Problems and solutions in chiral thin-layer chromatography: a two-phase “Pirkle” modified amino-bonded plate

A new bonded silica based on an amino acid derivative as a TLC stationary phase to solve nitrophenol structural isomerism and ibuprofen stereoisomerism

Background

Many amino acids and their derivatives have been used previously as chiral selectors in thin layer chromatography. Herein, we reported the synthesis, characterization, and chromatographic evaluation of a new modified stationary phase based on (S)-3-(aminomethyl)-5-methylhexanoic acid (also known as pregabalin) as an amino acid ligand bonded to the silica surface via triazine attachment.

Results

SC-2P (silica-cyanuric-2-pregabalin) is a bonded silica stationary phase prepared by direct binding of the pre-synthesized cyanuric-2-pregabalin organic ligand on the chlorinated silica surface. FT-IR and UV diffuse reflection spectroscopy (UV-DRS) were used to characterize the synthesized bonded phase, and the specific surface area was determined using the methylene blue Langmuir isotherm method to be 147.04 m2/g. TLC plates were prepared from a slurry of this synthesized material with 2 mM of Cu2+ as a selector additive, and the chromatographic characteristics of these plates were investigated to separate a ternary mixture of o-, m-, and p-nitrophenol as structural isomers, and a racemic mixture of (±)-ibuprofen as stereoisomers. Solvent systems of n-hexane:dichloroethane:n-propanol (75:20:5, v/v/v) and Dichloroethane:acetonitrile (90:10, v/v) were selected as mobile phases for nitrophenol and ibuprofen mixtures, respectively. The successful separation was densitometrically confirmed, and retardation factors (Rf) were determined for o-, m-, and p-nitrophenol at 320 nm to be 0.83, 0.45, and 0.28, and for R(−)-, S(+)-ibuprofen at 220 nm to be 0.43 and 0.63, respectively.

Conclusion

The synthesis, characterization, and chromatographic evaluation of SC-2P were reported in this article. SC-2P was used with copper ions to form in situ ligand exchange reagent (LER), which was successfully employed to solve an isomeric mixture of nitrophenol and a racemic mixture of (±)-ibuprofen. The synthesized stationary phase showed high repeatability with minimum Rf shifts between batches.

Application of thin-layer chromatography in enantiomeric chiral analysis-an overview

Several chiral drugs are produced and administered as pure enantiomers, whereas many others, especially of synthetic origin, are used mainly in the form of racemates. The biological and pharmacological activity of chiral compounds depends on their configuration. The racemic drugs may exhibit quite different activity from the optically pure drugs. Often only one of the enantiomers is pharmacologically active and/or even can be toxic. Since numerous enantiomers have been shown to behave differently from at least one point of view, whether pharmacokinetic, pharmacodynamic, toxicological or interaction, there seems to be hardly any exception to the general rule that a racemate cannot be considered as a single drug entity. A variety of chromatographic methods have been developed for optical resolution recently. Usually direct separation of the enantiomers is carried out on HPTLC chiral precoated plates or on plates impregnated with chiral substances. TLC techniques are a developing branch of separation and quantitation of drugs, both in pharmaceutical dosage forms and in biological material. This review presents an overview of the current successful enantioseparations of drugs by TLC and their potential in the analysis of the drug racemates.