Search

An efficient and concise approach for recognizing chirality and determining the absolute configuration of the diastereomers of ( RS )-Betaxolol, [( RS )-Bet] is described, which in turn is a measure of the enantiomeric purity of Bet since the diastereomers were separated and isolated using preparative TLC. A reverse phase HPLC method was developed and validated for the enantioseparation of ( RS )-Betaxolol. Diastereomers were synthesized, using a naproxen based chiral derivatizing reagent, under microwave irradiation. The absolute configuration of both diastereomers was established with the help of 1 H NMR spectroscopy and by developing the lowest energy optimized structures of the diastereomers based on density functional theory. The configuration of the diastereomers was established as [( S )-Nap-( R )-Bet] and [( S )-Nap-( S )-Bet]. The determination of the absolute configuration of the two diastereomers lent support to elution order and separation mechanism. The limits of quantitation were found to be 0.80 ng mL −1 for the diastereomers. [ABSTRACT FROM AUTHOR]

Five d -amino acids have been used for the first time to synthesize chiral derivatizing reagents (as variants of Marfey's reagent) by nucleophilic displacement of one of the fluorine atoms in 1,5-difluoro-2,4-dinitrobenzene as against the literature reports on application of only l -amino acids or their amides as chiral auxiliaries in dinitrobenzene (DNB) moiety. Five other DNB based reagents were also prepared by nucleophilic substitution of fluorine atom with the set of the same amino acids in l -configuration, as chiral auxiliaries. These reagents were characterized and used for synthesis of diastereomers of ( RS )-Mexiletine spiked in human plasma. Diastereomers were synthesized employing microwave irradiation and were separated on reversed-phase C 18 column. Performance of the two types of chiral derivatizing reagents was compared. The reagents containing d -amino acids provided enhanced separation of diastereomers than those containing l -amino acids. The best resolution was obtained using mobile phase consisting of acetonitrile and 0.1% trifluoroacetic acid in gradient mode. The detection was carried out at 340 nm. The method so developed was validated for linearity, accuracy and precision. The limit of quantitation was found to be approximately 25.2 ng mL −1 in human plasma. [ABSTRACT FROM AUTHOR]

Enantioseparation of (RS)-Bupropion was achieved as its diastereomers which were prepared with chiral derivatizing reagents (CDRs) synthesized from cyanuric chloride. Five new CDRs were prepared by nucleophilic substitution of Cl atom in 6‐butoxy derivative of cyanuric chloride with amino acids (namely, L‐Leu, L‐Val, L‐Phe, D‐Phg and L‐Ala) as chiral auxiliaries to give monochloro-s-triazine reagents and other five dichloro-s-triazine reagents were prepared by monosubstitution with the above-mentioned amino acids. The diastereomers were synthesized under microwave irradiation and by conventional method. Separation efficiencies of the two sets of CDRs, among themselves and among the two groups, have been compared on the basis of resolution (RS) and difference between the retention times of resolved diastereomers. The method was validated for detection limit, linearity, accuracy and precision. [ABSTRACT FROM AUTHOR]

Six dichloro- s-triazine (DCT) reagents having l-Leu, d-Phg, l-Val, l-Met, l-Ala and l-Met-NH as chiral auxiliaries in cyanuric chloride were introduced for enantioseparation of 13 proteinogenic amino acids. Four other DCTs and six monochloro- s-triazine (MCT) reagents having amino acid amides as chiral auxiliaries were also synthesized. These 16 chiral derivatizing reagents (CDRs) were used for synthesis of diastereomers of all the 13 analytes using microwave irradiation, which were resolved by reversed-phase high-performance liquid chromatography (RP-HPLC) using C18 column and gradient eluting mixture of aqueous TFA and acetonitrile with UV detection at 230 nm. It required only 60-90 s for derivatization using microwave irradiation. Better resolution and lower retention times were observed for the diastereomers prepared with CDRs having amino acids as chiral auxiliaries as compared to counterparts prepared with reagents having amino acid amides as chiral auxiliaries. As the best resolution of all the 13 analytes was observed for their diastereomers prepared using the DCT reagent having l-Leu as chiral auxiliary, this CDR was further employed for derivatization of Lys, Tyr, His and Arg followed by RP-HPLC analysis of resulting diastereomers. The results are discussed in light of acid and amide groups of chiral auxiliaries constituting CDRs, electronegativities of the atoms of achiral moieties constituting CDRs and hydrophobicities of side chains of amino acids constituting CDRs and analytes. [ABSTRACT FROM AUTHOR]

l-Ala-NH, l-Val-NH, l-Leu-NH, and d-Phg-NH were used as chiral auxiliaries to synthesize four chiral derivatizing reagents (CDRs) of each of the three categories, viz., difluoro dinitro benzene (DFDNB) based chiral variants, and cyanuric chloride (CC) based monochloro-s-triazine reagents (MCTs) and dichloro-s-triazine reagents (DCTs). DFDNB based chiral variants were synthesized by substituting one of the fluorine atoms of DFDNB with respective amino acid amides. The MCTs and DCTs were synthesized by substituting chlorine atom with aforesaid amino acid amide moieties in 6-methoxy dichloro-s-triazine and in CC, respectively. In total, 12 CDRs were characterized and used for microwave-assisted synthesis (45 s at 80% of 800 W using DFDNB-based chiral variants, 80 s at 90% of 800 W power using MCTs, and 50 s at 80% of 800 W power using DCTs) of diastereomers of (A) SeMet, and (B) mixture of (1) SeMet and Met, and (2) SeMet, Met, and Cys. The diastereomers were enantioseparated by reversed-phase high-performance liquid chromatography using gradient elution with mobile phases containing aq. TFA (0.1%)-MeCN in different compositions. The method was validated for accuracy, precision, and limit of detection. [ABSTRACT FROM AUTHOR]

Two new chiral monochloro- s-triazines (MCT) were synthesized [viz N-(4-chloro-6-piperidinyl-[1,3,5]-triazine-2-yl)- l-leucine amide and N-(4-chloro-6-piperidinyl-[1,3,5]-triazine-2-yl)- l-leucine) (CDR 1 and 2, respectively)] by the nucleophilic displacement of chlorine atoms in s-triazine moiety. One of the Cl atoms was replaced with piperidine, and the second Cl atom in the 6-piperidinyl derivative was replaced with amino acid amide (viz l-Leu-NH) and amino acid ( l-Leu). These reagents were characterized and used as CDRs for chiral separation of protein and non-protein amino acids, and were separated on a reversed-phase C column. The reaction conditions were optimized for the synthesis of diastereomers using one MCT reagent. The separation method was validated for limit of detection, linearity, accuracy, precision, and recovery. [ABSTRACT FROM AUTHOR]

Abstract: Four dichloro-s-triazine (DCT) and five monochloro-s-triazine (MCT) chiral derivatizing reagents (CDRs) were synthesized by incorporating amino acid amide moieties as chiral auxiliaries in trichloro-s-triazine and its 6-methoxy derivative, respectively. Another MCT reagent was synthesized by substitution of two chlorine atoms with two different amino acid amides in trichloro-s-triazine. These reagents were used for synthesis of diastereomers of (R,S)-baclofen under microwave irradiation (i.e. 60 s at 85% power using DCT reagents and 90 s at 85% power using MCT reagents). The diastereomers were separated on a reversed-phase C18 column using mixtures of methanol with aqueous trifluoroacetic acid (TFA) with UV detection at 230 nm. The separation behavior in terms of retention times and resolutions obtained for the two sets of diastereomers prepared with DCT and MCT reagents were compared among themselves and among the two groups. Longer retention times and better resolutions were observed with DCT reagents as compared to MCT reagents. The calibration curves were linear for both (R)- and (S)-baclofen in the concentration range 50-500 μg/ml. The average regression was 0.999 for both (R)- and (S)-baclofen. The RSD for (R)-baclofen was 0.40-0.86% for intra-day precision and 0.60-1.40% for inter-day precision and these values for (S)-baclofen were 0.52-0.75% and 0.64-1.32%, respectively. The recovery was 97.2-98.9% for (R)- and 97.0-98.9% for (S)-baclofen. The limit of detection was 1.63ng/ml and 1.52ng/ml for (R)- and (S)-baclofen, respectively. [Copyright &y& Elsevier]

Abstract: Four chiral derivatizing reagents (CDRs) having enantiomerically pure amines and two CDRs namely, [N-succinimidyl-(S)-2-(6-methoxynaphth-2-yl)propionate], and [dinitrophenyl-l-Pro-N-hydroxysuccinimide ester, DNP-l-Pro-SU] were synthesized and were used to prepare diastereomers of (R,S)-mexiletine (MEX); these were separated by reversed-phase high-performance liquid chromatography (RP-HPLC). The method was validated for linearity, accuracy, limit of detection (LOD) and limit of quantification (LOQ). [Copyright &y& Elsevier]

Abstract: Ten chiral derivatizing reagents (CDRs) were synthesized by replacing the l-Ala–NH2 moiety in Marfey''s reagent (MR) by seven l-amino acid amides and three l-amino acids employing microwave irradiation (MW) and were characterized. Ten racemic amino alcohols were derivatized with these CDRs under MW. The diastereomers were separated on a reversed-phase C18 column using binary mixtures of acetonitrile with aqueous trifluoroacetic acid (TFA) and triethylammonium phosphate buffer (TEAP). In general, amino acid variants of MR provided better separation of diastereomers in comparison to amino acid amide variants. The method was also found successful for the separation of 20 diastereomers from a mixture. [Copyright &y& Elsevier]

Abstract: Ten chiral dichloro- and monochloro-s-triazines were prepared by the nucleophilic displacement of chlorine atom(s) in s-triazine chloride and its 6-methoxy derivative with different amino acid amides. Dichloro-s-triazines (DCTs) were used as CDRs for derivatization of α-amino acids under basic conditions at room temperature (30°C) while derivatization with monochloro-s-traizine (MCT) reagents was carried out at 80°C. The resultant diastereomers were separated on a reversed-phase C18 column using mixtures of acetonitrile and aqueous-trifluoroacetic acid (TFA). The separation results for the two were compared. One DCT reagent was optimized for derivatization kinetics with respect to the effects of pH, reagent excess, temperature and reaction time on derivatization yield. In most of the cases, DCT reagents provided better separation of diastereomers in comparison to MCT reagents. One DCT reagent was also validated for limit of detection, linearity, recovery and robustness. Effects of structural modifications in reagents on chromatographic properties were investigated. Separation mechanism of diastereomers was proposed in light of both MCT and DCT reagents. [Copyright &y& Elsevier]

Direct chromatographic enantioseparations are among the most important practical tasks of chromatography. The accepted rules and concepts of stereochemistry anticipated one type of chromatographic systems applicable only to such enantioseparations consisting either of chiral stationary phase, or chiral selector present in mobile phase. In such a model of chromatographic system, both racemic and non-racemic mixtures could be enantioseparated. Over the years, however, reports on successful chiral enantioseparations in non-chiral chromatographic systems have cumulated as well. To adapt the initial model to novel experimental evidence, an assumption had to be added stating that these were only non-racemic mixtures (and not racemic ones), which could be enantioseparated in achiral chromatographic systems, by granting them an inevitable chirality from the enantiomeric excess of a more abundant enantiomer. There still exists an overlooked portion of experimental evidence for successful enantioseparations of racemic mixtures in the non-chiral chromatographic systems, which cannot be explained by the accepted model. Facing this incompatibility between the model and practical results, we reflect on how to reconcile these two by questioning the possibility of pinpointing a true racemate with the help of our imperfect analytical tools. [ABSTRACT FROM AUTHOR]