Enantiomeric resolution of kielcorin derivatives by HPLC on polysaccharide stationary phases using multimodal elution

Exploitation of the enantioselectivity space of coated amylose tris(3,5-dimethylphenylcarbamate) in mixtures of 2-propanol and acetonitrile

Chiral stationary phases (CSPs) with coated amylose tris(3,5-dimethylphenylcarbamate) (ADMPC) selector have long been recognized for their excellent chiral recognition ability in liquid chromatography. The conformational versatility behind this feature is the source of their known hysteretic behavior, which has been previously observed in polar organic (PO) mode eluents containing 2-propanol (IPA). Mixtures of IPA and acetonitrile (MeCN), a typical PO mode eluent system, have not been examined in this aspect yet, even though hysteresis is promising for finding unique unexplored enantioselectivities. Not only was the hysteresis detectable on ADMPC using mixtures of IPA and MeCN, but it was the typical behavior in a diverse set of test compounds. The difference in the retention time of the same analyte under conditions which only differed in the eluent history on the column can go up to 20-fold. The assumed hindered conformational changes of the selector were reflected in retention drift at certain eluent compositions. On the two sides of the transitions, distinct, useful states of the selector were detected. A series of IPA - MeCN compositions with defined pretreatment was selected and recommended as an extension of the preliminary, first choice method screening set that used only alcohols. The incorporation of a solvent possessing substantially different characteristics enhances the potential in practical applications, while keeping the technical simplicity. Stability and robustness of the additional states of the CSP were characterized. The examined columns of different brands shared the observed behavior. Kinetic stability of a column state is adequate for successful application. The evaluated states of ADMPC provide multiple enantiorecognition potential by using mixtures of IPA and MeCN also considering the pretreatment of the column. Unprecedented double and triple elution order reversals along the composition range supported the versatility of the available states. Our findings further enhance the usefulness of ADMPC-containing CSPs. We provide instructions for the application of the widespread chiral selector in common eluent mixtures to avoid pitfalls regarding reproducibility and robustness.

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

Chiral derivatives of xanthones and benzophenones: Synthesis, enantioseparation, molecular docking, and tumor cell growth inhibition studies

Liquid chromatography enantioseparation and determination of enantiomeric purity of synthetized xanthone and benzophenone derivatives comprising one or more chiral moieties are reported. High enantioselectivity and resolution were observed in (S,S)‐Whelk‐O1 chiral stationary phase (CSP) for the enantiomeric mixtures of compounds comprising an aromatic ring linked to the stereogenic center(s), with α values ranging from 1.35 to 4.15 and Rs values ranging from 2.22 to 13.87. Among all the tested enantiomeric mixtures, those comprising three chiral moieties positioned in the xanthone scaffold gave the best chromatographic results. Enantiomers comprising an alkyl chain linked to the stereogenic centers were enantioseparated on a Lux® Celullose‐2 CSP. For both CSPs, the elution was performed in polar organic mode. The enantiomeric ratio (e.r.) values were always higher than 99%. Additionally, assessment of chiral recognition mechanisms on (S,S)‐Whelk‐O1 CSP was performed by molecular docking approach, which are in accordance with the chromatographic parameters. The nature and number of chiral moieties in the central aromatic scaffold of either xanthone or benzophenone derivatives are proved to be crucial for enantiorecognition. The evaluation of the growth inhibition of human tumor cell lines revealed that (S,S)‐(+)‐5 was the most potent compound, with values of GI50 of 12.83 ± 2.09 μM for A375‐C5 melanoma, 12.40 ± 1.16 μM for MCF‐7 breast adenocarcinoma, and 13.06 ± 1.29 μM for NCI‐H460 non‐small cell lung cancer. In some cases, the growth inhibitory effects demonstrated to be dependent on the stereochemistry of the compounds.

From Natural Products to New Synthetic Small Molecules: A Journey through the World of Xanthones

This work reviews the contributions of the corresponding author (M.M.M.P.) and her research group to Medicinal Chemistry concerning the isolation from plant and marine sources of xanthone derivatives as well as their synthesis, biological/pharmacological activities, formulation and analytical applications. Although her group activity has been spread over several chemical families with relevance in Medicinal Chemistry, the main focus of the investigation and research has been in the xanthone family. Xanthone derivatives have a variety of activities with great potential for therapeutic applications due to their versatile framework. The group has contributed with several libraries of xanthones derivatives, with a variety of activities such as antitumor, anticoagulant, antiplatelet, anti-inflammatory, antimalarial, antimicrobial, hepatoprotective, antioxidant, and multidrug resistance reversal effects. Besides therapeutic applications, our group has also developed xanthone derivatives with analytical applications as chiral selectors for liquid chromatography and for maritime application as antifouling agents for marine paints. Chemically, it has been challenging to afford green chemistry methods and achieve enantiomeric purity of chiral derivatives. In this review, the structures of the most significant compounds will be presented.

Simultaneous determination of chiral and achiral impurities of ivabradine on a cellulose tris(3-chloro-4-methylphenylcarbamate) chiral column using polar organic mode

A high performance liquid chromatographic method was developed for the simultaneous determination of the related substances (R-ivabradine, dehydro-S-ivabradine, N-demethyl-S-ivabradine, ((S)-3,4-dimethoxy-bicyclo[4.2.0]octa-1,3,5-triene-7-yl-methyl)-methyl-amine) and 1-(7,8-dimethoxy-1,3,4,5-tetrahydro-2H-3-benzazepine-2-on-3-yl)-3-chloro-propane) of the heart-rate lowering drug, ivabradine. The separation capability of seven different polysaccharide-type chiral columns (Lux Amylose-1, Lux i-Amylose-1, Lux Amylose-2, Lux Cellulose-1, Lux Cellulose-2, Lux Cellulose-3 and Lux Cellulose-4) was investigated with a mobile phase consisting of 0.1% diethylamine in methanol, 2-propanol and acetonitrile. During the screnning experiments the best results were obtained on Lux Cellulose-2 (based on cellulose tris(3-chloro-4-methylphenylcarbamate) column with methanol with an ideal case, where all the impurities eluted before the S-ivabradine peak. Chromatographic parameters (flow rate, temperature and mobile phase constituents) were optimized by a full factorial screening design. Using optimized parameters (Lux Cellulose-2 column with 0.06% (v/v) diethylamine in methanol/acetonitrile 98/2 (v/v) with 0.45 mL/min flow rate at 12 °C) baseline separations were achieved between all compounds. The optimized method was validated according to the International Council on Harmonization Q2(R1) guideline and proved to be reliable, linear, precise and accurate for determination of at least 0.05% for all impurities in S-ivabradine samples. Method application was tested on a commercial tablet formulation and proved to be suitable for routine quality control of both chiral and achiral related substances of S-ivabradine.

Chiral Stationary Phases for Liquid Chromatography: Recent Developments

The planning and development of new chiral stationary phases (CSPs) for liquid chromatography (LC) are considered as continuous and evolutionary issues since the introduction of the first CSP in 1938. The main objectives of the development strategies were to attempt the improvement of the chromatographic enantioresolution performance of the CSPs as well as enlarge their versatility and range of applications. Additionally, the transition to ultra-high-performance LC were underscored. The most recent strategies have comprised the introduction of new chiral selectors, the use of new materials as chromatographic supports or the reduction of its particle size, and the application of different synthetic approaches for preparation of CSPs. This review gathered the most recent developments associated to the different types of CSPs providing an overview of the relevant advances that are arising on LC.

Synthetic Chiral Derivatives of Xanthones: Biological Activities and Enantioselectivity Studies

Many naturally occurring xanthones are chiral and present a wide range of biological and pharmacological activities. Some of them have been exhaustively studied and subsequently, obtained by synthesis. In order to obtain libraries of compounds for structure activity relationship (SAR) studies as well as to improve the biological activity, new bioactive analogues and derivatives inspired in natural prototypes were synthetized. Bioactive natural xanthones compromise a large structural multiplicity of compounds, including a diversity of chiral derivatives. Thus, recently an exponential interest in synthetic chiral derivatives of xanthones (CDXs) has been witnessed. The synthetic methodologies can afford structures that otherwise could not be reached within the natural products for biological activity and SAR studies. Another reason that justifies this trend is that both enantiomers can be obtained by using appropriate synthetic pathways, allowing the possibility to perform enantioselectivity studies. In this work, a literature review of synthetic CDXs is presented. The structures, the approaches used for their synthesis and the biological activities are described, emphasizing the enantioselectivity studies.

Enantiomeric Mixtures in Natural Product Chemistry: Separation and Absolute Configuration Assignment

Chiral natural product molecules are generally assumed to be biosynthesized in an enantiomerically pure or enriched fashion. Nevertheless, a significant amount of racemates or enantiomerically enriched mixtures has been reported from natural sources. This number is estimated to be even larger since the enantiomeric purity of secondary metabolites is rarely checked in the natural product isolation pipeline. This latter fact may have drastic effects on the evaluation of the biological activity of chiral natural products. A second bottleneck is the determination of their absolute configurations. Despite the widespread use of optical rotation and electronic circular dichroism, most of the stereochemical assignments are based on empirical correlations with similar compounds reported in the literature. As an alternative, the combination of vibrational circular dichroism and quantum chemical calculations has emerged as a powerful and reliable tool for both conformational and configurational analysis of natural products, even for those lacking UV-Vis chromophores. In this review, we aim to provide the reader with a critical overview of the occurrence of enantiomeric mixtures of secondary metabolites in nature as well the best practices for their detection, enantioselective separation using liquid chromatography, and determination of absolute configuration by means of vibrational circular dichroism and density functional theory calculations.

Enantioselective quantification of fluoxetine and norfluoxetine by HPLC in wastewater effluents

Microbial degradation is the most important process to remove organic pollutants in Waste Water Treatment Plants. Regarding chiral compounds this process is normally enantioselective and needs the suitable analytical methodology to follow the removal of both enantiomers in an accurate way. Thus, this paper describes the development and validation of an enantioselective High Performance Liquid Chromatography with Fluorescence Detection (HPLC-FD) method for simultaneous analysis of fluoxetine (FLX) and norfluoxetine (NFLX) in wastewater effluents. Briefly, this method preconcentrated a small volume of wastewater samples (50 mL) on 500 mg Oasis MCX cartridges and used HPLC-FD with a vancomycin-based chiral stationary phase under reversed mode for analyses. The optimized mobile phase was EtOH/aqueous ammonium acetate buffer (92.5/7.5, v/v) at pH 6.8. The effect of EtOH percentage, buffer concentration, pH, column oven temperature and flow rate on chromatographic parameters was systematically investigated. The developed method was validated within the wastewater effluent used in microcosms laboratory assays. Linearity (R(2)>0.99), selectivity and sensitivity were achieved in the range of 4.0-60 ng mL(-1) for enantiomers of FLX and 2.0-30 ng mL(-1) for enantiomers of NFLX. The limits of detection were between 0.8 and 2.0 ng mL(-1) and the limits of quantification were between 2.0 and 4.0 ng mL(-1) for both enantiomers of FLX and the enantiomers of its demethylated metabolite NFLX. The validated method was successfully applied and proved to be robust to follow the degradation of both enantiomers of FLX in wastewater samples, during 46 days.

Resolution and determination of enantiomeric purity of new chiral derivatives of xanthones using polysaccharide-based stationary phases

The enantioresolution and determination of the enantiomeric purity of 24 new chiral derivatives of xanthones (CDXs) were investigated on polysaccharide-based chiral stationary phases (CSPs). The tris-3,5-dimethylphenylcarbamates of cellulose and amylose and tris-3,5-dimethoxyphenylcarbamate of amylose were selected as CSPs for this work. The separation of enantiomeric mixtures of CDXs was explored under multimodal elution conditions. All the enantiomeric mixtures of CDXs were enantioseparated with very high enantioselectivity and resolution with α and R(S) ranging from 1.43 to 12.41 and from 1.48 to 10.29, respectively. The best performances were achieved on amylose tris-3,5-dimethylphenylcarbamate stationary phase under polar organic elution conditions. Furthermore the enantiomeric purity for all the CDXs was measured, achieving values higher than 99%. Based on the obtained results, the influence of the mobile phases and structural features of the CSPs and CDXs on chiral discrimination are also discussed.

Screening approach for chiral separation of pharmaceuticals

The aim of this work is to determine generic screening conditions and an initial simple separation strategy allowing the rapid separation of drug enantiomers in polar organic solvent chromatography (POSC). Four cellulose/amylose-based stationary phases were investigated in detail using two mobile phase basis solvents commonly applied in this mode, i.e. acetonitrile and methanol. Polar mode is interesting for use in purification of enantiomers. In a first step, the parameters potentially influencing the separation, such as addition of an alcohol to the polar organic solvent or the type of mobile phase additive(s), were examined by means of experimental designs. Afterwards, the factors found most important are investigated in more detail. Results showed that the cellulose- and amylose-based stationary phases have very broad and complementary enantiorecognition abilities in the POSC mode. The type of organic solvent for the mobile phase appeared to have a dramatic influence on the quality of the separation. Based on the results, a screening strategy was proposed. Enantioseparation was observed in more than 85% of the tested compounds and analysis times of last eluted peak were usually below 10 min.

Multimilligram enantioresolution of low-solubility xanthonolignoids on polysaccharide chiral stationary phases using a solid-phase injection system

Kielcorins are xanthonolignoids with protein kinase C inhibition and antitumor activities. Four racemates were enantioresolved at a multimilligram scale on tris-3,5-dimethylphenylcarbamate amylose phase using polar organic conditions as mobile phase. The low-solubility of these compounds conditioned the injection amount and consequently the productivity. A solid-phase injection system was developed to increase the production rate of the semipreparative process. The effects of the racemates and the related enantiomers on the in vitro growth of human breast cancer cell line MCF-7 were compared. Differences in their growth inhibitory activity were observed.

Chiral separations on polysaccharide stationary phases using polar organic mobile phases

About 30% of a chemically diverse set of compounds were found to separate on four polysaccharide chiral stationary phases using polar organic mobile phases. No structural features appeared to correlate to successful separations. Titrations between normal and polar organic mobile phases suggested that separation mechanisms do not differ between these mobile phases. Attempts made to control retention met with varying degrees of success. Addition of hexane to alcohols had minor effects on retention although this was occasionally beneficial. Addition of water to alcohols increased retention. Addition of water to acetonitrile decreased retention. Addition of alcohol to acetonitrile also proved beneficial to the separation of some compounds. Loading studies performed to mimic preparative separations indicated that the benefits of polar organic mobile phases are largely due to increased solubility.