Separation of planar chiral ferrocenes by capillary electrokinetic chromatography and liquid chromatography.

Capillary electrokinetic chromatography (CEKC) and liquid chromatography (LC) methods were explored for the enantiomeric separation of six unsymmetrically disubstituted ferrocene derivatives. In normal-phase mode liquid chromatography, the use of n-heptane, ethanol or isopropanol with 1% n-butylamine as mobile phase on six polysaccharide-based columns, allowed to fully separate the enantiomers of three compounds out of the six (i.e 7-chloro-N-(2-((dimethylamino)methyl)ferrocenyl)quinolin-4-amine (ferroquine) (compound 1), 1-[(1S)-(1-Aminoethyl)]-2-(diphenylphosphino)ferrocene (compound 5) and 1-[(1R)-1-(Dicyclohexylphosphino)ethyl]-2-(diphenylphosphino)ferrocene (compound 6). Among the columns used, the Lux i-Cellulose-5 was the most effective. In capillary electrokinetic chromatography, a phosphate buffer of 25 mM concentration and pH equal to 2.5 was chosen as background electrolyte, leading to cationic ferrocene derivatives. The addition of neutral cyclodextrins was undertaken first and native ß- or γ-cyclodextrins were found to resolve the enantiomers of two derivatives. Then, 15 mM of anionic cyclodextrins were added to the background electrolyte. The use of SBE-ß-CD, S-ß-CD or S-γ-CD have allowed the separation of the enantiomers for most of the ferrocene derivatives studied with high resolution values in short migration time. For instance, for 1-(R)-2-(Diphenylphosphino)ethyldi-tert-butylphosphine ferrocene (compound 2), the migration times were less than 2 minutes and the resolution value was equal to 3.52 in short-end mode with 15 mM S-ß-CD, at 25 kV and 25°C. Finally, a dual cyclodextrins system was tested using 15 mM of S-ß-CD plus 15 mM HP-γ-CD in the phosphate buffer. This system allowed the improved separation of two ferrocene derivatives with an unusual resolution value equal to 41.5 in long-end mode. Overall, CEKC showed better enantioseparating power of the six chiral ferrocenes studied than liquid chromatography.

Fluorocyclopropane-Containing Proline Analogue: Synthesis and Conformation of an Item in the Peptide Chemist's Toolbox.

Over the years, numerous modifications to the structure of proline have been made in order to tune its effects on bioactive compounds. Notably, the introduction of a cyclopropane ring or a fluorine atom has produced interesting results. Herein, we describe the synthesis of a proline containing fluorocyclopropane. This modified amino acid was inserted into a tripeptide, whose conformation was studied by nuclear magnetic resonance and density functional theory calculations.

Analytical and preparative enantioseparations in supercritical fluid chromatography using different brands of immobilized cellulose tris (3,5-dichlorophenylcarbamate) columns: Some differences.

The aim of this study was to determine the impact of the origin and the manufacturing processes of the chiral stationary phases (CSPs) on their chromatographic behaviors. Hence, four chiral stationary phases based on immobilized tris (3,5-dichlorophenylcarbamate) derivative of cellulose supplied by four different manufacturers were evaluated. A set of twenty-nine compounds, including commercially available and in-house synthesized compounds, with a broad range of lipophilicity and polarity was chosen. Three main parameters were evaluated on all stationary phases: retention factor, selectivity and loading capacity. This work highlighted that the retention factor strongly varied according to the manufacturer. Regardless of the characteristic of the tested compounds i.e. neutral, acidic or basic, there was a trend in retention ability of the four chiral stationary phases: retention was increasing from CHIRAL ART Cellulose-SC, REFLECT I-Cellulose C, Chiralpak IC to Lux i-Cellulose-5. On the contrary, selectivity did not follow the same trend as retention. The difference in selectivity between each column towards one compound was quite low while the difference in resolution depended on the nature of the compounds investigated and was significant in certain cases. Finally, the four different columns presented similar and high loading capacity.

Productivity and solvent waste in supercritical fluid chromatography for preparative chiral separations: a guide for a convenient strategy.

The advent of supercritical fluid chromatography (SFC) in the 90s has changed preparative liquid chromatography. SFC is an improved way for separating chiral compounds during drug discovery processes yielding upwards of one hundred grams of pure enantiomers or during clinical trials requiring higher quantities. The need to purify approximately 45 mg of racemic mixture raises concerns regarding processing parameters, including injection volumes and frequency, column size, chromatographic method, and feed composition. In this study, Chiralpak® AD-H amylose tris(3,5-dimethylphenylcarbamate) polysaccharide-based stationary phase columns of various dimensions were investigated for the purification of propranolol using EtOH (+0.3% triethylamine)/CO2 15/85 v/v as the mobile phase. Production rate (mg/h), productivity (kilograms of racemate separated per kilogram of chiral stationary phase per day; kkd), solvent usage (L/g) and environmental factor (E Factor) were calculated for four column sizes for sequence and stacked modes of injection. The parameters were optimized to determine a method yielding high productivity or reduced environmental impact. In the stacked mode of injection, which allows for rapid processing compared with the sequential mode, the shortest column presents the best productivity of 0.176 kkd. A semi-preparative column (30 mm i.d.) yielded the best production rate of 467 mg purified per hour but had the worst environmental impact with an E Factor of approximately 56,414 (due to the solvent volume used during column equilibration). At Column Dilution (ACD) and mixed stream injection mode were also compared to separate 495 mg of propranolol. With ACD injection, 915 mL of ethanol and approximately 48 min were required, whereas with mixed stream injection, 1200 mL of ethanol and 63 min were required.

Ion mobility-mass spectrometry analysis of diarylquinoline diastereomers: Drugs used for tuberculosis treatment.

Mycobacterium tuberculosis infection results in more than two million deaths per year and is the leading cause of mortality in people infected with HIV. A new structural class of antimycobacterials, the diarylquinolines, has been synthesized and is being highly effective against both M. tuberculosis and multidrug-resistant tuberculosis. As diarylquinolines are biologically active only under their ( R,S) stereoisomeric form, it is essential to differentiate the stereoisomers ( R,S) and ( R,R). To achieve this, tandem mass spectrometry and ion mobility spectrometry-mass spectrometry have been performed with 10 diarylquinoline diastereomers couples. In this study, we investigated cationization with alkali metal cations and several ion mobility drift gases in order to obtain diastereomer differentiations. We have shown that diastereomers of the diarylquinolines family can be differentiated separately by tandem mass spectrometry and in mixture by ion mobility spectrometry-mass spectrometry. However, although the structure of each diastereomer is close, several behaviors could be observed concerning the cationization and the ion mobility spectrometry separation. The ion mobility spectrometry isomer separation efficiency is not easily predictable; it was however observed for all diastereomeric couples with a significant improvement of separation using alkali adducts compared to protonated molecules. With the use of drift gas with higher polarizability only an improvement of separation was obtained in a few cases. Finally, a good correlation of the experimental collision cross section (relative to three-dimensional structure of ions) and the theoretical collision cross section has been shown.

The effect of high concentration additive on chiral separations in supercritical fluid chromatography.

Supercritical Fluid Chromatography is frequently used to efficiently handle separations of enantiomers. The separation of basic analytes usually requires the addition of a basic additive in the mobile phase to improve the peak shape or even to elute the compounds. The effect of increasing the concentration of 2-propylamine as additive on the elution of a series of basic compounds on a Chiralpak-AD stationary phase was studied. In this study, unusual additive concentrations ranging from 0.3% to 10% of 2-propylamine 2-propylaminein the modifier were explored and the effect on retention, peak shape, selectivity and resolution was evaluated. The addition of a large quantity of additive allowed to drastically improve the selectivity and the resolution, and even enantiomers elution order reversal was observed by changing the concentration of basic additive. The role of the ratio additive/modifier appeared a key to tune the enantioselectivity. Finally, the impact of these drastic conditions on the column material was evaluated.

Preparative supercritical fluid chromatography: A powerful tool for chiral separations.

In 2012, the 4 biggest pharmaceutical blockbusters were pure enantiomers and separating racemic mixtures is now frequently a key step in the development of a new drug. For a long time, preparative liquid chromatography was the technique of choice for the separation of chiral compounds either during the drug discovery process to get up to a hundred grams of a pure enantiomer or during the clinical trial phases needing kilograms of material. However the advent of supercritical Fluid Chromatography (SFC) in the 1990s has changed things. Indeed, the use of carbon dioxide as the mobile phase in SFC offers many advantages including high flow rate, short equilibration time as well as low solvent consumption. Despite some initial teething troubles, SFC is becoming the primary method for preparative chiral chromatography. This article will cover recent developments in preparative SFC for the separation of enantiomers, reviewing several aspects such as instrumentation, chiral stationary phases, mobile phases or purely preparative considerations including overloading, productivity or large scale chromatography.

Screening strategy for chiral and achiral separations in supercritical fluid chromatography mode.

Supercritical fluid (SF) was discovered 200 years ago, but the use of this fluid as a mobile phase in chromatography only became popular fifty years ago. The development of the supercritical fluid chromatography (SFC) was progressing slowly due to technological problems since ten years; the interest for this chromatographic mode has been growing up as the construction of the SFC instruments is more or less similar with HPLC instruments. The main difference in SFC is the installation of a back pressure regulator which is implemented to control the pressure above the critical pressure. SFC is widely used in chiral chromatography where Polysaccharide phases are the most versatile in use. The mobile phase is mainly composed by CO(2) but the polarity can be increased by adding alcohol. The nature of the alcohol can change drastically the selectivity. The choice of the best tandem stationary phase / mobile phase is difficult to predict. Hence a full screening with different stationary phases and mobile phase solvents is often mandatory. For the achiral separation, SFC is more and more used. Achiral SFC can be classified as normal phase mode, it means that stationary phases are more polar than mobile phase and retention times decrease as polarity of the mobile phase increases. Most popular stationary phases are silica linked with polar group such as aminopropyl, cyanoprpyl, diol or 2-ethylpyridine. Mobile phase are generally composed by CO(2) and methanol. SFC can be used as a complementary technique for reversed phase HPLC or sometimes even to replace HPLC.

Identification of a series of substituted 2-piperazinyl-5-pyrimidylhydroxamic acids as potent histone deacetylase inhibitors.

Pursuing our efforts in designing 5-pyrimidylhydroxamic acid anti-cancer agents, we have identified a new series of potent histone deacetylase (HDAC) inhibitors. These compounds exhibit enzymatic HDAC inhibiting properties with IC(50) values in the nanomolar range and inhibit tumor cell proliferation at similar levels. Good solubility, moderate bioavailability, and promising in vivo activity in xenograft model made this series of compounds interesting starting points to design new potent HDAC inhibitors.