Reversed-phase high-performance liquid chromatographic separation of diastereomers of ( R, S)-mexiletine prepared by microwave irradiation with four new chiral derivatizing reagents based on trichloro- s-triazine having amino acids as chiral auxiliaries and 10 others having amino acid amides

Abstract

A new series of chiral derivatizing reagents (CDRs) consisting of four dichloro- s-triazine reagents was synthesized by nucleophilic substitution of one chlorine atom in trichloro- s-triazine with amino acids, namely l-Leu, d-Phg, l-Val and l-Ala as chiral auxiliaries. Two other sets of CDRs consisting of four dichloro- s-triazine (DCT) and six monochloro- s-triazine (MCT) reagents were also prepared by nucleophilic substitution of chlorine atom(s) with different amino acid amides as chiral auxiliaries in trichloro- s-triazine and its 6-methoxy derivative, respectively. These 14 CDRs were used for the synthesis of diastereomers of ( R, S)-mexiletine under microwave irradiation ( i.e. 60 s and 90 s at 85% power (of 800 W) using DCT and MCT reagents, respectively), which were resolved by reversed-phase high-performance liquid chromatography using C18 column and gradient eluting mixtures of methanol with aqueous trifluoroacetic acid (TFA) with UV detection at 230 nm. The resolution ( R s), difference between retention times of resolved diastereomers (Δ t) and retention factors ( k) obtained for the three sets of diastereomers were compared among themselves and among the three groups. Explanations have been offered for longer retention times and better resolution of diastereomers prepared with DCT reagents in comparison of their MCT counterparts and, for the influence of hydrophobicity of the side chain R of the amino acid in the CDRs on retention times and resolution. The newly synthesized CDRs were observed to be superior as compared to their amide counterparts in terms of providing better resolution and cost effectiveness. The method was validated for limit of detection, linearity, accuracy and precision.