Abstract
The enantiomeric separation of 15 racemic 4-aryl-3,4-dihydropyrimidin-2(1H)-one (DHP) alkoxycarbonyl esters, some of which proved to be highly active as A2B adenosine receptor antagonists, was carried out by HPLC on ChirobioticTM TAG, a chiral stationary phase (CSP) bearing teicoplanin aglycone (TAG) as the chiral selector. The racemic compounds were separated under polar organic (PO) conditions. Preliminarily, the same selectands were investigated on three different Pirkle-type CSPs in normal-phase (NP) conditions. A baseline separation was successfully obtained on TAG-based CSPs for the majority of compounds, some of which achieved high enantioselectivity ratios (α > 2) in contrast with the smaller α values (1–1.5) and the lack of baseline resolution observed with the Pirkle-type CSPs. In particular, the racemic tetrazole-fused DHP ester derivatives, namely compounds 8 and 9, were separated on TAG-based HPLC columns with noteworthy α values (8.8 and 6.0, respectively), demonstrating the potential of the method for preparative purposes. A competition experiment, carried out with a racemic analyte (6) by adding N-acetyl-d-alanine (NADA) to the mobile phase, suggested that H-bonding interactions involved in the recognition of the natural dipeptide ligand d-Ala-d-Ala into the TAG cleft should be critical for enantioselective recognition of 4-aryl DHPs by TAG. The X-ray crystal structure of TAG was elucidated at a 0.77 Å resolution, whereas the calculation of molecular descriptors of size, polar, and H-bond interactions, were complemented with molecular docking and molecular dynamics calculations, shedding light on repulsive (steric effects) and attractive (H-bond—polar and apolar) interactions between 4-aryl DHP selectands and TAG chiral selectors.
Highlights
The development and optimisation of methods to obtain enantiomers with high optical purity remain an important goal to be achieved in the early stages of drug discovery, considering that single enantiomers and diastereoisomers could often have distinct profiles in pharmacodynamics, pharmacokinetics, metabolism, and toxicity [1]
A literature survey indicates that chiral stationary phases (CSPs) in enantioselective high-performance liquid chromatography (HPLC) is the method of choice as it allows the overcoming of limitations such as, to name a few, the demand on the enantiomeric purity of the derivatizing agent when opting for off-line diastereomeric formation, chiral selector consumption, and detection interference when adding a chiral auxiliary to the mobile phase
We report on the enantiomeric separation of a novel class of chiral bioactive 3,4-dihydropyrimidin-2(1H)-one (DHP) alkoxycarbonyl esters (Figure 1), mostly bearing an aryl group at C4, discovered by some of us as being potent and selective
Summary
The development and optimisation of methods to obtain enantiomers with high optical purity remain an important goal to be achieved in the early stages of drug discovery, considering that single enantiomers and diastereoisomers could often have distinct profiles in pharmacodynamics, pharmacokinetics, metabolism, and toxicity [1]. Within the variety of methods used, enantiomers’ separation by high-performance liquid chromatography (HPLC) has great impact in pharmaceutical research. A literature survey indicates that chiral stationary phases (CSPs) in enantioselective HPLC is the method of choice as it allows the overcoming of limitations such as, to name a few, the demand on the enantiomeric purity of the derivatizing agent when opting for off-line diastereomeric formation, chiral selector consumption, and detection interference when adding a chiral auxiliary to the mobile phase. We report on the enantiomeric separation of a novel class of chiral bioactive 3,4-dihydropyrimidin-2(1H)-one (DHP) alkoxycarbonyl esters (Figure 1), mostly bearing an aryl group at C4, discovered by some of us as being potent and selective. Further diversification of the original 4-aryl-DHP scaffold [5,12,13,14,15] produced new ligands with improved affinity and selectivity profiles
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