RP-LC Resolution of (R,S)-Atenolol via Diastereomerization with Marfey’s Reagent and Its Structural Variants Under Conventional and Microwave Heating

'Ab Ovo' Chiral Phases and Chiral Reagents for Liquid Chromatographic Separation and Isolation of Enantiomers

The de-novo approach of mixing chirally pure reagents or Cu(II)-L-amino acid complexes in the slurry of silica gel for preparing TLC plates was reported from author's laboratory and was successful for separation and isolation of enantiomers. Using high molar absorptivity molecules, e. g., 1,5-difluoro-2,4-dinitrobenzene and cyanuric chloride, more than 38 new chiral derivatizing reagents were synthesized in our laboratory by straightforward nucleophilic substitution with simple chiral auxiliaries. Besides, (S)-naproxen, (S)-ketoprofen, and (S)-levofloxacin were used as chiral platforms. A conceptual approach using both achiral phases in chromatography for enantioseparation was also adopted. ¹ H NMR and DFT based software were used to explain structures of non-covalent and covalent diastereomeric pairs and determination of configuration and separation mechanism. The methods can be easily used to determine and control enantiomeric purity with advantages over a variety of commercial chiral phases.

Reversed-phase high-performance liquid chromatographic enantioresolution of six beta-blockers using dinitrophenyl-L-Pro-N-hydroxysuccinimide ester, N-succinimidyl-(S)-2-(6-methoxynaphth-2-yl) propionate and twelve variants of Sanger's reagent as chiral derivatizing reagents

Twelve chiral derivatizing reagents (CDRs) were synthesized by substituting one of the fluorine atoms in 1,5-difluoro-2,4-dinitrobenzene (DFDNB) with three optically pure amines [(R)-(-)-1-cyclohexylethylamine, (+)-dehydroabietylamine and (S)-(-)-alpha,4-dimethylbenzylamine], six amino acid amides [L-Ala-NH(2), L-Phe-NH(2), L-Val-NH(2), L-Leu-NH(2), L-Met-NH(2) and D-Phg-NH(2)] and three amino acids [L-Ala, L-Val and L-Leu]. In addition, dinitrophenyl-L-Pro-N-hydroxysuccinimide ester and N-succinimidyl-(S)-2-(6-methoxynaphth-2-yl) propionate were also synthesized and used as CDR. Keeping in view the presence of an amino group, diastereomers of six beta-blockers (atenolol, propranolol, bisoprolol, metoprolol, salbutamol, and carvedilol) were synthesized by reaction with these 14 CDRs. The diastereomers were separated by RP-HPLC. The method was validated for linearity, accuracy, limit of detection and limit of quantification.

Different approaches of impregnation for resolution of enantiomers of atenolol, propranolol and salbutamol using Cu(II)-L-amino acid complexes for ligand exchange on commercial thin layer chromatographic plates

Atenolol and propranolol (the beta-blocking agents) and salbutamol (broncho- and vasodilator) were resolved into their enantiomers by adopting different modes of loading/impregnating the Cu(II) complexes of L-proline (L-Pro), L-phenylalanine (L-Phe), L-histidine (L-His), N,N-dimethyl-L-phenylalanine (N,N-Me(2)-L-Phe), and L-tryptophan (L-Trp) on commercial precoated normal phase plates. The three different approaches were (A) using the Cu(II)-L-amino acid complex as chiral mobile phase additive, (B) ascending development of plain commercial plates in the solutions of Cu complex, and (C) using a solution of Cu(II) acetate as mobile phase additive for the commercial TLC plates impregnated with ascending development of plates in the solutions of amino acid. Spots were located using iodine vapour. The results obtained for the three methods have been compared for their efficiency and the issue of involvement of the Cu(II) cation for the best performance of the three methods has been discussed with respect to the same mobile phase. The detection limit is 0.18 microg for each enantiomer.