(RS)‐Propranolol: enantioseparation by HPLC using newly synthesized (S)‐levofloxacin‐based reagent, absolute configuration of diastereomers and recovery of native enantiomers by detagging

Abstract

Diastereomers of (RS)-propranolol were synthesized using (S)-levofloxacin-based new chiral derivatizing reagents (CDRs). Levofloxacin was chosen as the pure (S)-enantiomer for its high molar absorptivity (εo ∼ 24000) and availability at a low price. Its -COOH group had N-hydroxysuccinimide and N-hydroxybenzotriazole, which acted as good leaving groups during nucleophilic substitution by the amino group of the racemic (RS)-propranolol; the CDRs were characterized by UV, IR, (1) H-NMR, high resolution mass spectrometry (HRMS) and carbon, hydrogen, nitrogen, and sulphur fundamental elemental components analyser (CHNS). Diastereomers were separated quantitatively using open column chromatography; absolute configuration of the diastereomers was established and the reagent moiety was detagged under microwave-assisted acidic conditions. (S)- and (R)-propranolol as pure enantiomers and (S)-levofloxacin were separated, isolated and characterized. Optimized lowest-energy structures of the diastereomers were developed using Gaussian 09 Rev. A.02 program and hybrid density functional B3LYP with 6-31G* basis set (based on density functional theory) for explanation of elution order and configuration. In addition, RP HPLC conditions for separation of diastereomers were optimized with respect to pH, concentration of buffer, flow rate of mobile phase and nature of organic modifier. HPLC separation method was validated as per International Conference on Harmonization guidelines. With the systematic application of various analytical techniques, absolute configuration of the diastereomers (and the native enantiomers) of (RS)-propranolol was established. Copyright © 2016 John Wiley & Sons, Ltd.

RP‐HPLC enantioseparation of β‐adrenolytics using micellar mobile phase without organic solvents

“…As discussed elsewhere (Alwera & Bhushan, , ) for diastereomers of propranolol prepared with ( S )‐Lfx and ( S )‐ketoprofen based CDRs, these ‘lowest energy structures’, show a spatial nearness due to rigid amide bond between the benzyl (of Bxl) and piperazinyl (of Lfx) in ( S , S )‐diastereomeric derivative (A2); the two hydrophobic groups, thus, cause a relatively more compact situation and the ( S , S )‐diastereomeric derivative (Figure S4), interacts more strongly with hydrophobic ODS material of the C 18 column (Bhushan & Nagar, ), resulting in a higher retention time and its elution after A1 with the mobile phase. Thus, it can be contended that the steric arrangement of the bulky groups at the stereogenic centre of diastereomeric derivatives causes a difference in hydrophobicity of the two isomers, which results in the differential interaction of the diastereomeric derivatives with the C 18 material of the column and their separation.…”

“…Synthesis of both CDR1 and CDR2 was carried out as per procedure reported earlier (Alwera & Bhushan, ) by reaction of ( S )‐(−)‐Lfx, with (1) N ‐hydroxysuccinimide, and (2) N ‐hydroxybenzotriazole. The CDRs so obtained were characterized (the method of synthesis and characterization data are given in Supporting information).…”

“…Synthesis of diastereomeric derivatives of each of the four β‐adrenolytics was carried out using both CDR1 and CDR2, under microwave, as per the procedure reported earlier (Alwera & Bhushan, ). The solutions of each of the racemic β‐adrenolytics (40 μL, 1 m m ), were mixed with solutions of CDR1 (80 μL, 1 m m ) and triethylamine (10 μL), and the resulting solutions were irradiated under microwave for 3 min using 80% power (800 W).…”

“…Literature reports on HPLC enantioseparation of different β‐adrenolytics by indirect approach using isothiocyanates, isocyanates, chloroformates, acid chlorides, aromatic anhydrides, 1,5‐difluoro‐2,4‐dinitrobenzene (DFDNB) and s ‐triazine based chiral derivatizing reagents (CDRs) for derivatization of the amine function, in particular, or by direct approach using various chiral stationary phases, and the recent reports from this laboratory on enantioseparation of a few commonly used β‐adrenolytics drugs [such as, ( RS )‐propranolol, ( RS )‐metoprolol and ( RS )‐atenolol] by indirect approach (Alwera & Bhushan, , ) show that the mobile phase used for HPLC separation of diastereomeric derivatives (of all the β‐adrenolytics prepared with different CDRs) do contain solvents such as trifluoroacetic acid, acetonitrile, methanol, dichloromethane and chloroform. Direct HPLC enantioseparation of carbobenzyloxy derivatives of several amino compounds, including ( RS )‐propranolol and other β‐adrenolytics, on polysaccharide and Pirkle‐type chiral stationary phases (Kraml, Zhou, Byrne, & McConnell, ) has been achieved by using several such organic modifiers.…”

“…Since the literature on such a CDR‐based approach of enantioseparation of several β‐adrenolytics has been covered in recent publications (Alwera & Bhushan, , ) it is not being cited herein to keep the overall length of the paper short and focus on the new aspect of investigations, i.e. change in the chemical nature of the mobile phase.…”