Shalini Andersson

Preparative chiral chromatographic resolution of enantiomers in drug discovery

A brief account is given of the role of preparative chromatography for the direct separation of enantiomers in the drug discovery process. Although it is not yet possible to predict the outcome of a chromatographic resolution attempt, and the optimisation procedure sometimes might be time-consuming, the technique is still indispensable as a means to obtain both enantiomers in pure form from a drug racemate for biological testing. The most suitable types of chiral stationary phases (CSPs) available for this purpose are discussed with special reference to loadability and compatibility with different mobile phase systems.

Proteins and peptides as chiral selectors in liquid chromatography

Abstract Considerable progress has been made recently in the application of amino acid-derived chiral selectors to direct enantiomer separations by liquid chromatography. A better understanding of the chiral discrimination exerted by proteins has been achieved through detailed analyses of the adsorption isotherms obtained under various mobile phase conditions and temperatures. Further insight into the complexity of protein-ligand interactions and the effect on retention have come from studies with the use of mobile phase additives causing allosteric interaction, sometimes resulting in dramatic effects on the enantiomeric separation factor. Further, the recent use of small, synthetic peptides as chiral selectors has shown promise for the future. The wide applicability of particularly the protein phases has led to a variety of novel experimental techniques, involving miniaturization (capillary LC and its combination with mass spectrometry), use for electrically driven separations (capillary electrophoresis), immobilization of proteins on so-called continuous gels for rapid enantiomer separations and use for studies of enzyme stereochemistry.

Unusual effects of separation conditions on chiral separations.

Unusual effects in liquid chromatographic separations of enantiomers on chiral stationary phases are reviewed with emphasis on polysaccharide phases. On protein phases and Pirkle phases reversal of the elution order between enantiomers due to variation of temperature and mobile phase composition has been reported. Most of the nonanticipated observations have dealt with the widely used polysaccharide phases. Reversed retention order and other stereoselective effects have been observed by variation of temperature, organic modifier and water content in nonpolar organic mobile phases.

Highly active iridium (I) complexes for catalytic hydrogen isotope exchange

Practically convenient methods have been developed for the preparation of new iridium complexes, possessing bulky N-heterocyclic carbene and phosphine ligands; these routinely handled complexes are highly active catalysts within directed hydrogen isotope exchange processes.

Direct liquid chromatographic separation of enantiomers on immobilized protein stationary phases. VIII: a comparison of a series of sorbents based on bovine serum albumin and its fragments

Abstract A mixture of three peptides (Mr 38 000), obtained by enzymatic cleavage of bovine serum albumin (BSA) was isolated, immobilized to silica and used as a chiral stationary phase in liquid chromatography. A comparison of sorbents containing these fragments and intact BSA, showed that the enantioselectivity is preserved for only a limited number of compounds. Under identical mobile phase conditions, retention on the columns based on BSA fragments was also much lower than on those containing intact BSA. The method used for immobilization has a great influence on the retentive and enantioselective properties of the sorbent obtained. When sorbents based on BSA entrapped in silica and 3-aminopropylsilica, by cross-linking with glutaraldehyde were compared under identical mobile phase conditions, the latter were generally found to give larger capacity factors and often also larger α values. These results indicate that the increased hydrophobicity, primarily caused by the aminopropyl groups, partly contributes to the overall retention and chiral discrimination process and that the situation may be different from that in a solution of the free protein.

Making medicinal chemistry more effective—application of Lean Sigma to improve processes, speed and quality

The pharmaceutical industry, particularly the small molecule domain, faces unprecedented challenges of escalating costs, high attrition as well as increasing competitive pressure from other companies and from new treatment modes such as biological products. In other industries, process improvement approaches, such as Lean Sigma, have delivered benefits in speed, quality and cost of delivery. Examining the medicinal chemistry contributions to the iterative improvement process of design-make-test-analyse from a Lean Sigma perspective revealed that major improvements could be made. Thus, the cycle times of synthesis, as well as compound analysis and purification, were reduced dramatically. Improvements focused on team, rather than individual, performance. These new ways of working have consequences for staff engagement, goals, rewards and motivation, which are also discussed.

Direct liquid chromatographic separation of enantiomers on immobilized protein stationary phases : V. optical resolution of N-(2,4-dinitrophenyl)- and dansyl-d,l-amino acids

Abstract Analytical-scale optical resolution of a series of N-(2,4-dinitrophenyl)- and dansyl- d , l -amino acids has been effected by the use of a bovine serum albumin silica column (Resolvosil). Decreasing retention was found for both types of amino acid with increasing pH or 1-propanol content of the mobile phase. In the dinitrophenyl series, the aspartic acid derivatives showed very large enantiomeric separation factors compared with the glutamic acid homologue, and an analogous, but less pronounced, efffect was found for the phenylglycine—phenylalanine pair. Fluorimetric studies of dansyl-alanine showed that by a simple post-column addition of 1-propanol the fluorescence yield can be increased by a factor of over 20,giving a very low detection limit. The usefulness of the analytical technique for the determination of the bacterial marker compounds, d -alanine and d -glutamic acid, present in cell wall hydrolysates, is emphasized.

Application of neutral iridium(I) N‐heterocyclic carbene complexes in ortho‐directed hydrogen isotope exchange

Bench-stable complexes of the type [Ir(COD)(NHC)Cl] (NHC = N-heterocyclic carbene) have been investigated within the field of hydrogen isotope exchange. By employing a sterically encumbered NHC within such complexes and catalyst loadings of only 5 mol%, moderate to high deuterium incorporations were achieved across a range of aromatic ketones and nitrogen-based heterocycles. The simple and synthetically accessible catalysts reported herein present alternatives to phosphine-based species and increase the available labelling systems with respect to established iridium-based isotope exchange methodologies.

Evaluation of the chiral recognition properties as well as the column performance of four chiral stationary phases based on cellulose (3,5-dimethylphenylcarbamate) by parallel HPLC and SFC.

The performance of four commercially available cellulose tris(3,5-dimethylphenylcarbamate) based chiral stationary phases (CSPs) was evaluated with parallel high performance liquid chromatography (HPLC) and super critical fluid chromatography (SFC). Retention, enantioselectivity, resolution and efficiency were compared for a set of neutral, basic and acidic compounds having different physico-chemical properties by using different mobile phase conditions. Although the chiral selector is the same in all the four CSPs, a large difference in the ability to retain and resolve enantiomers was observed under the same chromatographic conditions. We believe that this is mainly due to differences in the silica matrix and immobilization techniques used by the different vendors. An extended study of metoprolol and structure analogues gave a deeper understanding of the accessibility of the chiral discriminating interactions and its impact on the resolution of the racemic compounds on the four CSPs studied. Also, a clear difference in enantioselectivity is observed between SFC and LC mode, hydrogen bonding was found to play an important role in the differential binding of the enantiomers to the CSPs.

Preparative resolution of drug racemates to study the chiroptical properties of their enantiomers

The present work is focused on the resolution of ten racemates, in order to study their chiroptical properties and to test the validity of the requirement specified in the European Pharmacopeia (EP) for demonstrating that a drug entity is a racemate. This work shows that the optical purity of enantiomers and non racemic mixtures of a number of compounds can be determined more accurately by circular dichroic (CD) spectroscopy than by a measurement of the angle of rotation (AoR), the EP requirement. Using only the AoR, some of the racemates could not be distinguished from the enantiomers. CD spectroscopy or chiral chromatography should, therefore, be the technique of choice in the determination of optical purity of a chiral compound, especially for those exhibiting low AoR.