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

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 (ε_o ～ 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, ¹H‐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.     

Liquid chromatographic enantioseparation of three beta‐adrenolytics using new derivatizing reagents synthesized from (S)‐ketoprofen and confirmation of configuration of diastereomers

Diastereomers of racemic β‐adrenolytic drugs [namely (RS)‐propranolol, (RS)‐metoprolol and (RS)‐atenolol] were synthesized under microwave irradiation with (S)‐ketoprofen based chiral derivatization reagents (CDRs) newly synthesized for this purpose. (S)‐Ketoprofen was chosen for its high molar absorptivity (ε_o ~ 40,000) and its availability as a pure (S)‐enantiomer. Its ‐COOH group was activated with N‐hydroxysuccinimide and N‐hydroxybenzotriazole; these were easily introduced and also acted as good leaving groups during nucleophilic substitution by the amino group of the racemic β‐adrenolytics. The CDRs were characterized by UV, IR, ¹H‐NMR, HRMS and CHNS. Separation of diastereomers was achieved by RP HPLC and open column chromatography. Absolute configuration of the diastereomers was established with the help of ¹HNMR supported by developing their optimized lowest energy structures using Gaussian 09 Rev. A.02 program and hybrid density functional B3LYP with 6‐31G* basis set (based on density functional theory), and elution order was established. RP HPLC conditions for separation were optimized and the separation method was validated. The limit of detection values were 0.308 and 0.302 ng mL^?1. Copyright © 2016 John Wiley & Sons, Ltd.     

HPLC enantioseparation of racemic bupropion,baclofen and etodolac: modification of conventional ligand exchange approach by pre‐column formation of chiral ligand exchange complexes

Separation of racemic mixture of (RS)‐bupropion, (RS)‐baclofen and (RS)‐etodolac, commonly marketed racemic drugs, has been achieved by modifying the conventional ligand exchange approach. The Cu(II) complexes were first prepared with a few l ‐amino acids, namely, l ‐proline, l ‐histidine, l ‐phenylalanine and l ‐tryptophan, and to these was introduced a mixture of the enantiomer pair of (RS)‐bupropion, or (RS)‐baclofen or (RS)‐etodolac. As a result, formation of a pair of diastereomeric complexes occurred by ‘chiral ligand exchange’ via the competition between the chelating l ‐amino acid and each of the two enantiomers from a given pair. The diastereomeric mixture formed in the pre‐column process was loaded onto HPLC column. Thus, both the phases during chromatographic separation process were achiral (i.e. neither the stationary phase had any chiral structural feature of its own nor did the mobile phase have any chiral additive). Separation of diastereomers was successful using a C₁₈ column and a binary mixture of MeCN and TEAP buffer of pH 4.0 (60:40, v/v) as mobile phase at a flow rate of 1 mL/min and UV detection at 230 nm for (RS)‐Bup, 220 nm for (RS)‐Bac and 223 nm for (RS)‐Etd. Baseline separation of the two enantiomers was obtained with a resolution of 6.63 in <15 min. Copyright © 2016 John Wiley & Sons, Ltd.     

Liquid chromatographic enantioseparation of (RS)‐etodolac using (S)‐levofloxacin and determination of absolute configuration of the diastereomeric derivatives

(RS)‐Etodolac was isolated from commercial tablets and was purified and characterized to be used as racemic standard. A pair of diastereomeric derivatives was synthesized using (S)‐levofloxacin as a chiral derivatizing reagent. The derivatization reaction was carried out under conditions of stirring at room temperature (30°C for 1.5 h) as well as under microwave irradiation; the derivatives obtained by the two methods were compared. Reaction conditions for derivatization were optimized with respect to mole ratio of chiral derivatizing reagent and (RS)‐etodolac. No racemization was observed throughout the study. Separation of diastereomeric derivatives was successful using C₁₈ column and a binary mixture of methanol and triethyl ammonium phosphate buffer of pH 4.5 (80:20, v/v) as mobile phase at a flow rate of 1 mL min^?1 and UV detection at 223 nm. An efficient approach for recognizing chirality and determining the absolute configuration of the diastereomeric derivatives of (RS)‐etodolac is described, which in turn is a measure of the enantiomeric purity of (RS)‐etodolac since the diastereomeric derivatives were separated and isolated using preparative thin‐layer chromatography.     

A Solid State Polymer‐Coated Electrode Containing a Chiral Crown Ether Derivative for Enantioselective Detection of Phenylglycine Methyl Ester Isomer

All solid‐state enantioselective electrode (ASESE) based on a newly synthesized chiral crown ether derivative ((R)‐(?)‐(3,3′‐diphenyl‐1,1′‐binaphthyl)‐23‐crown‐6 incorporating 1,4‐dimethoxybenzene) was prepared and characterized by potentiometry. The ASESE clearly showed enantiomer discrimination for methyl esters of alanine, leucine, valine, phenylalanine, and phenylglycine, where the enantioselectivity for phenylglycine methyl ester was the highest (K_R,S=8.5±7.1%). Experimental parameters of ASESE for the analysis of (R)‐(?)‐phenylglycine methyl ester were optimized. The optimized ASESE showed a slope of 55.3±0.2 mV/dec for (R)‐(?)‐phenylglycine methyl ester in the concentration range of 1.0×10^?5–1.0×10^?2 M and the detection limit was 9.0×10^?6 M. The ASESE showed good selectivity for (R)‐(?)‐phenylglycine methyl ester against inorganic cations and various amino acid methyl esters. The concentration of (R)‐(?)‐phenylglycine methyl ester was determined in the mixture of (R)‐(?) and (S)‐(+)‐phenylglycine methyl ester, which ratios varied from 2 : 1 to 1 : 9. The lifespan of the electrode was alleged to be 30 days.     

Methods and approaches for determination and enantioseparation of (RS)‐propranolol

Propranolol, a β‐adrenergic receptor antagonist, is a chiral compound that is marketed as a racemate, but only the (S)‐(?)‐enantiomer is responsible for the β‐adrenoceptor blocking activity. Different chromatographic methods have been applied for separation and determination of enantiomers of (RS)‐propranolol. In this article a review is presented on different liquid chromatographic methods used for enantioseparation of (RS)‐propranolol, using both HPLC and TLC. In addition, some aspects of enantioseparation under achiral phases of liquid chromatography have been briefly mentioned.     

High‐performance liquid chromatographic enantioseparation of (RS)‐bupropion using isothiocyanate‐based chiral derivatizing reagents

A high‐performance liquid chromatographic (HPLC) method for enantioseparation of bupropion was developed using two isothiocyanate‐based chiral derivatizing reagents, (S)‐1‐(1‐naphthyl) ethyl isothiocyanate, (S)‐NEIT, and (R)‐α‐methyl benzyl isothiocyanate, (R)‐MBIT. The diastereomers synthesized with (S)‐NEIT were enantioseparated by reversed‐phase HPLC using gradient elution with mobile phase containing water and acetonitrile, whereas diastereomers synthesized with (R)‐MBIT were enantioseparated using triethyl amine phosphate buffer and methanol. Derivatization conditions were optimized and the method was validated for accuracy, precision and limit of detection. The limit of detection was found to be 0.040–0.043 µg/mL for each of the diastereomers prepared with (S)‐NEIT. Copyright © 2013 John Wiley & Sons, Ltd.     

An efficient method for the determination of enantiomeric purity of racemic amino acids using micellar chromatography,a green approach

A simple, sensitive and green micellar liquid chromatographic method (RP-HPLC) was developed for enantioseparation of four racemic amino acids, namely, (RS)-selenomethionine, (RS)-methionine, (RS)-cysteine and (RS)-penicillamine. An aqueous solution of sodium dodecyl sulfate and Brij-35 was prepared and used as mobile phase for HPLC analysis. Activated esters of (S)-ibuprofen, (S)-ketoprofen and (S)-levofloxacin were synthesized by reacting them with N-hydroxybenzotriazole. These esters were characterized by UV, IR, ¹HNMR, HRMS and elemental analysis. These chiral reagents (activated esters) were used for the synthesis of diastereomeric derivatives of the chosen amino acids. The diastereomeric derivatives were separated on a C₁₈ column by micellar liquid chromatography. Chromatographic conditions were optimized by varying concentration of surfactant in aqueous solution, and by varying the concentration and pH of the buffer. The green assessment score was calculated for the developed method (78, an excellent green method score). In addition, density functional theory calculations were performed, using Gaussian 09 rev. A.02 and hybrid density functional B3LYP with a 6-31G* basis set program, in order to develop lowest energy optimized structures of diastereomeric derivatives. The method was validated according to International Conference on Harmonization guidelines and the retention factor (k), selectivity factor (α), resolution factor (R_S) and limit of detection (0.295 ng ml⁻¹) and limit of quantification (0.896 ng ml⁻¹) were calculated.     

Elementary reaction analysis of the radical polymerization of α‐ethyl β‐N‐(α′‐methylbenzyl)itaconamates carrying (RS)‐ and (S)‐α‐methylbenzylaminocarbonyl groups

The polymerizations of α‐ethyl β‐N‐(α′‐methylbenzyl)itaconamates carrying (RS)‐ and (S)‐α‐methylbenzylaminocarbonyl groups (RS‐EMBI and S‐EMBI) with dimethyl 2,2′‐azobisisobutyrate (MAIB) were studied in methanol (MeOH) and in benzene kinetically and with electron spin resonance (ESR) spectroscopy. The initial polymerization rate (R_p) at 60 °C was given by R_p = k[MAIB]^{0.58 ± 0.05}[RS‐EMBI]^{2.4 ± 0.l} and R_p = k[MAIB]^{0.61 ± 0.05}[S‐EMBI]^{2.3 ± 0.l} in MeOH and R_p = k[MAIB]^{0.54 ± 0.05}[RS‐EMBI]^{1.7 ± 0.l} in benzene. The rate constants of initiation (k_df), propagation (k_p), and termination (k_t) as elementary reactions were estimated by ESR, where k_d is the rate constant of MAIB decomposition and f is the initiator efficiency. The k_p values of RS‐EMBI (0.50–1.27 L/mol s) and S‐EMBI (0.42–1.32 L/mol s) in MeOH increased with increasing monomer concentrations, whereas the k_t values (0.20?7.78 × 10⁵ L/mol s for RS‐EMBI and 0.18?6.27 × 10⁵ L/mol s for S‐EMBI) decreased with increasing monomer concentrations. Such relations of R_p with k_p and k_t were responsible for the unusually high dependence of R_p on the monomer concentration. The activation energies of the elementary reactions were also determined from the values of k_df, k_p, and k_t at different temperatures. R_p and k_p of RS‐EMBI and S‐EMBI in benzene were considerably higher than those in MeOH. R_p of RS‐EMBI was somewhat higher than that of S‐EMBI in both MeOH and benzene. Such effects of the kinds of solvents and monomers on R_p were explicable in terms of the different monomer associations, as analyzed by ¹H NMR. The copolymerization of RS‐EMBI with styrene was examined at 60 °C in benzene. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1819–1830, 2003     

Micellar liquid chromatography for enantioseparation of β-adrenolytics using (S)-ketoprofen-based reagents

Micellar liquid chromatographic method has been developed for enantioseparation of four β-adrenolytics, namely, (RS)-salbutamol, (RS)-carvedilol, (RS)-bisoprolol, and (RS)-betaxolol. Both sodium docyl sulfate and Brij-35 were used as the surfactants in water as the mobile phase. Advantages for using both the surfactants in combination were investigated. Two (S)-ketoprofen-based activated esters were synthesized by activating its carboxylic group with N-hydroxybenzotriazole and N-hydroxysuccinimide, respectively. The esters were characterized by UV, IR, ¹H-NMR, HRMS, and elemental analyses. These reagents were used for the synthesis of diastereomeric derivatives of the chosen β-adrenolytics. These diastereomeric derivatives were enantioseparated on C₁₈ column by high-performance liquid chromatography. Chromatographic conditions were optimized by varying concentration of surfactant and buffer, and pH. The method was validated according to International Conference of Harmonization guideline and the retention factor (k), selectivity factor (α), resolution factor (R_S), and limit of detection and limit of quantification were calculated.     

Sensitive enantioseparation and determination of isoprenaline in human plasma and pharmaceutical formulations

A simple, sensitive and fast RPHPLC method was developed and validated for the enantioselective determination of (RS)‐isoprenaline (Ipn) in human plasma. The enantiomers were converted to diastereomeric derivatives using s‐triazine (cyanuric chloride) based chiral derivatizing reagents. l ‐isoleucine and l ‐methionine were introduced as chiral auxiliary in s‐triazine and two new monochloro‐s‐triazine reagents were synthesized. These reagents were characterized and used for synthesis of diastereomeric derivatives of (RS)‐Ipn spiked in human plasma. (RS)‐Ipn was isolated (purified and characterized) from a commercial pharmaceutical formulation and was used as the standard racemic sample. Structures of the two diastereomeric derivatives were optimized for lowest energy using the Gaussian 09 Rev A. 02 program and hybrid density functional B3LYP with 6‐31G* basis set which showed the spatial orientation of hydrophobic groups on stereogenic centers in the diastereomeric derivatives. The results were correlated with the mechanism of separation and elution order. Limit of detection values were found to be 24.6 and 26.8 ng mL^?1 for the first and second eluting diastereomeric derivatives, respectively.     

Über Pterinchemie. 72. Mitteilung. Trennung der Diastereomeren (6R)- und (6S)-5,6,7,8-Tetrahydro-L-biopterin

Separation of the Diastereomers (6R)- and (6S)-5,6,7,8-Tetrahydro-L -biopterin The mixture of the diastereomers of the tetraacetylderivative IV of (6RS)-5,6,7,8-tetrahydro-L -biopterin could be separated by fractional crystallisation in methanol into the diastereomers IV A and IV B. Hydrolysis with hydrochloric acid gives the pure, diastereomeric, (6R)- and (6S)-5,6,7,8-tetrahydro-L -biopterins.     

Über Pterinchemie. 71. Mitteilung. Trennung der Diastereomeren (6R)- und (6S)-5,6,7,8-Tetrahydro-L-neopterin

Separation of the Diastereomers (6R) and (6S)-5,6,7,8-Tetrahydro-L -neopterin The mixture of the diastereomers of the pentaacetylderivative IV of (6RS)-5,6,7,8-tetrahydro-L -neopterins could be separated by fractional crystallisation in methanol into the diastereomers IV A and IV B. Hydrolysis with hydrochloric acid gives the pure, diastereomeric, (6R)- and (6S)-5,6,7,8-tetrahydro-L -neopterins.     

On the Reactivity of (−)‐(R)‐Carvone and (−)‐4aα,7α,7aβ‐Nepetalactone: Synthesis of New Heterocycles

The 1,3‐dipolar cycloaddition of 4‐chlorobenzonitrile oxide to the unsaturated system of (?)‐(R)‐carvone occurred exclusively at C(8) to give a new isoxazoline derivative. This derivative reacts with NH₂OH to yield a new heterocycle, observed for the first time. On the other hand, the addition of 4‐chlorobenzonitrile oxide to the unsaturated lactone (?)‐4aα,7α,7aβ‐nepetalactone gave, in a good yield, also a new heterocycle, again obtained for the first time. The terpenoid (?)‐(R)‐carvone and iridoid (?)‐4aα,7α,7aβ‐nepetalactone were isolated from Moroccan species Mentha viridis (L.) and Nepeta tuberosa (L.), respectively. The new heterocycles obtained were identified by combination of chromatographic and spectroscopic methods.     

A liquid chromatography-tandem mass spectrometry method for the quantitative determination of diastereomers of a phosphoramidate nucleotide prodrug (PSI-7851) in human plasma

A rapid and stereospecific method using high performance liquid chromatography–tandem mass spectrometry (HPLC‐MS/MS) for the separation and determination of PSI‐7851 diastereomers in human K₂EDTA plasma has been developed. The analytical method involves direct protein precipitation with acetonitrile, followed by separation of the diastereomers on a Luna C₁₈ column, positive mode electrospray ionization and selected reaction monitoring mode mass spectrometry detection. The mobile phase composition and pH were investigated for the resolution of the two diastereomers of PSI‐7851. The optimized method showed good resolution (R_s = 4.8) within short analysis time (approximately 8 min). The assay range was 5–2500 ng/mL for both diastereomers using a 1/x² weighted linear regression analysis for standard curve fitting. Replicate sample analysis indicated that intra‐ and inter‐day accuracy and precision were within ±15.0%. The recovery of diastereomers from human plasma was greater than 85% and no significant matrix effect was observed. The method was demonstrated to be sensitive, selective and robust, and was successfully used to support clinical studies. Copyright © 2011 John Wiley & Sons, Ltd.     

β-Cyclodextrin-based chiral nanocomposite for thin-layer chromatographic detection of enantiomers of fluoxetine

Enantioselective analysis or separation is very essential for improved therapeutic effects of drugs as the pure enantiomeric drug formulations display potential benefits over racemates. In this work, we carried out (i) the synthesis of a nanocomposite of β-cyclodextrin and 3D graphene (G/β-CD NC), and (ii) its application for the detection of fluoxetine enantiomers [(RS)-FLX)] using a thin-layer chromatographic method. The synthesized nanocomposite was introduced into silica gel slurry while preparation of thin-layer plates. The separation conditions were optimized by altering pH, temperature, and mobile phase composition. The method is simple and easy to be optimized, and it can therefore be exploited to assess and monitor routine work of enantiomeric purity of drug enantiomers. The average precision (as measured by RSD) was in the region of 1.35‒1.65% for the enantiomers of (RS)-FLX. The measured limit of detection and limit of quantification for (RS)-FLX enantiomers were 1.8 and 5.4 mg mL^‒1, respectively.

     

Liquid chromatographic enantioseparation of β-blocking agents with (1R,2R)-1,3-diacetoxy-1-(4-nitrophenyl)-2-propyl isothiocyanate as chiral derivatizing agent

The applicability of (1R,2R)-1,3-diacetoxy-1-(4-nitrophenyl)-2-propyl isothiocyanate [(R,R)-DANI] as a recently developed chiral derivatizing agent for the enantioseparation of a series of β-blockers is described. The thiourea diastereomers formed were analyzed by reversed-phase high-performance liquid chromatography, mixtures of water and methanol or acetonitrile being used for elution. Conditions of derivatizations (temperature, reagent excess and reaction time) were optimized, and the effects of organic modifiers on the retention and separation were investigated; the diastereomers could readily be baseline separated with methanol-containing mobile phases with resolutions between 1.58 and 2.72.     

Dibothrioclinin I and II,epimers from Gerbera piloselloides (L.) Cass

Dibothrioclinin I and II, namely (+)‐(11R,12S,25R,27S)‐ and (±)‐­(11RS,12RS,25RS,27SR)‐3,3,7,17,21‐penta­methyl‐4,12,18,26‐tetraoxahepta­cyclo­[15.11.1.0^2,15.0^5,14.0^6,11.0^19,28.0^20,25]­nona­cosa‐5(14),6,8,10,19(28),20,22,24‐octaene‐13,27‐dione, respectively, are C₃₀H₂₈O₆ epimers which are derived from two bothrioclinin moieties joined so as to create an additional six‐membered ring. Structurally, the epimers differ only by inversion at one C atom of a central ring junction and the corresponding six‐membered rings have similar conformations in each mol­ecule, except for one ring adjacent to this inversion site.     

Pd(II)‐catalyzed polymerization of optically active norbornene carboxylic acid esters

(?)‐(1S,2R)‐Norbornene‐2‐carboxylic acid alkyl esters (alkyl = Me, Bz, L ‐menthyl, or D ‐menthyl) were successfully prepared by the Diels–Alder reaction of cyclopentadiene with (R)‐(?)‐pantolactone‐O‐yl acrylate followed by epimerization and column chromatography. The enantiomeric excess was 99.9%. These monomers were polymerized by Pd(II)‐based catalysts, and high yields of the polymers were obtained. The methyl ester gave an optically active polymer of high optical rotation (monomer [α]_D = ?24.7, polymer [α]_D = ?98.5). This high rotation value of the polymer was attributed to the isotactic chain regulation of the polymer. This high rotation was not observed with methyl esters prepared by the transesterification of menthyl esters. The stereoregular polymer exhibited notable resonance peaks at 39 ppm in ¹³C NMR spectra. No crystallinity was observed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1263–1270, 2006     

Beitrag zur Analytik und Synthese von 3-Hydroxy-4-oxocarotinoiden

Contribution to the Analytical Separation and the Synthesis of 3-Hydroxy-4-oxocarotenoids (3RS,3′RS)-Astaxanthin (= 3,3′-dihydroxy-β,β-carotene-4,4′-dione, 1:1-mixture of racemate and meso-form; 1 ) can be separated into its optical isomers (3S,3′S)- 1a , (3R,3′R)- 1b and meso-(3R,3′S)- 1c via the corresponding diastereomeric di-(?)-camphanates. Some aspects of the configurational stability of astaxanthin are discussed. - HPLC. analysis of the (?)-camphanates of 3-hydroxy-4-oxocarotenoids provides, in suitable cases and supported by spectroscopic data, an analytical method for the simultaneous determination of constitution and chirality.