Stereoretention in the Bulk ROP of l-Lactide Guided by a Thermally Stable Organocatalyst.

Polylactide (PLA) has emerged as one of the most promising bio-based alternatives to petroleum-based plastics, mainly because it can be produced from the fermentation of naturally occurring sugars and because it can be industrially compostable. In spite of these benefits, the industrial ring-opening polymerization (ROP) of l-lactide (L-LA) still requires the use of highly active and thermally stable metal-based catalysts, which have raised some environmental concerns. While the excellent balance between activity and functional group compatibility of organic acid catalysts makes them some of the most suitable catalysts for the metal-free ROP of L-LA, the majority of these acids are highly volatile and subject to decomposition at high temperature, which limits their use under industrially relevant conditions. In this work we exploit the use of a nonstoichiometric acid–base organocatalyst to promote the solvent-free and metal-free ROP of L-LA at elevated temperatures in the absence of epimerization and transesterification. To do so, a stable acidic complex was prepared by mixing 4-(dimethylamino)pyridine (DMAP) with 2 equiv of methanesulfonic acid (MSA). Both experimental and computational results indicate that DMAP:MSA (1:2) not only is highly thermally stable but also promotes the retention of stereoregularity during the polymerization of L-LA, leading to PLLA with a molar mass of up to 40 kg mol–1 and a chiral purity in excess of 98%. This result provides a new feature to exploit in organocatalyzed polymerization and in the design of new catalysts to facilitate the path to market.

The effect of water on the large-scale supercritical fluid chromatography purification of two factor XIa active pharmaceutical ingredients

BMS-962212, a parenteral Factor XIa inhibitor, was scaled-up for toxicity studies. Two steps of supercritical fluid chromatography (SFC) were developed for the chiral resolution of the penultimate and achiral purification of final active pharmaceutical ingredient (API), BMS-962212. A robust SFC process using Chiralcel OD-H with methanol-acetonitrile as modifier in CO2 was established to achieve a stable and uninterrupted operation with reduced mobile phase viscosity and system pressure drop. More than 230 g of the racemic penultimate was chirally resolved to reach >99% chiral purity, ready for final tert-butyl ester deprotection to provide the API. There were a significant number of impurities in BMS-962212 generated from the final step that needed to be removed. In contrast to conventional SFC conditions, an SFC method exploiting water and ammonia as additives in both the mobile phase and sample solution was developed to accomplish purification and desalting (i.e. removing TFA) of the zwitterionic API in one step. Water as an additive eliminated salt precipitation and improved the resolution while ammonia contributed to the desalting, details of which will be discussed in this article. A throughput of 2 g/h was achieved, and >80 g of the crude API was purified. The same strategy was applied to another Factor XIa API (compound A) and its penultimate.

(Invited) Combining Absorption, Wavelength-Dependent Raman and Fluorescence-Excitation Spectroscopy with High-Resolution Transmission Electron Microscopy for Determining the Chiral Purity of Sorted SWCNT Samples

Single-wall carbon nanotubes (SWCNTs) possess unique electronic and optical properties that depend strongly on their exact chiral structure. Recent progress in the structure sorting of specific SWCNT chiralities, with increasing chiral purity[1], demands an effective characterization methodology to be developed to accurately determine the chiral composition of any SWCNT chirality. Very often, optical spectroscopy is used to assess the chiral composition of a sample, but absorption cross-sections, PL quantum efficiencies and Raman cross-sections are all depending on the exact chiral structure,[2] and can be strongly influenced by other factors such as the specific internal and external environment of the SWCNTs.[3] In this work, we systematically compare the chirality distribution obtained from 3 different optical spectroscopic techniques, i.e. absorption, wavelength-dependent Raman and fluorescence-excitation spectroscopy with the chirality distribution obtained from high-resolution transmission electron microscopy for both chirality-sorted and unsorted SWCNT samples. This combined approach demonstrates the importance of using a multi-technique characterization strategy for a reliable determination of SWCNT chirality distribution.[4][1] J.A. Fagan, Nanoscale Adv. 1, 3307 (2019) and references therein[2] V.N. Popov, Nano Lett. 4, 1795 (2004)[3] S. Cambré et al, Angew. Chem. – Int Ed. 50, 2764 (2011); ACS nano 6, 2649 (2012)[4] A. Castan et al, Carbon 171, 968 (2021)

Radiosynthesis and Evaluation of Talazoparib and Its Derivatives as PARP-1-Targeting Agents.

Poly (ADP-ribose) polymerase-1 (PARP-1) is a critical enzyme in the DNA repair process and the target of several FDA-approved inhibitors. Several of these inhibitors have been radiolabeled for non-invasive imaging of PARP-1 expression or targeted radiotherapy of PARP-1 expressing tumors. In particular, derivatives of olaparib and rucaparib, which have reduced trapping potency by PARP-1 compared to talazoparib, have been radiolabeled for these purposes. Here, we report the first radiosynthesis of [18F]talazoparib and its in vitro and in vivo evaluation. Talazoparib (3a″) and its bromo- or iodo-derivatives were synthesized as racemic mixtures (3a, 3b and 3c), and these compounds exhibit high affinity to PARP-1 (Ki for talazoparib (3a″): 0.65 ± 0.07 nM; 3a: 2.37 ± 0.56 nM; 3b: 1.92 ± 0.41 nM; 3c: 1.73 ± 0.43 nM; known PARP-1 inhibitor Olaparib: 1.87 ± 0.10 nM; non-PARP-1 compound Raclopride: >20,000 nM) in a competitive binding assay using a tritium-labeled PARP-1 radioligand [3H]WC-DZ for screening. [18F]Talazoparib (3a″) was radiosynthesized via a multiple-step procedure with good radiochemical and chiral purities (98%) and high molar activity (28 GBq/μmol). The preliminary biodistribution studies in the murine PC-3 tumor model showed that [18F]talazoparib had a good level of tumor uptake that persisted for over 8 h (3.78 ± 0.55 %ID/gram at 4 h and 4.52 ± 0.32 %ID/gram at 8 h). These studies show the potential for the bromo- and iodo- derivatives for PARP-1 targeted radiotherapy studies using therapeutic radionuclides.

Development of industrial efficient asymmetric synthesis of endothelin-A antagonist ABT-546: A case study based on green chemistry perspective

The catalytic, extremely effective enantioselective conjugate-addition strategy was approved by via 4 mol% of bis(oxazoline)-Mg(OTf)2 along with amine complex as co-catalyst. The magnicium triflate based catalysis afford 88% selectivity in the molecule and provide less dependence on the tartrate salt for enantioselectivity, which was previously prepared for attaining chiral purity for the synthesis of the endothelin-A antagonist ABT-546. Along with enantioselective conjugate-addition the new strategy involves green solvents in the final steps of ABT-546 drug preparation. The use of green solvent owes its own advantage of minimizing the residues remaining in the API synthesized. The article includes the case study of the ABT-546 to discuss the green chemistry objectives fulfilled by pharma industry.

Role of Chromatograph-based Analytical Techniques in Quantification of Chiral Compounds: An Update

Background: Chiral purity is a critical quality attribute of pharmaceutical materials as chiral compounds are known to exhibit different pharmacological and toxicological properties as compared to those of their enantiomers (or diastereomers for molecules with multiple chiral centers). It is important to note that about 40% of the synthetic drugs are chiral, 60% of all pharmaceuticals are chiral and 45% chiral drugs are sold as racemates. The objective of the current review is to discuss various chromatographic techniques used for the separation of chiral compounds. Methods: Various bibliographic databases of previously published peer-reviewed research papers were explored and systematic data has been compiled in terms of various chromatographic techniques used for chiral compounds’ separation. A comparison of different techniques as well as their advantages is also discussed. Results: A comprehensive review of 130 papers including both, research and review articles, was carried out. The analytical techniques have been discussed in detail. Apart from chromatographic techniques, other techniques such as circular dichroism, nuclear magnetic resonance, UV-visible spectroscopy using cyclodextrin derivatives have also been highlighted. Conclusion: The pharmaceutical industries need analytical methods to conclude enantiomeric concentration and obtain a drug with a single stereo configuration. Sensitive techniques such as HPLC, GCMS and LCMS, etc. are used for identification and quantification of single enantiomers, specifically in drug discovery and development.

Facile Solvent Mixing Strategy for Extracting Highly Enriched (6,5)Single-Walled Carbon Nanotubes in Improved Yield

Abstract Selective sorting of semiconducting single-walled carbon nanotubes (SWNTs) of single chirality is critical for constructing electronic nanodevices with high performance. Nevertheless, trade-off between yield and chirality purity often exists in conventional SWNTs sorting processes. To solve this problem, we present an extremely facile method for enriching SWNTs with both high yield and high chiral selectivity. Simply by using a solvent mixed with toluene and chlorobenzene at certain volume ratio, the sorting yield of (6,5)SWNTs was largely enhanced via polymer extraction, while the chirality purity of sorted SWNTs could be kept high as well. We further proved that the sorting efficiency would not be lowered even after multiple extractions, while the chirality selectivity of the obtained SWNTs was dependent on exact ratio between the two solvents. Solvent polarity-controlled polymer (or solvent molecule)-SWNTs interaction was believed responsible for our results, which was later successfully testified by molecular dynamics simulations. Our proposed strategy will simplify the selecting process of single chiral SWNTs and benefit the optimization of SWNTs-integrated devices in near future.

Chiron approach and the [4 + 2] Diels-Alder cycloaddition of 2-pyrones for the synthesis of cis-(-)-aminoindan-2-ol

A cascade Diels-Alder reaction coupled with an oxidative aromatization reaction was developed and applied to a chiral 2-pyrone for the synthesis of the relevant chiral synthon/chiral auxiliary (-)-cis-aminoindan-2-ol. Chirality and optical purity were provided by the privileged chiron 7,3-LXF.

Collection and Characterization by Mass Spectrometry of the Neutral Serine Octamer Generated upon Sublimation.

Serine forms neutral octameric clusters during sublimation, as demonstrated by electrostatically deflecting thermally ionized serine species from the sublimate, then gently ionizing the remaining neutrals for examination by mass spectrometry (MS). The MS results demonstrate a strong homochiral preference in the neutral octamer (measured after its gentle ionization), while the smaller serine clusters are achiral. In the initial stages of its sublimation, nonracemic solid serine generates a neutral serine monomer as the principal species in the vapor phase, with a significant enantiomeric enrichment relative to the solid. The serine monomer, when the flux is sufficient, assembles into the octamer, which displays a much higher chiral purity than the monomer. The serine octamer is separated from other neutral clusters in the sublimate by a new method based on the different distances that the clusters travel in an inert gas stream before they condense in a cooled collector. The deposited octamer is subsequently dissolved, and the solution is investigated by MS. The spectrum confirms that the collected serine octamer has undergone chiral enrichment relative to the starting solid used in the sublimation. The chiral enrichment observed in going from the serine monomer to octamer can be accommodated using a chemical model, grounded on the homochiral preference of the neutral serine octamer. Using the enantiomeric excess (ee %) of the vapor-phase monomer as the input, the model output matches the experimental octamer ee % when subliming solid serine with various initial ee % values.

Rotigotine: A Review of Analytical Methods for the Raw Material, Pharmaceutical Formulations, and Its Impurities.

Rotigotine is a dopaminergic agonist developed for the treatment of Parkinson's disease and restless leg syndrome. The pure levorotatory enantiomer is marketed in several countries as a transdermal patch. Reports of oxidation and instability in a previous formulation indicate the need to evaluate impurities in both the raw material and pharmaceutical dosage forms of rotigotine to ensure product quality. This review examines the main analytical methods for analyzing rotigotine in raw material and its transdermal patches with the aim of assisting the development of new pharmaceutical formulations and stability studies. Analytical methods based on high-performance liquid chromatography for rotigotine from pharmacopoeias and literature were evaluated. A comparison was made between the methods found in the literature and official rotigotine monographs described by the United States, European, and British Pharmacopoeias, including a discussion of their acceptance limits for impurities related to the drug. The different impurities from the synthesis processes and degradation studies of rotigotine were also evaluated, as well as the main articles that describe methods for assessing their chiral purity. Qualified and unofficial official impurities found in forced degradation studies were verified. The methods presented show adequate specificity and selectivity in determining the drug in the presence of its impurities. The approached methods are promising, but more detailed studies on the stability of rotigotine are still lacking, mainly in the pharmacokinetic and toxicological characterization of its impurities.

Supercritical fluid chromatography versus liquid chromatography for the enantiomeric separation of itraconazole

Separation of the stereoisomers of the antifungal drug itraconazole using supercritical fluid chromatography and normal-phase liquid chromatography is presented. The performances of the two methods are evaluated based on their speed, resolution, and the analytical method greenness score. Both methods achieve baseline separation and meet the United States Pharmacopeia-recommended resolution of ≥1.5 between the four stereoisomers. The methods are validated following the International Conference on Harmonization guidelines and evaluated in terms of specificity, precision, linearity, accuracy, and range; they are thus appropriate for purposes of quantification. Supercritical fluid chromatography achieves separation more rapidly and is found to be more eco-friendly than normal-phase liquid chromatography. To emphasize the use of ecofriendly alternatives, the assessment is performed using a metric termed as the analytical method greenness score. The method can thus be implemented for determination of chiral purity in various Quality control laboratories.

Validated stability-indicating HPLC method for the separation of pindolol enantiomers and its related substances

A simple, rapid and stability-indicating reverse-phase liquid chromatographic (RP-LC) method was developed for the separation of pindolol (PDL) and its related substances in the presence of its degradation products generated from forced decomposition studies. The chromatographic separation was achieved on a C18 Inertsil ODS-3V column (250 mm x 4.6 mm i.d.), using a mobile phase of 20 mM sodium dihydrogen orthophosphate-acetonitrile containing orthophosphoric acid to maintain pH at 4.0. Gradient elution was used at a flow rate of 1.0 mL/min. The UV detector was operated at 205 nm and the column temperature was maintained at ambient. Separation of PDL enantiomers was achieved on normal phase liquid chromatography (NP-LC) using a chiral pack AD-H (250 mm x 4.6 mm i.d.) column, with mobile phase consisting of n-hexane : ethanol : diethylamine (860 : 140 mL : 0.05%, v/v). A flow rate of 0.9 mL/min and detection wavelength of 215 nm were used. The method was validated for specificity, linearity and range, precision, accuracy, solution stability, robustness, limit of detection (LOD) and limit of quantification (LOQ). Selectivity of the RP-LC method was validated by subjecting the stock solution of PDL to acidic, basic, photolysis oxidative and thermal degradation. The calibration curve was found to be linear in the concentration range of 0.01 - 100 μg/mL (r2 = 0.9998) and 0.001 - 50 μg/mL (r2 = 0.9982) of PDL for the RP-LC and NP-LC method, respectively. The peaks of degradation products did not interfere with that of pure PDL. The utility of the developed method was examined for chemical and chiral purity by analyzing the tablets containing PDL and also by determining the related substances of PDL. The results of analysis were supported by statistical parameters.

Practical Early Development Synthesis of Nav1.7 Inhibitor GDC-0310

The concise early development route to the Nav1.7 inhibitor GDC-0310 is described. The active pharmaceutical ingredient (API) contains one stereocenter, which was obtained with high enantiomeric excess (>99:1) by using an SN2 displacement approach to connect two intermediates: a chiral benzyl alcohol and a piperidine. The synthesis of the piperidine building block proceeded via a regioselective SNAr reaction on 1-chloro-2,4-difluorobenzene by N-Boc-4-piperidinemethanol, followed by installation of the methyl ester group by electrophilic aromatic bromination and a palladium-catalyzed alkoxycarbonylation. A subsequent Suzuki–Miyaura cross-coupling reaction was then telescoped directly into cleavage of the Boc group to provide the advanced piperidine intermediate. The key feature of the synthesis is the highly selective SN2 displacement of the chiral mesylate of (R)-1-(3,5-dichlorophenyl)ethan-1-ol with the piperidine intermediate, followed by a chiral purity upgrade via the corresponding (1S)-(+)-camphorsulfonic acid salt. After standard hydrolysis of the methyl ester and CDI mediated amidation to couple the resulting acid with methanesulfonamide, enantiomerically pure GDC-0310 was obtained in high overall yield (37%) on a 6.5 kilogram scale.

Rapid and efficient chiral method development for lamivudine and tenofovir disoproxil fumarate fixed dose combination using ultra-high performance supercritical fluid chromatography: A design of experiment approach

Fixed dose combination (FDC) of tenofovir disoproxil fumarate (TDF) and lamivudine (3TC) is one of the most preferred FDC for the treatment of acquired immunodeficiency syndrome (AIDS)/human immunodeficiency virus (HIV) infection. To the best of authors’ knowledge there are no reported methods for chiral purity estimation of both drugs simultaneously from a FDC. The current study was focused on the development of a single chiral method uisng supercritical fluid chromatography (SFC) for separation of stereoisomers of TDF and 3TC combination employing design of experiment (DoE) approach. Method development was planned in three steps by using different experimental designs for each step. I-optimal, Taguchi orthogonal array and face-centred central composite designs (CCD) were employed for primary parameter selection, secondary parameter screening and final method optimization, respectively. All six stereoisomers were separated in a 10 minute run on Chiralpak IA column with carbon di-oxide /methanol (containing 0.5 % v/v n-butylamine) as mobile phase at 1.5 mL/min in gradient mode. The optimized method was verified for performance through establishing specificity, precision, linearity, accuracy, limit of quantification, and solution stability. Resolution between each isomeric pair was more than 1.5. The method was found to be linear from 1.5 µg/mL to 7.5 µg/mL for 3TC and 7.5 µg/mL to 37.5 µg/mL for TDF stereoisomers. The R2 values for all the linearity curves for undesired isomers were greater than 0.995. The method proved to be rapid, reproducible and efficient to quantify stereoisomers of both drugs in a single run.