Resolution Of Racemic Mandelic Acid Research Articles

Biosynthesis of Phenylglyoxylic Acid by LhDMDH, a Novel d-Mandelate Dehydrogenase with High Catalytic Activity.

d-Mandelate dehydrogenase (DMDH) has the potential to convert d-mandelic acid to phenylglyoxylic acid (PGA), which is a key building block in the field of chemical synthesis and is widely used to synthesize pharmaceutical intermediates or food additives. A novel NAD+-dependent d-mandelate dehydrogenase was cloned from Lactobacillus harbinensi (LhDMDH) by genome mining and expressed in Escherichia coli BL21. After being purified to homogeneity, the oxidation activity of LhDMDH toward d-mandelic acid was approximately 1200 U·mg-1, which was close to four times the activity of the probe. Meanwhile, the kcat/ Km value of LhDMDH was 28.80 S-1·mM-1, which was distinctly higher than the probe. By coculturing two E. coli strains expressing LhDMDH and LcLDH, we developed a system for the efficient synthesis of PGA, achieving a 60% theoretical yield and 99% purity without adding coenzyme or cosubstrate. Our data supports the implementation of a promising strategy for the chiral resolution of racemic mandelic acid and the biosynthesis of PGA.

The pH-dependency of Diastereomeric Salt Resolutions with Amphoteric Resolving Agents

We have analysed the role of pH in the diastereomeric resolution of racemic mandelic acid with (S)-phenylalanine and (R)-3-(aminomethyl)-5-methylhexanoic acid [(R)-Pregabalin] enantiomers. The adjusted pH had an effect on the optical purity and yield of the mandelic acid enantiomeric mixtures isolated from the diastereomeric salts. We showed that the pKa of the carboxyl group of the amphoteric resolving agents should be considered during the pH optimisation of diastereomeric salt resolutions.

Effect of alcohol chain length on the enzymatic resolution of racemic mandelic acid and kinetic study

Asymmetric esterification of racemic mandelic acid (MA) was conducted in 1,2-dichloroethane using lipase Novozym 435 as catalyst. Different alcohols were screened for esterification to achieve high conversion and enantioselectivity. The results show that ethanol is the best alcohol substrate among tested alcohols, and long-chain alcohols show lower enantioselectivities. The effect of substrate concentrations on esterification with ethanol was studied, and the result indicates an alcohol inhibition. To examine the mechanism of the alcohol chain length effect, the ping-pong bi-bi mechanism with competitive alcohol inhibition was applied to build a model. By comparing the kinetic parameters of methanol, ethanol, butanol, heptanol, and octanol, it was found that both the Michaelis constant and the inhibition constant of alcohol increase with the increase in alcohol chain length, indicating that the balance between nucleophilicity and the inhibition effect is the possible reason for the highest conversion and enantioselectivity as shown by ethanol.

Highly efficient resolution of mandelic acid using lipase from Pseudomonas stutzeri LC2-8 and a molecular modeling approach to rationalize its enantioselectivity

Mandelic acid, a key precursor of chiral synthons, was successfully acylated in diisopropyl ether. The reaction was catalyzed by the lipase from Pseudomonas stutzeri LC2-8, and vinyl acetate was employed as acyl donor. Under the optimized reaction conditions, a resolution of 180 mM (55 g/L) mandelic acid was achieved. (S)-O-Acetyl mandelic acid was enantioselectivity formed in >99% ee at a yield close to the maximum theoretical value for kinetic resolution (50%). The highly substrate tolerable and enantioselective nature of lipase LC2-8 suggests that it is of great potential for the practical resolution of racemic mandelic acid. Additionally, the high enantiopreference of lipase LC2-8 toward (S)-mandelic acid in acetylation was also rationalized through molecular docking and molecular dynamics simulations.

Rationally re-designed mutation of NAD-independent l-lactate dehydrogenase: high optical resolution of racemic mandelic acid by the engineered Escherichia coli

BackgroundNAD-independent l-lactate dehydrogenase (l-iLDH) from Pseudomonas stutzeri SDM can potentially be used for the kinetic resolution of small aliphatic 2-hydroxycarboxylic acids. However, this enzyme showed rather low activity towards aromatic 2-hydroxycarboxylic acids.ResultsVal-108 of l-iLDH was changed to Ala by rationally site-directed mutagenesis. The l-iLDH mutant exhibited much higher activity than wide-type l-iLDH towards l-mandelate, an aromatic 2-hydroxycarboxylic acid. Using the engineered Escherichia coli expressing the mutant l-iLDH as a biocatalyst, 40 g·L-1 of dl-mandelic acid was converted to 20.1 g·L-1 of d-mandelic acid (enantiomeric purity higher than 99.5%) and 19.3 g·L-1 of benzoylformic acid.ConclusionsA new biocatalyst with high catalytic efficiency toward an unnatural substrate was constructed by rationally re-design mutagenesis. Two building block intermediates (optically pure d-mandelic acid and benzoylformic acid) were efficiently produced by the one-pot biotransformation system.

Chromatographic resolution and isotherm determination of (R,S)‐mandelic acid on Chiralcel‐OD column

Resolution of racemic mandelic acid ((R,S)-MA) and numerical determination of binary competitive isotherm of (R,S)-MA on Chiralcel-OD column have been investigated in this study. The effects of the alcohol modifier and acidic additive in the mobile phase on the retention and enantioseparation of (R,S)-MA were studied at first. The inverse method was then used to determine the competitive isotherm parameters of (R,S)-MA by minimizing the sum of square deviations of the model predictions from the measured elution profiles. The results indicate that the mobile phase with 85% hexane/15% isopropanol/0.3% trifluoroacetic acid mixture gives the best resolution of (R,S)-MA and competitive-modified Langmuir isotherm provides the more accurate sorption mechanism of (R,S)-MA on the cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase.

High Resolution of Racemic Mandelic Acid through a Method of Bubble Fractionation

AbstractA method of bubble fractionation was developed for the resolution of racemic mandelic acid (MA), using 2‐hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD) as chiral collector. The influences of concentration of HP‐β‐CD, reflux equilibriation time, gas flow rate, packing height of column and pH of buffer on resolution performance were investigated, respectively. According to the similar physical behavior of bubble fractionation and chemical reaction processes, the equivalent chemical reaction constant was introduced. The resolution process was preliminarily analyzed by means of kinetics. The results show that the enantiomeric excess of 60.7% can be obtained under the optimal conditions. The process could be regarded as a first order chemical reaction, where the equivalent speed constant was kl=0.00376. This method is helpful for realizing high resolution and linear amplification of device.

TiCl4 promoted menthyl ester chiral auxiliary mediated synthesis of chiral syn-β-amino esters and applications of a representative syn-β-amino ester

β-Amino esters were obtained in up to 78% yield with 72:28–96:4 diastereomeric ratios by the reaction of the chiral titanium enolate of menthyl esters, prepared using the TiCl 4 /Et 3 N reagent system with prochiral imines. A representative syn -β-amino ester derivative has been used for the resolution of racemic mandelic acid to obtain a sample with >99% ee in a single step. A representative syn -β-amino ester was converted to the corresponding N-deprotected amino ester using the Pd–C/HCOOH reagent system, and then to the corresponding β-amino acid using the glacial CH 3 COOH/HCl reagent system, and to the corresponding β-lactam derivative with partial epimerization by the reaction using C 2 H 5 MgBr.

Extractive Resolution of racemic mandelic acid through a bulk liquid membrane containing binary chiral carrier

A method of bulk liquid membrane (BLM) using complex of di(2-ethylhexyl) phosphoric acid (D2EHPA) and O,O'-dibenzoyl-(2R, 3R)-tartaric acid (L-(-)-DBTA) was developed for the extractive resolution of racemic mandelic acid (rac MA). The effects of resolution time, concentration of D2EHPA in the organic phase, concentration of rac MA in the feed phase, stirring speed and temperature on extractive resolution were studied. The experimental results have shown that the greatest separation factor (α) 2.74 and the enantiomeric excess value (ee, %) 46.47% were obtained when the resolution time was about 4 h, the molar concentration ratio of rac MA, D2EHPA and L-(-)-DBTA 0.05:0.2:0.3, the stirring speed 400 r min-1 and the temperature 35 ºC. The developed method is helpful for optimizing the transport of BLM systems and for large-scale production of pure enantiomer.

Mechanism of optical resolutions via diastereoisomeric salt formation. Part 6. Thermoanalytical investigation of the optical resolution of racemic mandelic acid with S-(+)-2-benzylamino-butanol

The optical resolution of racemic mandelic acid (I) by S-2-benzylamino-butanol (II) was performed in water, ethyl acetate, and water saturated ethyl acetate. It was found that the efficiency of the resolution is three times higher in water saturated ethyl acetate than in either water or ethyl acetate. The salt mixtures produced during the resolutions and the pure diastereoisomeric salts were analyzed by TG, DSC and X-ray powder diffraction measurements. The R-(-)-I×S-(+)-II salt has the higher melting point and heat of fusion value which indicates that this is the more stable salt. Though the general assumption is that diastereoisomeric salt pairs of successful optical resolutions form eutectic systems, the R-mandelic acid-S-2-benzylamino-butanol and the S-mandelic acid-S-2-benzylamino-butanol system was found to behave in a different way. Melting did not start at or near the estimated eutectic temperature. The difference can be explained either by miscibility in the solid phase (solid solution) or by a blocked interaction between the crystals of the two solid salts. This unusual behaviour of the salt pair should be responsible for the unusual difference in the efficiency of the resolutions performed in different solvents

Optical resolution of mandelic acid by cinchonine in different solvents

During optical resolution of racemic mandelic acid by cinchonine the S-mandelate as a hydrate, the R-mandelate or the R-mandelate with ethylacetate solvate precipitates depending on the solvent.

Guaran: A Novel Polysaccharide for Racemate Resolution

Abstract The polysaccharide, guaran, as a chromatographic media has been shown to exhibit weak chiral discrimination for the enantiomers in a racemate. On covalent binding of quinine to the polysaccharide, the chiral selectivity of guaran is further enhanced. This paper describes the method for covalent binding of quinine as a chiral selector on guaran back bone. Successful application of this new chiral media has been made for effective resolution of racemic mandelic acid and cotton seed pigment, gossypol. Probable mechanism for the chiral selectivity of underivatized and derivatized guaran, has been suggested.

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

AbstractCrosslinked 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.