Mandelic Acid Methyl Ester Research Articles

Chemoenzymatic Preparation of Enantiomerically Enriched (R)‐(–)‐Mandelic Acid Derivatives: Application in the Synthesis of the Active Agent Pemoline

The enantioselective resolution of several racemic derivatives of mandelic acid methyl ester catalyzed by lipases from Pseudomonas fluorescens (Amano AK) or Burkholderia cepacia (Amano PS‐C II and Amano PS‐IM) has been achieved. A gram‐scale lipase‐mediated kinetic resolution approach has been developed that allows the facile synthesis of the corresponding methyl (R)‐(–)‐mandelates with excellent enantiomeric excesses (up to >99 % ee) and reaction enantioselectivity (E values up to >200). The dopaminergic agent pemoline, used in the treatment of attention‐deficit hyperactivity disorder (ADHD) and narcolepsy, was synthesized with 98 % ee in a straightforward route by condensing the prepared methyl (R)‐(–)‐mandelate with guanidine hydrochloride under basic conditions. The desired (R)‐(+)‐pemoline in optically pure form (>99 % ee) was obtained after two recrystallizations from ethanol. However, it was confirmed by chiral HPLC that optically active pemoline undergoes racemization in methanol solution.

Synthesis, post-modification and characterization of linear polystyrene-based supports for interaction with immobilized biocatalysts

A series of linear polystyrene-based supports has been synthesized using conventional or controlled (reversible addition fragmentation chain transfer or nitroxide-mediated) free radical polymerizations, in order to modulate their molecular weight characteristics. Polymer post-modification via esterification of the primary hydroxyl alcohol functionalities, pendant from the polymer backbone, with a mandelic acid derivative introduces efficiently (>85% yield) racemic, enzyme-cleavable moieties. A study of the interactions of these polymeric substrates with immobilized penicillin G acylase, in mixed organic/aqueous buffer solvent systems, results in a modest enantioselectivity (comparable with that of racemic mandelic acid methyl ester), but reveals that the biocatalytically active moieties on the linear polymers can be cleaved with first-order rate kinetics, as observed with small molecules in solution; the reaction rates (which can differ by more than one order of magnitude) are influenced both by the synthetic polymer length and by the polymer structure resulting from the method of polymerization used. Copyright © 2012 Society of Chemical Industry

Assessment of immobilized PGA orientation via the LC-MS analysis of tryptic digests of the wild type and its 3K-PGA mutant assists in the rational design of a high-performance biocatalyst

The mutant penicillin G acylase (PGA) 3K-PGA contains three additional Lys residues on the surface opposite the active site. This protein was designed to selectively drive its immobilization on aldehyde supports. We describe here a modified bottom-up proteomic method to assess the orientation of the immobilized wild-type and mutant proteins to verify our hypothesis of a driven immobilization induced by the mutations introduced. Tryptic digestion of the immobilized enzymes followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of released peptides was performed. This protocol generated peptides from the most accessible surface areas of the immobilized protein, thus not directly bound to the solid support, providing direct evidence of the areas involved in the linkage to the solid matrix. The results obtained suggest that 72 % of the wild-type PGA is immobilized on aldehyde agarose mainly through the Lys residues on the same side of the active site, whereas 3K-PGA reacted with the same support preferentially through the additional Lys residues introduced by mutation on the opposite side. This demonstrates that the active site of the 3K-PGA faces mostly (63 %) toward the reaction medium, resulting in significantly improved accessibility to the substrates. This finding is supported by the catalytic properties of the immobilized biocatalysts. The two immobilized preparations were tested in the synthesis of mandelyl-7-aminocephalosporanic acid (mandelyl-7-ACA) by N-acylation of the β-lactam nucleus (7-aminocephalosporanic acid) with mandelic acid methyl ester: upon immobilization, the synthetic properties of wild-type PGA strongly decreased, whereas those of 3K-PGA were unaffected. Furthermore, the activity of 3K-PGA was not influenced by the physicochemical nature of the support used for immobilization (glyoxyl agarose or aldehyde Sepabeads) unlike that of wild-type PGA, whose active site is close to the matrix. The results obtained from the analytical characterization correlate well with those obtained by investigation of the synthetic properties of the immobilized enzymes both in the synthesis of mandelyl-7-ACA and in the preparative synthesis of cefazolin. This work highlights the effect exerted by site-directed mutagenesis on the orientation of PGA upon immobilization on solid matrices and suggests how protein engineering tools can be exploited in a synergistic fashion to rationally develop efficient biocatalysts.

Enantioselective enzymatic hydrolysis of racemic drugs by encapsulation in sol–gel magnetic sporopollenin

Candida rugosa lipase was encapsulated within a sol-gel procedure and improved considerably by fluoride-catalyzed hydrolysis of mixtures of octyltriethoxysilane and tetraethoxysilane in the presence of magnetic sporopollenin. The catalytic properties of the immobilized lipases were evaluated into model reactions, i.e., the hydrolysis of p-nitrophenylpalmitate (p-NPP), and the enantioselective hydrolysis of racemic naproxen methyl ester, mandelic acid methyl ester or 2-phenoxypropionic acid methyl ester that were studied in aqueous buffer solution/isooctane reaction system. The encapsulated magnetic sporopollenin (Spo-M-E) was found to give 319 U/g of support with 342% activity yield. It has been observed that the percent activity yields and enantioselectivity of the magnetic sporopollenin encapsulated lipase were higher than that of the encapsulated lipase without support. The substrate specificity of the encapsulated lipase revealed more efficient hydrolysis of the racemic naproxen methyl ester and 2-phenoxypropionic acid methyl ester than racemic mandelic acid methyl ester. It was observed that excellent enantioselectivity (E > 400) was obtained for encapsulated lipase with magnetic sporopollenin with an ee value of S-Naproxen and R-2 phenoxypropionic acid about 98%.

Enhancement of Novozym-435 catalytic properties by physical or chemical modification

Physical (ionic exchange of ionic polymers) or chemical (aminoethylamidation, succinylation, hydroxyethylamidation) modifications of Novozym 435 have been performed and the resulting biocatalysts have been assayed in diverse reactions. The coating of the immobilized enzyme with dextran-sulphate via ionic exchange permitted to increase the asymmetric factor of the biocatalyst from A = 13 (ee = 83%) to 24 (ee > 90%) in the hydrolysis of 3-phenylglutaric acid dimethyl diester, producing the (R)-monomethyl ester. The chemical succinylation of Novozym 435 permitted to enhance the biocatalyst enantiospecificity from E = 1 to 13 in the hydrolysis of (±)-mandelic acid methyl ester. In the hydrolysis of (±)-2- O-butyryl-2-phenylacetic acid, the enantiospecificity of Novozym 435 was very high towards the S-enantiomer ( E > 100) but it was inverted after the chemical hydroxyethylamidation of the immobilized enzyme ( E = 6.6 towards R-enantiomer). Thus, these simple protocols seem to be a very powerful tool to generate a library of biocatalysts from Novozym 435 with very different catalytic properties.

TiCl4-Complexation by Mandelic Acid Methylester

The complexes of trinuclear TiCl3-O-TiCl3/TiCl2(O-(R)-mandelic acid methylester)2 I, C18H18Cl8O7Ti3, and dimeric TiCl3O-rac-mandelic acid methylester II, C18H18Cl6O6Ti2, were determined unambiguously by X-ray diffraction. Both crystal structures contain one solvent molecule toluene per complex. In structure I there are five independent complexes in the asymmetric unit cell. The formation of both the crystal structure as well as the coordination sphere of titanium is controlled sensitively by the chirality of mandelic acid methylesters.

Purification of different lipases from Aspergillus niger by using a highly selective adsorption on hydrophobic supports

In this manuscript, we have purified three different lipases from crude preparations from Aspergillus niger in a simple fashion, secluding the esterases and other enzymes presented in the preparation. Firstly, the crude was offered at low ionic strength to octyl agarose. The support specifically adsorbed two lipases, with molecular weights of 43 and 65 kDa. Desorption with a gradient of Triton X-100 permitted to fully purify both lipases. The addition of octadecyl-Sepabeads support to the non-adsorbed proteins on octyl-agarose permitted to selectively adsorb a third lipase, having a molecular weight of 31 kDa. Desorption of the enzyme using Triton X-100 permitted to have also a pure sample of this enzyme. A significant percentage of esterase activity remains in the supernatant, derived from esterases or lipases unable to become adsorbed on the employed supports. Furthermore, these purified lipases were immobilized via ionic adsorption on DEAE-Sepharose and their selectivity was analyzed in the kinetic resolution of (+/-)-O-2-butyryl-2-phenylacetic acid and (+/-)-mandelic acid methyl ester. In the resolution of (+/-)-O-2-butyryl-2-phenylacetic acid, the crude extract preparation gave a low enantioselectivity value (E = 9), whereas the three immobilized preparations of purified lipases exhibited an increase in E-value from 11 (43 kDa lipase) to > 100 (31 kDa lipase). When (+/-)-mandelic acid methyl ester was used, the crude extract preparation presented low enantioselectivity hydrolyzing the S enantiomer quicker, while the purified lipase preparations preferred the R one. In this case, the 65 kDa lipase was the most selective enzyme (E = 20).

Electron-Impact Ionization of Mandelic Acid and Mandelic Acid Methyl Ester as Prototypes for the C6H5CH(OH)-X System: Ionization and Appearance Energies, Activation Energies and Enthalpies of Formation

Electron-impact ionization mass spectra, the decay of metastable ions, ionization and appearance energies and bond energies, as dissociation energies, are reported for mandelic acid (MA) and mandelic acid methyl ester (MAME) . An ionization energy of 8.55 eV was obtained for both compounds. For the formation of C6H5+ ions, appearance energies of 14.25 eV or 14.40 eV were determined for MA or MAME, respectively. On the main fragmentation route from molecular ions to these C6H5+ ions, an activation energy of (1.1±0.1)eV in excess of the calculated total reaction enthalpy was found for the elimination of CO from the prominent C7H7O+ ions to C6H7+ ions, involving the transfer of two H-atoms. The second (minor) fragmentation route via C7H6O+ions resembles the benzaldehyde decomposition pathway and the individual steps were found to be near to their calculated thermochemical reaction enthalpies. Gas-phase enthalpies of formation of (–4.652±0.3 eV) [≡ (–448.8±30) kJ mol−1] for MA and (–4.243±0.3) eV [≡ (–409.4±30) kJ mol−1] for MAME are derived, based on the appearance energy of their C7H7O+ ions at m/z 107 and the formation enthalpy adopted from protonated benzaldehyde. The results are compared with data for benzylalcohol.

A glycopeptide antibiotic chiral stationary phase for the enantiomer resolution of hydroxy acid derivatives by capillary electrochromatography

Separation of hydroxy acid enantiomers was achieved by using capillary electrochromatography (CEC) employing a chiral stationary phase (CSP) based on MDL 63,246 (Hepta-Tyr), a macrocyclic antibiotic of the teicoplanin family. The chiral selector was chemically bonded to 5 num diol-modified silica particles and the CSP mixed with amino silica (3:1 w/w) was packed into a 75 num ID fused-silica capillary. The CEC experiments were carried out by using an aqueous reversed-phase mode for the enantiomeric resolution of hydroxy acid compounds. Good enantioresolution was achieved for mandelic acid (MA), m-hydroxymandelic acid (m-OH-MA), p-OH-MA, and 3-hydroxy-4-methoxymandelic acid (3-OH-4-MeO-MA). The CEC system was less enantioselective towards 2-phenyllactic acid (2-PhL) and 3-PhL while mandelic acid methyl ester (MA-Et-Est) enantiomers were not resolved. Several experimental parameters, such as organic solvent type and concentration, buffer pH, capillary temperature, on enantioresolution factor, retention time, and retention factor were studied.

The presence of methanol exerts a strong and complex modulation of the synthesis of different antibiotics by immobilized penicillin G acylase

The yields in the synthesis of antibiotics catalyzed by Penicillin G acylase under very mild experimental conditions by using esters of side chains as acylating agents depend on the ratio between synthetase and hydrolase activities of penicillin G acylase. The presence of low concentrations of methanol in the reaction medium exerts complex and interesting changes in the catalytic properties of immobilized penicillin G acylase. When phenylglycine methyl ester is used as acylating agent, the presence of methanol has a double effect. It increased the ratio synthesis/hydrolysis of antibiotic, but decreased the ratio between synthesis of antibiotic and direct hydrolysis of the acyl donor ester. In this way, synthetic yields are increased when using an excess of acylating substrate (e.g., from 63% to 73%) because the key point in this reaction is to decrease the hydrolysis of the already synthesized antibiotic. When using an excess of antibiotic nucleus, the hydrolysis of antibiotic is strongly inhibited and then the ratio synthesis of antibiotic/hydrolysis of the acyl donor is the key point and the yields suffered a decrease (e.g., from 90 to 60%). On the contrary, when using mandelic acid methyl ester as acylating agent, methanol increases both ratios (hydrolysis/synthesis of the antibiotic and synthesis of antibiotic/hydrolysis of the acyl donor); thus, the yields increased by almost a twofold factor (e.g., from 38 to 70%). Finally, the contradictory effects observed in the presence of methanol can be modulated by using different enzyme derivatives. In fact, the effect of methanol on the Penicillin G acylase structure could be cleared out by increasing the stability of the enzyme derivative or by “fixing” the enzyme conformation in the presence of the organic solvent during the immobilization process.

Liquid chromatographic resolution of enantiomers containing single aromatic rings with β-cyclodextrin-bonded phases

Racemic compounds containing two or more ring moieties are thought to be of optimal size for complexation and resolution on β-cyclodextrin (β-CD) bonded-phase liquid chromatographic columns. There are few reports on the separation of racemates containing one or no rings. Here, seventeen single-ring-containing racemates are resolved via β-CD complexation: (±)-1-phenyl- ethyl propionate; (±)-α-methylbenzyl acetate, (±)-2-chloro-2-phenylacetic acid, (±)-ethyl- 3-phenylglycidate, d, l-3,4-dihydroxyphenyl-α-propylacetamide; (±)-2-methoxy-α-methylbenzyl alcohol, d, l-2-phenylpropionaldehyde, d, l-α-amino-3-thiopheneacetic acid, (±)-mandelic acid, (±)-mandelic acid methyl ester, (±)-O-acetylmandelic acid, (±)-ephedrine; d, l-phenyl- alanineamide, d, l-tyrosine, d, l-tyrosine methyl ester, 3-hydroxy- d, l-kynurenine and N-(3,5- dinitrobenzoyl)- d, l-leucine. These compounds can be separated on β-CD media, and have certain common structural properties that enhance chiral recognition. The factors that permit the resolution of single-ring-containing racemates on β-CD columns are discussed.