Enantioselective HPLC Analysis to Assist the Chemical Exploration of Chiral Imidazolines.

Bruno Cerra,Emidio Camaioni,Antonio Macchiarulo,Ina Varfaj,Roccaldo Sardella,Antimo Gioiello,Andrea Carotti

doi:10.3390/molecules25030640

Abstract

In the present work, we illustrate the ability of high-performance liquid chromatography (HPLC) analysis to assist the synthesis of chiral imidazolines within our medicinal chemistry programs. In particular, a Chiralpak® IB® column containing cellulose tris(3,5-dimethylphenylcarbamate) immobilized onto a 5 μm silica gel was used for the enantioselective HPLC analysis of chiral imidazolines synthesized in the frame of hit-to-lead explorations and designed for exploring the effect of diverse amide substitutions. Very profitably, reversed-phase (RP) conditions succeeded in resolving the enantiomers in nine out of the 10 investigated enantiomeric pairs, with α values always higher than 1.10 and RS values up to 2.31. All compounds were analysed with 50% (v) water while varying the content of the two organic modifiers acetonitrile and methanol. All the employed eluent systems were buffered with 40 mM ammonium acetate while the apparent pH was fixed at 7.5. Based on the experimental results, the prominent role of π-π stacking interactions between the substituted electron-rich phenyl groups outside of the polymeric selector and the complementary aromatic region in defining analyte retention and stereodiscrimination was identified. The importance of compound polarity in explaining the retention behaviour with the employed RP system was readily evident when a quantitative structure-property relationship study was performed on the retention factor values (k) of the 10 compounds, as computed with a 30% (v) methanol containing mobile phase. Indeed, good Pearson correlation coefficients of retention factors (r - log k1st = −0.93; r - log k2nd = −0.94) were obtained with a water solubility descriptor (Ali-logS). Interestingly, a n-hexane/chloroform/ethanol (88:10:2, v/v/v)-based non-standard mobile phase allowed the almost base-line enantioseparation (α = 1.06; RS = 1.26) of the unique compound undiscriminated under RP conditions.

Highlights

Among the numerous chromatographic techniques available today for chiral analysis, high-performance liquid chromatography (HPLC) excels at this task due to its simplicity and effectiveness
At the end of this process, a structure-based drug design approach was used to drive the synthesis of a small library of chiral imidazolines, including compounds
With the aim to obtain the enantioseparation of the investigated compounds (Figure 2) with the cellulose-based Chiralpak® IB® (Figure 1), chiral stationary phase (CSP) and eluent systems coherent with the most commonly applied normal phase (NP) conditions were initially used

Summary

Introduction

Among the numerous chromatographic techniques available today for chiral analysis, high-performance liquid chromatography (HPLC) excels at this task due to its simplicity and effectiveness. HPLC is an easy-to-use, versatile technique to assist synthesis in view of the wide array of chemicals detection and low implementation time, and when integrated with flow synthesizers and high-throughput experimentation (HTE) systems. Thanks to the continuous technological advancements in the production of Chiral Stationary Phases (CSPs), HPLC-based methods have gotten faster and more effective for the enantiopurity assessment of chiral drugs [1,2,3,4,5]. It needs to be clearly stated that a universal CSP enabling the enantioseparation of all classes of racemic compounds. In some instances, selecting the proper CSP for the enantioseparation of a chiral compound can be rather difficult

Methods

Results

Conclusion