Indirect enantioresolution of (R,S)‐mexiletine by reversed‐phase high‐performance liquid chromatography via diastereomerization with [(S,S)‐O,O'‐di‐p‐toluoyl tartaric acid anhydride], (S)‐naproxen and nine chiral reagents synthesized as variants of Marfey's reagent

Eleven chiral derivatizing reagents (CDRs) were used for preparation of diastereomers of (R,S)‐mexiletine containing a primary amino group in close proximity to the stereogenic center. One anhydride, namely [(S,S)‐O,O'‐di‐p‐toluoyl tartaric acid anhydride] was synthesized and (S)‐naproxen was used as such as the chiral derivatizing reagent. The other nine CDRs were synthesized by substituting one of the fluorine atoms in 1,5‐difluoro‐2,4‐dinitrobenzene with six amino acid amides and three amino acids. The diastereomers were separated by reversed‐phase high‐performance liquid chromatography. The method was validated for linearity, accuracy, limit of detection and limit of quantification. The limit of detection was found in the range of 10–30 pmol. Copyright © 2010 John Wiley & Sons, Ltd.     

(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.     

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.     

Reversed‐phase liquid chromatographic determination of enantiomers of atenolol in rat plasma using derivatization with Marfey's reagent

An HPLC method was established for enantioseparation of (R,S)‐atenolol (ATE) and determination of enantiomers in rat plasma. Marfey's reagent (1‐fluoro‐2,4‐dinitrophenyl‐5‐L‐alanine amide, FDNP‐L‐Ala‐NH₂, MR) was used as chiral derivatizing reagent with detection of diastereomers at 340 nm. It was shown that the R‐isomer eluted before the S‐isomer. The method was validated for linearity, repeatability, limits of detection and limit of quantification (LOQ). Recovery of ATE at LOQ was 92.8% for (R)‐ATE and 92.6% for (S)‐ATE. Copyright © 2009 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.     

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.     

Indirect enantioseparation of proteinogenic amino acids using naproxen‐based chiral derivatizing reagent and HPLC

Diastereomers of 18 proteinogenic amino acids were synthesized under microwave irradiation and by vortexing using (S)‐naproxen–benzotriazole as chiral derivatizing reagent. The diastereomers synthesized by two approaches were found to be identical in terms of their characterization and chromatographic data. A linear gradient of triethylammonium phosphate (pH 3.5)–acetonitrile (30–65%, within 35 min) was found to be successful using reversed‐phase high‐performance liquid chromatography for their separation. Detection was carried out at 231 nm and sharp peaks were obtained. The method was validated for accuracy, precision and limit of detection. Copyright © 2012 John Wiley & Sons, Ltd.     

HPLC法拆分(-)-2(S)-苄基-4-酮-4-(顺式-全氢化异吲哚-2-基)丁酸钙对映体

目的：建立刺激胰岛素分泌的新型降糖药物(-)-2 (S)-苄基-4-酮-4-(顺式-全氢化异吲哚-2-基)丁酸钙对映体的HPLC拆分方法。方法：采用Sumichiral OA-3300手性柱(250 × 4.6 mm I.D., 5 μm), 柱温35℃,以0.05 mol·L-1醋酸铵的甲醇溶液为流动相,检测波长为210 nm。结果：本品两对映体在22分钟内实现良好分离,分离度达3以上,S－异构体分别在0.028 ~ 5.6 μg mL-1和0.03 ~ 6.0 μg mL-1范围内线性关系良好,回归方程分别为：Y=1.32×103x-2.54 (r=0.9997)和Y=1.15×103x-1.78 (r=0.9998),最低检测限分别为0.15 ng和0.10 ng,方法精密度RSD低于1.0% (n=5)。结论：建立的对映体分离方法可用于本品光学异构体的质量控制。     

Indirect reversed‐phase high‐performance liquid chromatographic and direct thin‐layer chromatographic enantioresolution of (R,S)‐Cinacalcet

Enantioresolution of the calcimimetic drug (R,S)‐Cinacalcet was achieved using both indirect and direct approaches. Six chiral variants of Marfey's reagent having l ‐Ala‐NH₂, l ‐Phe‐NH₂, l ‐Val‐NH₂, l ‐Leu‐NH₂, l ‐Met‐NH₂ and d ‐Phg‐NH₂ as chiral auxiliaries were used as derivatizing reagents under microwave irradiation. Derivatization conditions were optimized. Reversed‐phase high‐performance liquid chromatography was successful using binary mixtures of aqueous trifluoroacetic acid and acetonitrile for separation of diastereomeric pairs with detection at 340 nm. Thin silica gel layers impregnated with optically pure l ‐histidine and l ‐arginine were used for direct resolution of enantiomers. The limit of detection was found to be 60 pmol in HPLC while in TLC it was found to be in the range of 0.26–0.28 µg for each enantiomers. Copyright © 2010 John Wiley & Sons, Ltd.     

Efficient asymmetric addition of diethylzinc to aldehydes using C2‐novel chiral pyridine β‐amino alcohols as chiral ligands

A series of novel C₂‐symmetric chiral pyridine β‐amino alcohol ligands have been synthesized from 2,6‐pyridine dicarboxaldehyde, m‐phthalaldehyde and chiral β‐amino alcohols through a two‐step reaction. All their structures were characterized by ¹H NMR, ¹³C NMR and IR. Their enantioselective induction behaviors were examined under different conditions such as the structure of the ligands, reaction temperature, solvent, reaction time and catalytic amount. The results show that the corresponding chiral secondary alcohols can be obtained with high yields and moderate to good enantiomeric excess. The best result, up to 89% ee, was obtained when the ligand 3c (2S,2′R)‐2,2′‐((pyridine‐2,6‐diylbis(methylene))bisazanediyl))bis(4‐methyl‐1,1‐diphenylpentan‐1‐ol) was used in toluene at room temperature. The ligand 3g (2S,2′R)‐2,2′‐((1,3‐phenylenebis(methylene))bis(azanediyl))bis(4‐methyl‐1,1‐diphenylpentan‐1‐ol) was prepared in which the pyridine ring was replaced by the benzene ring compared to 3c in order to illustrate the unique role of the N atom in the pyridine ring in the inductive reaction. The results indicate that the coordination of the N atom of the pyridine ring is essential in the asymmetric induction reaction. Copyright © 2014 John Wiley & Sons, Ltd.     

Optical Resolution of 5‐Oxo‐1‐Phenyl‐Pyrazolidine‐3‐Carboxylic Acid as a New Organocatalyst for Organic Reactions

Optical resolution of racemic 5‐oxo‐1‐phenyl‐pyrazolidine‐3‐carboxylic acid 2 with L‐amino acid methyl ester via the diastereomers formation was investigated. Treatment of racemic 5‐oxo‐1‐phenyl‐pyrazolidine‐3‐carboxylic acid 2 with L‐valine methyl ester gave diastereomers with a total yield of 86%. The diastereomeric dipeptides can be easily separated by flash column chromatography. Acidic cleavage of the derived diastereomers gave both the optically pure (+)‐(R)‐ and (‐)‐(S)‐5‐oxo‐1‐phenyl‐pyrazolidine‐3‐carboxylic acid ((+)‐(R)‐ 2 and (‐)‐(S)‐ 2 ) with a total yield of 94% and 95%, respectively.     

Resolution and isolation of enantiomers of (±)‐isoxsuprine using thin silica gel layers impregnated with l‐glutamic acid,comparison of separation of its diastereomers prepared with chiral derivatizing reagents having l‐amino acids as chiral auxiliaries

Thin silica gel layers impregnated with optically pure l ‐glutamic acid were used for direct resolution of enantiomers of (±)‐isoxsuprine in their native form. Three chiral derivatizing reagents, based on DFDNB moiety, were synthesized having l ‐alanine, l ‐valine and S‐benzyl‐l ‐cysteine as chiral auxiliaries. These were used to prepare diastereomers under microwave irradiation and conventional heating. The diastereomers were separated by reversed‐phase high‐performance liquid chromatography on a C₁₈ column with detection at 340 nm using gradient elution with mobile phase containing aqueous trifluoroacetic acid and acetonitrile in different compositions and by thin‐layer chromatography (TLC) on reversed phase (RP) C₁₈ plates. Diastereomers prepared with enantiomerically pure (+)‐isoxsuprine were used as standards for the determination of the elution order of diastereomers of (±)‐isoxsuprine. The elution order in the experimental study of RP‐TLC and RP‐HPLC supported the developed optimized structures of diastereomers based on density functional theory. The limit of detection was 0.1–0.09 µg/mL in TLC while it was in the range of 22–23 pg/mL in HPLC and 11–13 ng/mL in RP‐TLC for each enantiomer. The conditions of derivatization and chromatographic separation were optimized. The method was validated for accuracy, precision, limit of detection and limit of quantification. Copyright © 2014 John Wiley & Sons, Ltd.     

Asymmetric hydration of alkenes catalyzed by wool‐palladium complex

A wool‐palladium complex has been found to be able to catalyze the asymmetric hydration of 1‐octene to (S)‐(+)‐2‐octanol and 1‐decene to (R)‐(+)‐2‐decanol under 1 atm N₂ and at 70°C. The optical yields were greatly affected by Pd content in wool‐palladium complex, reaction time and so on, when the proper conditions were selected, (S)‐(+)‐2‐octanol and (R)‐(+)‐2‐decanol could be obtained in 83.2 and 75.6%e.e. optical yield respectively. This chiral natural biopolymer‐palladium complex catalyst was very easy to prepare and could be reused several times without appreciable change in catalytic activity. Copyright © 2004 John Wiley & Sons, Ltd.     

Development of a high‐performance liquid chromatography method for the simultaneous determination of chiral impurities and assay of (S)‐clopidogrel using a cellulose‐based chiral stationary phase in methanol/water mode

A simple reversed‐phase high‐performance liquid chromatography method for the chiral separation of the active pharmaceutical ingredient (S)‐clopidogrel has been developed on the cellulose‐based Chiralcel OJ‐RH chiral stationary phase. The S enantiomer was baseline resolved from its R impurity (impurity C) with a mobile phase consisting of methanol/water (100:15) without any interference coming from the other two potential chiral impurities A and B. The enantio‐ and chemoselective method was partially validated and compared with that reported in the United States Pharmacopoeia for the drug product. The versatility of the Chiralcel OJ‐RH allowed separating the enantiomers of the impurity B also under normal phase and setting up an efficient strategy to convert the racemic sample into the enantiomeric S form on a semipreparative scale.     

High‐performance liquid chromatographic enantioseparation of isoxazoline‐fused 2‐aminocyclopentanecarboxylic acids on a chiral ligand‐exchange stationary phase

The application of a chiral ligand‐exchange column for the direct high‐performance liquid chromatographic enantioseparation of unusual β‐amino acids with a sodium N‐((R)‐2‐hydroxy‐1‐phenylethyl)‐N‐undecylaminoacetate‐Cu(II) complex as chiral selector is reported. The investigated amino acids were isoxazoline‐fused 2‐aminocyclopentanecarboxylic acid analogs. The chromatographic conditions were varied to achieve optimal separation. The effects of temperature were studied at constant mobile phase compositions in the temperature range 5–45°C, and thermodynamic parameters were calculated from plots of lnk or lnα versus 1/T. Δ(ΔH°) ranged from –2.3 to 2.2 kJ/mol, Δ(ΔS°) from –3.0 to 7.8 J mol^?1 K^?1 and –Δ(ΔG°) from 0.1 to 1.7 kJ/mol, and both enthalpy‐ and entropy‐controlled enantioseparations were observed. The latter was advantageous with regard to the shorter retention and greater selectivity at high temperature. Some mechanistic aspects of the chiral recognition process are discussed with respect to the structures of the analytes. The sequence of elution of the enantiomers was determined in all cases.     

Chiral separation of amides of amino acid on a (S)‐N‐(3,5‐dinitrobenzoyl)leucine‐N‐phenyl‐N‐propylamide‐bonded silica using nonaqueous capillary electrochromatography

(S)‐N‐(3,5‐dinitrobenzoyl)leucine‐N‐phenyl‐N‐propylamine‐bonded silica was used as a chiral stationary phase for separation of a set of racemic π‐acidic and π‐basic α‐amino acid amides in electrolyteless ACN‐water eluents by CEC in the RP and polar organic (PO) modes. The effect of the amount of water in the ACN‐water eluent on chiral separation was examined. As water is added to ACN, retention was shortened but resolution and selectivity deteriorated severely. Retention, enantioselectivity, and resolution factors obtained in 100% ACN were compared with those in an n‐hexane‐isopropanol eluent with a small amount of water by normal phase (NP) CEC. Much shorter retention times with comparable enantioselectivities were observed with 100% ACN, demonstrating the advantage of separation on (S)‐N‐(DNB)leucine‐N‐phenyl‐N‐propylamine‐bonded silica in PO‐CEC over NP‐CEC.     

Enantiomeric separation of d‐ and l‐serine using high‐performance liquid chromatography with electrochemical detection following pre‐column diastereomer derivatization

Enantiomeric separation of d ‐ and l ‐serine on an octadecylsilica column was investigated using (2R)‐2,5‐dioxopyrrolidin‐1‐yl‐2,5,7,8‐tetramethyl‐6‐(tetrahydro‐2H‐pyran‐2‐yloxy)chroman‐2‐carboxylate (R‐NPCA), which was developed for a pre‐column derivatization reagent for electrochemical detection. In addition, (2S)‐2,5‐dioxopyrrolidin‐1‐yl‐2,5,7,8‐tetramethyl‐6‐(tetrahydro‐2H‐pyran‐2‐yloxy)chroman‐2‐carboxylate (S‐NPCA) was newly synthesized from (S)‐(?)‐6‐hydroxy‐2,5,7,8‐tetramethylchroman‐2‐carboxylic acid (S‐α‐CA), and the enantiomeric separation of d ‐ and l ‐serine using S‐NPCA was also examined. The enantiomeric separation of d ,l ‐serine was achieved using the R‐ or S‐NPCA as a chiral derivatization reagent, and the elution orders of the enantiomers were reversed between R‐ and S‐NPCA. The elution orders of d ‐ and l ‐serine unexpectedly reversed between the phosphate buffer at pH 4.0 and pH 2.2, both of which were used in the mobile phase. Separation factors obtained using R‐ and S‐NPCA were similar—1.09 and 1.07, respectively. The detection limit was approximately 940 fmol on the column (signal‐to‐noise ratio 3) when the applied voltage was +650 mV. Copyright © 2009 John Wiley & Sons, Ltd.     

HPLC enantioseparation of β2‐homoamino acids using crown ether‐based chiral stationary phase

RP high‐performance liquid chromatographic methods were developed for the enantioseparation of eleven unusual β²‐homoamino acids. The underivatized analytes were separated on a chiral stationary phase containing (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid as chiral selector. The effects of organic (alcoholic) and acidic modifiers, the mobile phase composition and temperature on the separation were investigated. The structures of the substituents in the α‐position of the analytes substantially influenced the retention and resolution. The elution sequence was determined in some cases: the S enantiomers eluted before the R enantiomers.     

Stereocontrolled anionic alternating copolymerization of ethylphenylketene with benzaldehyde by a bisoxazoline ligand

Anionic copolymerization of ethylphenylketene with benzaldehyde with butyllithium or diethylzinc as the initiator proceeded in a perfect 1:1 alternating manner to produce the corresponding polyester, whose repeating unit had two adjacent chiral centers. The relative stereochemistry between these two chiral centers was successfully controlled by the addition of (S,S)‐(‐)‐2,2′‐isopropylidenebis(4‐tert‐butyl‐2‐oxazoline), producing the corresponding polyester that had excellent diastereoselectivity (erythro‐configuration : threo‐configuration = 4:96). The diastereomeric ratio was determined by high‐performance liquid chromatography analysis of the diol, which was obtained by reductive degradation of the polyester while maintaining the configuration of the repeating unit. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5384–5388, 2004     

A novel approach for enantioseparation as applied to (RS)‐etodolac from pharmaceutical formulations: LC MS and density functional theory support for confirmation of diastereomers so separated

In the present studies formation of diastereomers of (RS)‐etodolac was confirmed using LC‐MS when [M + H]⁺ or [M]⁺ were recorded for the diastereomers. The lowest energy optimized structures of two diastereomers were drawn, which confirmed the three‐dimensional geometry of the diastereomers. This supports the optimized analytical separation conditions. In addition, separation of diastereomers was successful using a 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. The separation method was validated as per International Conference on Harmonization guidelines. (RS)‐Etodolac was isolated from commercial tablets and purified and characterized to be used as racemic standard. Three pairs of diastereomers were synthesized using enantiomerically pure amines, namely, (R)‐(+)‐α‐methyl benzyl amine, (S)‐(?)‐α,4‐dimethylbenzylamine and (R)‐(?)‐1‐cyclohexylethylamine. Derivatization reactions were carried out under conditions of stirring at room temperature (30 °C for 2 h) as well as under microwave irradiation (MWI), and the two types of diastereomers were compared. Reaction conditions for derivatization were optimized with respect to mole ratio of chiral derivatizing agent and (RS)‐etodolac and MWI time. No racemization was observed throughout the study. Copyright © 2015 John Wiley & Sons, Ltd.