Racemic Carboxylic Acids Research Articles

Non-enzymatic dynamic kinetic resolution of racemic α-arylalkanoic acids: an advanced asymmetric synthesis of chiral nonsteroidal anti-inflammatory drugs (NSAIDs)

An efficient protocol was developed to produce chiral 2-arylalkanoic esters in high yields (up to 99%) from racemic carboxylic acids utilizing the racemization of the mixed-anhydrides generated from acid components with pivalic anhydride in the presence of an acyl-transfer catalyst. The present DKR involves the enantio-discriminating esterification of the racemic 2-arylalkanoic acids and the rapid racemization of the chiral 2-arylalkanoic acids under suitable reaction conditions using pivalic anhydride, diisopropylethylamine, and benzotetramisole (BTM) in a polar solvent, and this method was successfully applied for the preparation of pharmacodynamically active (S)-enantiomers of nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and naproxen.

Carboxylic acid functionalized ortho-linked oxacalix[2]benzene[2]pyrazine: synthesis, structure, hydrogen bond and metal directed self-assembly

Cyclooligomerization of 2,6-dichloropyrazine 4 and benzyl 2,3-dihydroxybenzoate 5 under microwave irradiation resulted in a racemic pair of ester functionalized ortho-linked oxacalix[2]benzene[2]pyrazine 6, which was further transformed to the corresponding racemic carboxylic acid functionalized ortho-linked oxacalix[2]benzene[2]pyrazine 3. Both enantiomers of 3 adopt 1,3-alternate conformations with their two carboxylic acid groups pointing to opposite directions in the solid state. Enantiomers of 3 form a step-like one-dimensional supramolecular polymer via intermolecular hydrogen bond interactions between the carboxylic acids for crystals obtained in methanol. No hydrogen bonds were formed between the carboxylic acids for crystals of 3 obtained in pyridine and aqueous guanidine solutions; instead, intermolecular hydrogen bonds between the carboxylic acid groups of 3 and pyridine, as well as guanidinium ions were formed. Under metal-mediated self-assembly conditions, the pyrazinyl nitrogen atoms in 3 interacted with transition metal ions, such as Ag(I), Cu(II) and Zn(II), and resulted in the formation of four new metal-containing supramolecular complexes. Metallomacrocycles 7, 8 and 9 were formed by reactions of 3 with Ag(I) or Cu(II) ions by bridging two ligands 3 in the equatorial region via M-N coordination bonds. A one-dimensional coordination polymer 10 was generated by reaction between ligand 3 and Zn(II) ions, and a cage-based structure is presented in 10 by bridging of the cyclophane units by Zn(2+) ions via Zn-N and Zn-O bonds.

Resolution of Carboxylic Acids Using Copper(I)-Promoted Removal of Propargylic Esters under Neutral Conditions

A method for the optical resolution of carboxylic acids is described. Condensation of racemic carboxylic acids with chiral terminal propargyl alcohols gave separable diastereomeric esters. Chromatographic separation followed by heating the individual diastereomers in methanol with catalytic copper(I) halide regenerated the carboxylic acids in good yields and in enantiomeric ratios of ≥94%. This method is particularly useful for the resolution of carboxylic acids that are incompatible with conventional ester hydrolysis.

Amino alcohol based chiral solvating agents: synthesis and applications in the NMR enantiodiscrimination of carboxylic acids

Four optically active amino alcohols were synthesized via the ring opening of ( R)- N-(2,3-epoxypropyl)phthalimide with ( R)-2-phenyl glycinol, (1 R,2 S)- cis-1-amino-2-indanol, ( R)-2-amino-1-butanol and ( S)-phenyl ethylamine in 73–93% yields. The enantioselective recognition of these receptors towards the enantiomers of racemic carboxylic acids was studied by 1H NMR spectroscopy. The molar ratio and the association constants of the chiral compounds with each of the enantiomers of the guests were determined by using Job plots and a non-linear least-squares fitting method, respectively. Large non-equivalent chemical shifts (up to 30.0 Hz) can be achieved in the presence of chiral amino alcohols 2 and 5. Amongst the chiral receptors used, compound 5 was found to be the best chiral shift reagent, and was effective in the determination of the enantiomeric excess of chiral carboxylic acids.

Highly Efficient Kinetic Resolution of Racemic Carboxylic Acids

Shiina and co-workers report a high­y efficient kinetic resolution of racemic α-aryl-alkanoic acids using acyl transfer Birman catalyst 1 (V. B. Birman, X. Li Org. Lett. 2006, 8, 1351) to afford optically enriched esters utilizing achiral ­alcohols (s-factor: up to 484). The obtained bis-(α-naphthyl)-methyl esters could be easily transformed into the corresponding chiral acids, such as ibuprofen and naproxen, without loss of optical purity.

ChemInform Abstract: Highly Diastereoselective Inter-Esterification Reactions Involving a Racemic Acetate and a Racemic Carboxylic Acid Catalyzed by Lipase Enzymes.

ChemInform Abstract: Highly Diastereoselective Inter-Esterification Reactions Involving a Racemic Acetate and a Racemic Carboxylic Acid Catalyzed by Lipase Enzymes.

Kinetic Resolution of Racemic α-Arylalkanoic Acids with Achiral Alcohols via the Asymmetric Esterification Using Carboxylic Anhydrides and Acyl-Transfer Catalysts

A variety of optically active carboxylic esters are produced by the kinetic resolution of racemic alpha-substituted carboxylic acids using achiral alcohols, aromatic or aliphatic carboxylic anhydrides, and chiral acyl-transfer catalysts. The combination of 4-methoxybenzoic anhydride (PMBA) or pivalic anhydride with the modified benzotetramisole-type catalyst ((S)-beta-Np-BTM) is the most effective for promotion of the enantioselective coupling reaction between racemic carboxylic acids and a novel nucleophile, bis(alpha-naphthyl)methanol, to give the corresponding esters with high ee's. This protocol was successfully applied to the production of nonracemic nonsteroidal anti-inflammatory drugs from racemic compounds utilizing the transacylation process to generate the mixed anhydrides from the acid components with the suitable carboxylic anhydrides.

Enantiomeric excess detection with (S)-3-Phenyl-2-(selenophenyl)propan-1-ol derivatizing agent via mix and shake 77Se NMR

In this article, we demonstrate that (S)-3-phenyl-2-(selenophenyl)propan-1-ol can be successfully employed as chiral derivatizing agent for enantiomeric excess determination of chiral carboxylic acid substrates via 77Se NMR. The required derivatives are obtained by mixing the chiral derivatizing agent with the non racemic carboxylic acid substrate directly in the NMR tube and determining the enantiomeric excess with just one spectrum. The best results were obtained with (S)-3-phenyl-2- (selenophenyl)propan-1-ol, which showed good anysochronies of the selenium signals, ranging from Δδ = 22 Hz to Δδ = 211 Hz, even with the stereocenter and the selenium atom five bonds apart.

Enantiopure tert-butyl(phenyl)phosphine oxide. Chirality-recognition ability and mechanism

When enantiopure tert-butyl(phenyl)phosphine oxide 1 was used as a resolving agent, it showed an acceptable to good chirality-recognition ability for several kinds of racemic carboxylic acids 2. A study on a chirality-recognition mechanism based on X-ray crystallographic analyses of the diastereomeric complexes of 2 with 1 revealed that the complex crystals consisted of helical columns and that 1 was not responsible for the formation of the helical column and occupied a void between the columns; although 1 interacted with 2 via a hydrogen bond to primarily form a pair with 2, the complex crystals were mainly stabilized by the accumulation of weak interactions, such as CH/π, π/π and CH⋯O interactions, between 1/ 1, 1/ 2 and 2/ 2.

ChemInform Abstract: 3‐Pyrroline‐1‐carbonyl (Pyroc) Group: A Removable Protecting Group for the Kinetic Resolution of Racemic Carboxylic Acids and Alcohols Through Catalytic Asymmetric Acylation.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

3-Pyrroline-1-carbonyl (Pyroc) Group: A Removable Protecting Group for the Kinetic Resolution of Racemic Carboxylic Acids and Alcohols through Catalytic Asymmetric Acylation

The O-3-pyrroline-1-carbonyl (O-Pyroc) group and 3-pyrrolinamide are useful removable protecting groups for the kinetic resolution of racemic α-hydroxycarboxylic acids, β-hydroxycarboxylic acids, 1,2-dicarboxylic acids, and 1,2-diols using the L -histidine-derived bifunctional catalysts. The Pyroc group can be easily introduced from Pyroc chloride. Selective deprotection of the Pyroc group and 3-pyrrolinamide can be carried out via DDQ oxidation followed by hydrolysis using sodium hydroxide, without epimerization.

Design, Synthesis, and Application of Enantioselective Coupling Reagent with a Traceless Chiral Auxiliary

Stable chiral N-triazinylbrucinium tetrafluoroborate enantioselectively activates racemic carboxylic acids yielding enantiomerically enriched amides, esters, and dipeptides with er from 8:92 to 0.5:99.5. Due to the departure of a chiral auxiliary after the activation of the carboxylic function, all of the subsequent stages of the coupling reaction proceed without any perturbation caused by a chirality discriminator (traceless). Therefore, the advantageous coupling conditions, configuration, and enantiomeric purity of the final product are entirely predictable from the model experiment.

Kinetic Resolution of Racemic Carboxylic Acids Using Achiral Alcohols by the Promotion of Benzoic Anhydrides and Tetramisole Derivatives: Production of Chiral Nonsteroidal Anti‐Inflammatory Drugs and Their Esters

AbstractA variety of optically active carboxylic esters were produced by kinetic resolution of racemic carboxylic acids by using achiral alcohols, benzoic anhydrides, and Birman's tetramisole‐type catalysts. Bis(α‐naphthyl)methanol is a very effective reagent for producing the corresponding esters with high ee values in the presence of 4‐methoxybenzoic anhydride (PMBA) as the coupling reagent by promotion of benzotetramisole derivatives (BTM, α‐Np‐BTM, and β‐Np‐BTM). This protocol directly provides chiral carboxylic esters from free carboxylic acids and achiral alcohols by utilizing a transacylation process to generate the mixed anhydrides from the acid components with benzoic anhydride derivatives in the presence of chiral catalysts.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Synthesis of new chiral calix[4]azacrowns for enantiomeric recognition of carboxylic acids

Two novel chiral calix[4]azacrown ethers 4 and 5 bearing a furfuryl group on the nitrogen atom were developed by the reaction of dibromo- or ditosyl derivatives of p- tert-butylcalix[4]arenes 2 and 3 with a chiral diol, 1. The enantioselective recognition of these receptors towards the enantiomers of racemic carboxylic acids has been studied by 1H NMR spectroscopy. The molar ratio and the association constants of the chiral compounds 4 and 5 with each of the enantiomers of guest molecules were determined by using Job plots and a nonlinear least-squares fitting method, respectively. The Job plots indicate that both of the hosts form 1:1 instantaneous complexes with ( R)- or ( S)-mandelic acid and ( l)- or ( d)-dibenzoyltartaric acid. The receptors exhibited different chiral recognition abilities towards the enantiomers of racemic guests.

Kinetic Resolution of Racemic Carboxylic Acids by anl-Histidine-Derived Sulfonamide-Induced Enantioselective Esterification Reaction

The direct and catalytic kinetic resolution of racemic carboxylic acids bearing a Brønsted base such as O-protected alpha-hydroxy carboxylic acids and N-protected alpha-amino acids has been accomplished through an L-histidine-derived sulfonamide-induced enantioselective esterification reaction with tert-butyl alcohol for the first time. Highly asymmetric induction [S( k fast/ k slow) = up to 56] has been achieved under the equilibrium between a chiral catalyst and two diastereomeric acylammonium salts through an intramolecular hydrogen-bonding interaction.

Enantioselective Recognition for Carboxylic Acids by Novel Chiral Macrocyclic Polyamides Derived from L-/D-tartaric Acid

The enantioselectivity of chiral macrocyclic polyamides 1–3 derived from L-/D-tartaric acid was investigated by using 1H NMR. All the macrocycles exhibited certain chiral recognition towards the enantiomers of the racemic carboxylic acids we had chosen. As a chiral solvating agent, the compound 3 has the excellent enantiomeric discriminating ability for mandelic acids and its derivatives, containing an α-OH at the chiral carbon, while the compound 2 has the best enantioselectivity towards dibenzoyltartaric acid. The molar ratio and the association constants of the compound 3 with each of the enantiomers of some guest molecules were determined by using the Job's plots and a nonlinear leastsquares fitting method, respectively. The effect of the structure of the hosts or guests on the enantioselectivity of the compound 1–3 has been explored.

Total synthesis of (–)-xyloketal D and its enantiomer — Confirmation of absolute stereochemistry

The total synthesis of (–)-xyloketal D and its enantiomer have been achieved by the reaction of an ortho-quinone methide with (4R)- and (4S)-4,5-dihydro-2,4-dimethylfuran via a diastereoselective inverse electron demand Diels–Alder reaction. This total synthesis confirmed the absolute stereochemistry of the natural product. The ortho-quinone methide was generated by reaction of an appropriately functionalized Mannich base with methyl iodide. The Mannich base was prepared in one step from 2,4-dihydroxyacetophenone, formaldehyde, and morpholine. The enantiomeric dihydrofurans were prepared from (2R)- and (2S)-2-methylpent-4-ynoic acid via a three-step reaction sequence. These chiral nonracemic synthetic precursors were prepared from the corresponding (R)-phenylglycinol-derived diastereomeric amides of the readily available racemic carboxylic acid. The absolute stereochemistry of these carboxylic acids was firmly established by conversion to a known compound that had been previously prepared from a chiral pool starting material.Key words: xyloketal D, inverse electron demand Diels–Alder reaction, ortho-quinone methide, dihydrofuran.

Novel rigid chiral macrocyclic dioxopolyamines derived from l-proline as chiral solvating agents for carboxylic acids

1H NMR was employed to investigate the chiral recognition ability of two novel l-proline derived rigid chiral macrocyclic dioxopolyamines, (12 S)-1,4,7,10-tetraazadicyclo[10.3.0]pentadecane-3,11-dione 1 and (8 S,18 S)-1,4,10,13,16-pentaazatricyclo[16.3.0.0 4,8]heneicosane-9,17-dione 2, the latter of which is newly synthesized. The crystal structures of the two macrocyclic compounds were determined by X-ray single crystal structure analysis. The Job plots indicate that compound 1 forms a 1:1 instantaneous complex with ( R)- or ( S)-mandelic acid while compound 2 forms a 1:2 instantaneous complex with each of the two guests above. Association constants of compound 1 with ( R)- and ( S)-mandelic acids were determined by a nonlinear least-squares fitting method. Both of the macrocycles exhibited good chiral recognition toward the enantiomers of the racemic carboxylic acids we chose. It was shown that the macrocyclic dioxopolyamine of C 2 symmetry had a better enantiomeric discriminating ability than that of C 1 symmetry.

2000 Alfred Bader Award LectureIn the footsteps of Pasteur: asymmetric induction in the photochemistry of crystalline ammonium carboxylate salts

This review describes the development of a new, synthetically useful method of asymmetric synthesis in organic photochemistry. Similar in many ways to the Pasteur procedure for resolving racemic carboxylic acids and organic amines, the method relies on the use of crystalline organic salts in which the enantioselectivity of a photochemical reaction of an achiral organic ion (for example, a carboxylate anion) is governed in the solid state by the presence of an optically pure counterion (for example, an optically active ammonium ion). Such optically pure counterions are termed ionic chiral auxiliaries. Salts containing ionic chiral auxiliaries are required to crystallize in chiral space groups, which provide the asymmetric environment necessary for chiral induction. Using this methodology, we have obtained near-quantitative optical yields in a wide variety of photochemical reactions.Key words: photochemistry, solid state, chiral auxiliaries, asymmetric synthesis, crystal structure–reactivity relationships.

Resolution of racemic carboxylic acids via the lipase-catalyzed irreversible transesterification of vinyl esters.

The lipase-catalyzed irreversible transesterification procedure using vinyl esters was applied to the resolution of racemic 2-phenoxypropanoic acids. Aspergillus niger lipase showed high enantioselectivities and reasonable reaction rates. The enantioselectivity was found to be affected profoundly by several variables, e.g., the alcohol as nucleophile, the organic solvent used, and the reaction temperature. A gram-scale resolution of (RS)-2-phenoxypropanoic acid was achieved after optimization of the reaction conditions. Then this irreversible transesterification procedure was applied to the resolution of some related 2-substituted carboxylic acids. Thus, racemic 2-methoxy-2-phenylacetic acid was resolved via the A. niger lipase-catalyzed transesterification of the corresponding vinyl ester. 2-Phenylpropanoic acid and 2-phenylbutanoic acid were resolved using Pseudomonas sp. lipase. A gram-scale resolution of 2-phenylbutanoic acid was achieved by this procedure coupled with the porcine liver esterase-catalyzed hydrolysis of the resulting methyl ester.