Static vs. dynamic electrostatic repulsion reversed phase liquid chromatography: solutions for pharmaceutical and biopharmaceutical basic compounds

Many efforts have been made to separate basic compounds, which are challenging to resolve in reversed phase liquid chromatography. In this process, they are strongly retained and the peak shape undergoes significant distortion. The principal origin of this has been identified with the non-negligible interaction with residual deprotonated silanols. Consequently, all solutions that efficiently shield silanols are being sought. This review is an upgrade on the use of the electrostatic repulsion reversed phase (ERRP) approach: retention of bases, in protonated form, can be achieved by modulating the charge repulsion caused by the presence of positive charges in the chromatographic system. This study successfully (i) introduced fixed positive charges in the structure of stationary phases, (ii) used cationic and hydrophobic additives in the mobile phase, and (iii) used the ERRP-like approach employed at the preparative level for peptide purification

Molecular recognition of the HPLC Whelk-O1 selector towards the conformational enantiomers of nevirapine and oxcarbazepine

The presence of stereogenic elements is a common feature in pharmaceutical compounds, and affording optically pure stereoisomers is a frequent issue in drug design. In this context, the study of the chiral molecular recognition mechanism fundamentally supports the understanding and optimization of chromatographic separations with chiral stationary phases. We investigated, with molecular docking, the interactions between the chiral HPLC selector Whelk-O1 and the stereoisomers of two bioactive compounds, the antiviral Nevirapine and the anticonvulsant Oxcarbazepine, both characterized by two stereolabile conformational enantiomers. The presence of fast-exchange enantiomers and the rate of the interconversion process were studied using low temperature enantioselective HPLC and VT-NMR with Whelk-O1 applied as chiral solvating agent. The values of the energetic barriers of interconversion indicate, for the single enantiomers of both compounds, half-lives sufficiently long enough to allow their separation only at critically sub-ambient temperatures. The chiral selector Whelk-O1 performed as a strongly selective discriminating agent both when applied as a chiral stationary phase (CSP) in HPLC and as CSA in NMR spectroscopy

Determination of enantiomerization barriers of hypericin and pseudohypericin by dynamic high-performance liquid chromatography on immobilized polysaccharide-type chiral stationary phases and off-column racemization experiments

Direct enantiomer separation of hypericin, pseudohypericin, and protohypericin was accomplished by high-performance liquid chromatography (HPLC) using immobilized polysaccharide-type chiral stationary phases (CSPs). Enantioselectivities up to 1.30 were obtained in the polar-organic elution mode whereby for hypericin and pseudohypericin Chiralpak IC [chiral selector being cellulose tris(3,5-dichlorophenylcarbamate)] and for protohypericin Chiralpak IA (chiral selector being the 3,5-dimethylphenylcarbamate of amylose) gave favorable results. Enantiomers were distinguished by on-line electronic circular dichroism detection. Optimized enantioselective chromatographic conditions were the basis for determining stereodynamic parameters of the enantiomer interconversion process of hypericin and pseudohypericin. Rate constants delivered by computational simulation of dynamic HPLC elution profiles (stochastic model, consideration of peak tailing) were used to calculate averaged enantiomerization barriers (DG] enant) of 97.6–99.6 kJ/mol for both compounds (investigated temperature range 25–458C). Complementary variable temperature off-column (i.e., in solution) racemization experiments delivered DG] enant 5 97.1–98.0 kJ/mol (27–458C) for hypericin and DG] enant 5 98.9–101.4 kJ/mol (25–558C) for pseudohypericin. An activation enthalpy of DH# 5 86.0 kJ/mol and an activation entropy of DS# 5 237.7 J/(K mol) were calculated from hypericin racemization kinetics in solution, whereas for pseudohypericin these figures amounted to 74.1 kJ/mol and 282.6 J/(K mol), respectively. Although the natural phenanthroperylene quinone pigments hypericin and pseudohypericin as well as their biological precursor protohypericin are chiral and can be separated by enantioselective HPLC low enantiomerization barriers seem to prevent the occurrence of an excess of one enantiomer under typical physiological conditions—at least as long as stereoselective intermolecular interactions with other chiral entities are absen

Asymmetric Hydroarylation Reactions Catalyzed by Transition Metals: Last 10 Years in a Mini Review

Hydroarylation reactions play a pivotal role in organic chemistry due to their versatility and efficiency. In the last 10 years, the scientific production around this reaction has been very high, but in its asymmetric version, the results are less. In this mini review, selected literature examples are considered to draw attention to directions of the asymmetric hydroarylation reaction mediated by transition metal catalysts. The selected works were grouped in two main sections. In the first, we reported examples relating the narrower definition of hydroarylation, namely the metal-catalyzed processes where inactivated aryl moiety undergoes a direct functionalization via insertion of an unsaturated compound. In the second part, hydroarylation reactions take place with the use of pre-activated aryl substrates, usually aryl-iodides or aryl-boronated

Tetrasubstituted cyclopentadienones as suitable enantiopure ligands with axial chirality.

A series of thermally stable atropisomeric phencyclone ligands (ΔG‡ rac > 35 kcal mol−1), bearing two chiral axes, has been successfully synthesized, taking into account the results of DFT calculations on model systems. The absolute configurations of the novel atropisomers have been assigned using TD-DFT simulation of ECD spectra. Atropisomeric phencyclones herein presented pave the way towards new ruthenium- based enantioselective hydrogenation catalyst

Recognition mechanism of aromatic derivatives resolved by argentation chromatography: The driving role played by substituent groups

Argentation chromatography is widely used nowadays as a powerful tool to separate complex mixtures of analytes containing unsaturated and/or aromatic fragments. Here we present the results of chromatographic and computational studies on a silver-thiolate stationary phase, in which the silver metal is covalently bonded to mercaptopropyl silica particles. The exceptionally high selectivity displayed by this organometallic moiety prompted us to deeply investigate its molecular recognition properties. The interactions of the silver atom with a series of benzene derivatives was investigated to gain information on the mechanism by which the different ring substituents modulate retention factors and selectivity. The experimental trend was fully rationalized by means of quantum-mechanical Density Functional Theory (DFT) calculations, which allowed us to elucidate the chromatographic results in the light of unusual and unexpected substituent effects

Characterization and applications in seconds time scale of new totally porous sub-2micron CSPs: brush-type and macrocyclic selectors.

Over the last ten years, the technological progress has led to the development of stationary phases on ever smaller silica particles and instruments (UHPLC/UHPSFC) with a reduced extra-column volume able to reach very high pressure. These innovations allow higher efficiencies, resolutions and permit to reduce the analysis time and the eluent consumption. For these reasons also chiral stationary phases (CSPs) are moving to sub-2µm particles diameter. This talk concerns the development of two different sub-2µm CSPs based on the WhelkO-1[1] and on the teicoplanin selectors[2]. The first selector was covalently bonded on fully porous 1.8 µm Kromasil and the second one on totally porous and monodispersed 1.9 µm Titan silica particles. Both CSPs were packed in columns with an I.D. of 4.6 mm and different lengths, from 10 cm down to 1 cm, the latter geometry permitting very short analysis time. The UHPLC columns packed with the WhelkO-1-CSP were evaluated using normal phase and supercritical fluid eluents. Kinetic performances were estimated using trans-stilbene oxide as a probe, and resulted in efficiencies up to 250’000 plates/m under normal phase conditions. To evaluate the thermodynamic performances a large library screening[3] was performed under sub-critical fluid conditions: in one working day, 81 out of 129 randomly collected racemates were resolved under identical eluting conditions. The new UHPC-Titan120-Chirobiotic-TZWIT-1.9 showed a broad field of application in different environments (reversed phase, polar organic mode, HILIC, sub-critical fluid and normal phase). The thermodynamic performances of the new TEICO-Titan 1.9 µm have been evaluated with several N-protected amino acids, aryloxy acids, pharmaceutical compounds, sulfoxides and phosphine oxides. This CSP frequently showed high enantio-selectivity values: downsizing in column length, from 10-cm down to 1-cm was easily possible maintain high efficiency obtaining baseline separations and providing a considerable reduction of the analysis time. Ultra-fast enantiomeric separations in less than 60 seconds could get a routine in the chiral screening methods. References [1] D. Kotoni, A. Ciogli, C. Molinaro, I. D’Acquarica, J. Kocergin, T. Szczerb, H. Ritchie, C. Villani, F. Gasparrini, Anal. Chem., 84 (2012), 6805 [2] Berthod A., Chen X., Kullman J.P., Armstrong D.W., Gasparrini F., D’Acquarica I., Villani C., Carotti A., Anal. Chem. 72 (2000) 1767-1780; [3] L. Sciascera, O. Ismail, A. Ciogli, D. Kotoni, A. Cavazzini, L. Botta, T. Szczerba, J. Kocergin, C. Villani, F. Gasparrini, JCA, 1383 (2015) 160–16

A Compendium of the Principal Stationary Phases Used in Hydrophilic Interaction Chromatography: Where Have We Arrived?

Hydrophilic interaction liquid chromatography (HILIC) today is a well-known and largely applied technique to analyse polar compounds such as pharmaceuticals, metabolites, proteins, peptides, amino acids, oligonucleotides, and carbohydrates. Due to the large number of stationary phases employed for HILIC applications, this review aims to help the reader in choosing a proper stationary phase, which often represents the critical point for the success of a separation. A great offer is present for achiral applications in contrast to the chiral phases developed for HILIC enantioseparations. In the last case, up-to-date solutions are presented

Atropisomers of arylmaleimides: stereodynamics and absolute configuration

4-Aryl-3-bromo-N-benzylmaleimides and 3,4- biaryl-N-benzylmaleimides have been synthesized by a modified Suzuki cross-coupling reaction from 3,4-dibromo-Nbenzylmaleimide. The conformational studies by dynamic NMR and DFT calculations showed that the interconversion barrier between the two available skewed conformations is under steric control. When the aryl group was a 2- methylnaphthyl, thermally stable atropisomers were isolated by enantioselective HPLC and their absolute configurations were assigned by TD-DFT simulations of the ECD spectr