Optical resolution of mandelic acid derivatives by column chromatography on crosslinked cyclodextrin gels

Crosslinked α- and β-cyclodextrin gels (α-CD-E and β-CD-E) were used for the chromatographic resolution of racemic mandelic acid and its derivatives. β-CD-E bound L -(+)-isomers preferentially over D -(?)-isomers and resolved DL -methyl mandelate to give a D -(?)-isomer of 100% optical purity in the first fraction. Mandelic acid, ethyl mandelate, and O-methylated mandelic acid yielded resolutions of 65–83% in initial fractions. α-CD-E, on the contrary, bound D -(?)-isomers more strongly than L -(+)-isomers, resolving DL -methyl mandelate to a smaller extent than β-CD-E. Binding of DL -mandelic acid and DL -methyl mandelate on β-CD-E was studied quantitatively by the equilibrium method. β-CD-E has a similar binding capacity to starch with 1:1 stoichiometry but bound much more strongly than starch. β-CD-E has the same mode of selectivity as starch for the asymmetric binding of the mandelic acid derivatives, but α-CD-E has a reverse selectivity to β-CD-E and starch.     

The effect of pH on the HPLC separation of some diastereomeric tripeptides

Summary In the course of the estimation of the impurity profile of the immunostimulant peptide RGH-0205 (Arg-Lys-Asp) its main impurity was found to be Arg-Lys-DAsp. In order to optimize its separation and quantitative determination and to check the possibility of the presence of other diastereomeric impurities a systematic study was carried out using various HPLC systems. It was found that the most important factor determining the resolution of the diastereomers is the pH of the solution. The presence of an anionic ionpairing reagent is also essential. While the separation at neutral and slightly acidic pH values generally used for the reversed-phase separation of diastereomeric peptides was very poor, acceptable separation of all the four possible diastereomers (L-Arg-L-Lys-L-Asp, L-Arg-L-Lys-D-Asp, L-Arg-D-Lys-L-Asp and D-Arg-L-Lys-L-Asp) has been achieved at the unusually high pH of 9–9.5.     

A new hydrogen-bonding motif for chiral recognition in the diastereomeric salts of racemic 1-phenylethylamine derivatives with enantiopure O-ethyl phenylphosphonothioic acid

An enantiopure phosphonothioic acid showed a unique and superior chiral recognition ability, arising from its P-stereogenicity, for racemic 1-phenylethylamine derivatives through diastereomeric crystallization. Spherical molecular clusters, associated by hydrogen bonds and CH/pi interactions, aggregated with high symmetry in the less-soluble diastereomeric salts.     

Possible factors influencing the separation mechanism of diastereomeric amino acid derivatives obtained from s-triazine type reagents

Diastereomeric derivatives prepared from an amino acid and an amino amide using trichloro s-triazine as a coupling platform are known to produce good chromatographic diastereoselectivity for many amino acid analytes. The chromatographic diastereoselectivity of these derivatives is difficult to rationalize based on the possibility of four possible conformational isomers, which can interconvert by rotation about the C-N bonds between the amino substituents and the triazine ring. The observed diastereoselectivity implicates an unobvious but significant driving force which causes one of several conformations to be favored over the others. Several possibilities are discussed. Intramolecular hydrogen bonding between acid and amide substituents was explored using computer aided molecular modeling. While such hydrogen bonding may be geometrically possible between the amino acid and the amide substituents, it does not explain why derivatives produced from other chiral compounds which are not capable of the same hydrogen bonding interaction nevertheless exhibit substantial diastereoselectivity. Two other more general effects, steric hindrance to solvation and ion pairing, are therefore suggested as possible contributing factors to the chromatographic diastereoselectivity. Based on the conformational equilibrium behavior of related triazine compounds as reported in the literature, either one of these effects could influence the conformation of the diastereomeric derivatives even in the absence of intramolecular hydrogen bonding interactions between the two chiral substituents, and these effects may therefore be a contributing factor for the observed elution order of the diastereomers.     

An excellent new resolving agent for the diastereomeric resolution of rac-mandelic acid

Chiral mandelic acid (S)-1, which is an important precursor for stereoselective transformations and a versatile intermediate for pharmaceuticals, was resolved with the Pope and Peachey method. Enantiopure 1-amino-3-phenoxypropan-2-ol (S)-2, a key intermediate for pharmaceuticals, was used to resolve rac-mandelic acid rac-1 successfully for the first time. The less soluble salt (S)-1·(S)-2·H₂O could be obtained in 77% yield and 98% de (E 75%) using (S)-2 and LiOH in water. The crystal structure of the less soluble salt (S)-1·(S)-2·H₂O showed that the water molecule played a key role in forming the crystals.     

The Dutch Resolution variant of the classical resolution of racemates by formation of diastereomeric salts: family behaviour in nucleation inhibition

The resolution of racemates through their diastereomeric salts can be positively affected by the addition of small amounts of suitable nucleation inhibitors. This discovery is a logical extension of "Dutch Resolution", in which equimolar amounts of resolving agents that are members of the same family (i.e., structurally related) are used. We conducted a systematic search for nucleation inhibitors of the resolving agent 1-phenylethylamine. A wide range of amines that bear possible family resemblances to 1-phenylethylamine was investigated. It was found that (R)-1-phenylbutylamine is a good inhibitor of (R)-1-phenylethylamine. Results of turbidity measurements showed that, for the model case of mandelic acid resolution, the chief effect of this inhibitor was to widen the metastable zone for the more soluble diastereomer. This observation is in accordance with previous experience. Further scouting for possible family members revealed a wide variation in the effectiveness of inhibitors, dependent on their structure. By far the most effective inhibitors are bifunctional 1-phenylethylamine and/or 1-phenylbutylamine analogues. The effect of racemic inhibitors was found to approach that of enantiomerically pure inhibitors of the same absolute configuration of the 1-phenylethylamine used for resolution. The most effective inhibitors were tested for the resolution of a structural variety of racemates, and were shown to be broadly applicable.     

The effect of photomask resolution on separation efficiency on microfabricated devices

Separation quality on glass microfluidic devices fabricated from photomasks of different optical resolutions was compared by measuring the dispersion (apparent diffusion) coefficients of a set of standard compounds separated on these devices. Currently, the channel manifolds of most microfluidic devices are patterned using chrome photomasks. A much cheaper, more robust alternative to chrome photomasks are laser photoplotted masks. The primary disadvantage to using laser photoplots is that the optical resolution of these masks is not as high as that of chrome masks, and this feature increases the side-wall roughness of etched channel manifolds patterned using such masks. The increased wall roughness may affect the fluid flow within the channels and, therefore, the separation quality. To determine the effect of increased sidewall channel roughness, microchip channel manifolds were patterned in soda lime glass using a chrome photomask and laser photoplots printed at resolutions of 620, 1240, 3100 and 6200 dots per centimetre (dpc). Separations were performed on these devices using dilute solutions of fluorescently labeled amino acids. The peak variances of the amino acids were calculated at increasing distances down the separation channel and plotted as a function of migration time. From this plot, dispersion coefficients of the analytes were measured. This allowed for a reliable, relatively easy, direct separation analysis among microchips fabricated from the various photomasks. After multiple separations using microchips fabricated from each resolution mask, we found that the change in sidewall surface roughness did not significantly affect the dispersion coefficients measured, and thus the separation quality. The lower mask resolution limit, rather, was governed by the fidelity to which the mask could capture the original CAD design.     

Racemate resolution via diastereomeric helicates in hydrogen-bonded co-crystals: the case of BINOL–diamine complexes

The structures of a series of co-crystals formed by enantiomers of BINOL (2,2′-dihydroxy-1,1′-binaphthalene) with (R,R)-1,2-diaminocyclohexane (DACH) and (R,R)-1,2-diamino-1,2-diphenylethane have been determined by X-ray diffraction to investigate the factors responsible for diastereoselective co-crystallization, the process responsible for the highly effective resolution of rac-BINOL by DACH. The absolute configurations of the molecules and directional preferences of the hydrogen bonds in the supramolecular aggregates have been analyzed. The supramolecular aggregation of hydrogen-bonded BINOL and diamines shows considerable distortions from the helical shape, which indicates that cohesion forces other than hydrogen bonds play a significant role in the diastereoselective resolution.     

Modeling the retention mechanism for high‐performance liquid chromatography with a chiral ligand mobile phase and enantioseparation of mandelic acid derivatives

The chromatographic retention mechanism describing relationship between retention factor and concentration of Cu²⁺(l ‐phenylalanine)₂ using chiral ligand mobile phase was investigated and eight mandelic acid derivatives were enantioseparated by chiral ligand exchange chromatography. The relationship between retention factor and concentration of the Cu²⁺(l ‐phenylalanine)₂ complex was proven to be in conformity with chromatographic retention mechanism in which chiral discrimination occurred both in mobile and stationary phase. Different copper(II) salts, chiral ligands, organic modifier, pH of aqueous phase, and conventional temperature on retention behavior were optimized. Eight racemates were successfully enantioseparated on a common reversed‐phase column with an optimized mobile phase composed of 6 mmol/L of l ‐phenylalanine or N,N‐dimethyl‐l ‐phenylalanine and 3 mmol/Lof copper(II) acetate or copper(II) sulfate aqueous solution and methanol.     

Probability of spontaneously resolvable conglomerates for racemic acid/racemic amine salts predicted on the basis of the results of diastereomeric resolutions

The salts of racemic acids with racemic amines, of which the enantiopure component is a good resolving agent for the corresponding counterpart in diastereomeric resolution, have an extremely high tendency to be conglomerates: the results of diastereomeric resolutions are highly informative for the prediction of conglomerates, which can be enantioseparated by preferential crystallization developed independently from diastereomeric resolution over one and a half centuries.     

ESR study of conductors made up of entirely organic materials: radical cation salts of tetrathiafulvalene derivatives with the monoanion of isocyanuric acid

The radical cation salts of tetrathiafulvalene derivatives with the cyanuric acid monoanion were studied by the ESR method. The ESR data are consistent with the earlier obtained structural characteristics, electric conductivity, and optical properties of the studied salts.     

Interaction of cinnamic acid derivatives with serum albumins: a fluorescence spectroscopic study

Cinnamic acid (CA) derivatives are known to possess broad therapeutic applications including anti-tumor activity. The present study was designed to determine the underlying mechanism and thermodynamic parameters for the binding of two CA based intramolecular charge transfer (ICT) fluorescent probes, namely, 4-(dimethylamino) cinnamic acid (DMACA) and trans-ethyl p-(dimethylamino) cinnamate (EDAC), with albumins by fluorescence spectroscopy. Stern-Volmer analysis of the tryptophan fluorescence quenching data in presence of the added ligand reveals fluorescence quenching constant (κ(q)), Stern-Volmer constant (K(SV)) and also the ligand-protein association constant (K(a)). The thermodynamic parameters like enthalpy (ΔH) and entropy (ΔS) change corresponding to the ligand binding process were also estimated. The results show that the ligands bind into the sub-domain IIA of the proteins in 1:1 stoichiometry with an apparent binding constant value in the range of 10(4) dm(3) mol(-1). In both the cases, the spontaneous ligand binding to the proteins occur through entropy driven mechanism, although the interaction of DMACA is relatively stronger in comparison with EDAC. The temperature dependence of the binding constant indicates the induced change in protein secondary structure.     

GC resolution of various D,L-amino acid derivatives on a chirasil-pval capillary column

An enantiomeric mixture of amino acids is derivatized to give the respective N(O)-perfluoroacyl alkyl esters which can be resolved directly by gas chromatography on a Chirasil-Val capillary column. The retention times, resolution numbers, and elution order are reported.     

Tautomerism of peroxyacetic acid derivatives: a quantum chemical study

The electronic and molecular structures and the relative stabilities of organic peracids X=C(R)-COOH and their cyclic tautomers, dioxiranes

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, with R = Me, CF₃; X = O, NH, were studied using the ab initio Hartree-Fock method and the density functional theory (B3LYP version) as well as at the MP2-MP4 Møller-Plesset levels of perturbation theory. Geometry optimization was performed by the UHF and B3LYP methods with the 6-31G** basis set and at the MP2/cc-pvtz level of theory. The acyclic form of the peracid is more stable than the cyclic dioxirane form irrespective of the nature of the substituent. The energy difference between these tautomers increases as the CF₃ and NH groups are replaced by Me and O, respectively. Parameters of the activation barrier to tautomeric conversion increase in parallel with enhancement of the electron-accepting properties of both substituents. The transition state of tautomeric interconversion, which is topologically similar to the acyclic structure of the carbonyl oxide derivative R(HX)C=O⁺-O^?, was found and characterized taking peroxyacetic acid as an example. The characteristic features of the transition state are an intramolecular “multicenter” H-bond and the charge distribution that is inconsistent with the canonical structure mentioned above. An appropriate reaction coordinate for the transformation of the quasi-tetrahedral dioxirane structure into a planar peroxyacetic acid structure is provided by the dihedral angle. Deprotonated anionic systems are characterized by much smaller differences between the relative stabilities of the open and closed forms of isomers and by much lower activation barriers to isomeric conversions.

     

Fast separation of amino acid phenylthiohydantion derivatives by HPLC on a non-porous stationary phase

Summary A new method is described for the separation of phenylthiohydantoin (PTH) derivatives of the 20 common amino acids. The analysis requires approximately 7 minutes and good resolution is obtained by RP-HPLC on columns packed with a non-porous stationary phase (Kovasil-C14; 33×4.6 mm). Gradient elution was chosen with eluents containing either sodium acetate/ acetic acid buffers (moderately acidic conditions) or a heptafluorobutyric acid modifier (strongly acidic eluent). A slightly different elution order of the PTH-amino acids was found in the two systems. Low detection limits (in the femtomol range) were achieved with simple commercial HPLC equipment. Presented at the Balaton Symposium on High Peformance Separtion Methods, Siófok, Hungary, september 1–3, 1999     

Isopropylidene glycerol hydrogen phthalate as a resolving agent: a study of its diastereomeric salts with (S)- and (R)-1-phenylethylamine

An insight into the mechanism of the highly efficient resolution of 1-phenylethylamine with enantiomerically pure isopropylideneglycerol hydrogen phthalate is provided by comparison of physicochemical and X-ray crystallographic data of the two diastereomeric salts formed by the amine with the S acid. In the nearly identical structures of these salts, the different disposition of the isopropylidene glycerol moiety stands out drawing attention to the critical role played by the chiral part of the acid in the discrimination between the amine enantiomers.