Racemic Carboxylic Acids Research Articles

Kinetic Resolution of α‐Allyl‐α‐Cinnamyl Carboxylic Acids Bearing an All‐Carbon Quaternary Stereocenter via Bromolactonization Catalyzed by a Chiral Bifunctional Sulfide

AbstractCatalytic kinetic resolutions of racemic carboxylic acids are important methods for preparing chiral carboxylic acids in their optically active forms. While several effective methods for the catalytic kinetic resolution of chiral α‐tertiary carboxylic acids have been reported, the efficient kinetic resolution of α‐quaternary carboxylic acids remains a significant challenge. In this context, we aimed to develop methods for the kinetic resolution of α‐quaternary carboxylic acids. Our approach involves a BINOL‐derived chiral bifunctional sulfide‐catalyzed bromolactonization technique, which was developed by our research group. In this work, we report the kinetic resolution of α‐allyl‐α‐cinnamyl carboxylic acids bearing an α‐quaternary stereocenter via chemo‐ and stereoselective bromolactonizations. The use of a BINOL‐derived chiral bifunctional sulfide catalyst with a hydroxy group was key for the efficient kinetic resolution of racemic α‐allyl‐α‐cinnamyl carboxylic acids.

Organocatalyzed Kinetic Resolution of Racemic Carboxylic Acids

AbstractThe kinetic resolution of racemic compounds is a reliable method to prepare important chiral molecules in highly optically enriched forms. Although methods for the catalytic kinetic resolution of chiral alcohols, amines, and epoxides have been extensively investigated, the kinetic resolution of racemic carboxylic acids has remained under‐developed despite the importance of chiral carboxylic acids. Excellent catalytic approaches for the kinetic resolution of racemic carboxylic acids via esterification using chiral organocatalysts have been reported by several research groups. Furthermore, the organocatalyzed lactonization of alkenyl and alkynyl carboxylic acids has also been adapted to achieve kinetic resolution. Herein, the methods typically used to produce the organocatalyzed kinetic resolution of racemic carboxylic acids are reviewed to elucidate the current status of this research.

Resolution by diastereomeric salts: Access to both enantiomers of racemic acid using the same enantiomer of resolving base.

Racemic carboxylic acid, a Diels-Alder cycloadduct derived from 5-bromo-3-phenyl-α-pyrone and acrylate dienophile, was resolved into enantiomers by diastereomeric salt crystallization. Quinidine was used as a sole resolving base. The salt of (+)-acid crystallized from aqueous acetonitrile solution. Once this salt was separated by filtration, quinidine salt with (-)-acid crystallized from mother liquor. As a result, both enantiomers of Diels-Alder cycloadduct were isolated in high enantiomeric purity.

Optical Resolution of Carboxylic Acid Derivatives of Homoleptic Cyclometalated Iridium(III) Complexes via Diastereomers Formed with Chiral Auxiliaries.

We report on a facile method for the optical resolution of cyclometalated iridium(III) (Ir(III)) complexes via diastereomers formed with chiral auxiliaries. The racemic carboxylic acids of Ir(III) complexes (fac-4 (fac-Ir(ppyCO2H)3 (ppy: 2-phenylpyridine)), fac-6 (fac-Ir(tpyCO2H)3 (tpy: 2-(4'-tolyl)pyridine)), and fac-13 (fac-Ir(mpiqCO2H)3 (mpiq: 1-(4'-methylphenyl)isoquinoline))) were converted into the diastereomers, Δ- and Λ-forms of fac-9 (from fac-6), fac-10 (from fac-4), fac-11 (from fac-6), and fac-14 (from fac-13), respectively, by the condensation with (1R,2R)-1,2-diaminocyclohexane or (1R,2R)-2-aminocyclohexanol. The resulting diastereomers were separated by HPLC (with a nonchiral column) or silica gel column chromatography, and their absolute stereochemistry was determined by X-ray single-crystal structure analysis and CD (circular dichroism) spectra. Spectra of all diastereomers of the Ir(III) complexes are reported. Hydrolysis of the ester moieties of Δ- and Λ-forms of fac-10, fac-11, and fac-14 gave both enantiomers of the corresponding carboxylic acid derivatives in the optically pure forms, Δ-fac and Λ-fac-4, -6, and -13, respectively.

N‐Boc‐Protected α‐Amino Acids by 1,3‐Migratory Nitrene C(sp3)−H Insertion

AbstractN‐Boc‐protected α‐amino acids are synthesized in two steps from linear or branched carboxylic acid feedstocks. In the first step, the carboxylic acid is coupled with tert‐butyl aminocarbonate (BocNHOH) to generate azanyl ester (acyloxycarbamate) RCO2NHBoc. In the second step, this azanyl ester undergoes a stereocontrolled iron‐catalyzed 1,3‐nitrogen migration to generate the N‐Boc‐protected non‐racemic α‐amino acid. This straightforward protocol is applicable to the catalytic asymmetric synthesis of α‐monosubstituted α‐amino acids with aryl, alkenyl, and alkyl side chains. Furthermore, α,α‐disubstituted α‐amino acids are accessible in an enantioconvergent fashion from racemic carboxylic acids. The new method is also advantageous for the synthesis of α‐deuterated α‐amino acids. N‐Boc‐protected α‐amino acids synthesized using this two‐step protocol are ready‐to‐use building blocks.

Chiral quaternary ammonium ionic liquids derived from natural dehydroabietylamine and their potential application in chiral molecular recognition

A series of chiral ionic liquids (CILs) based on a quaternary ammonium core substituted with (+)-dehydroabietylamine (DAA) moiety have been synthesized, which are used for the chiral discrimination of non-ionic racemic carboxylic acid and derivatives. The enantioselective recognition ability of the obtained DAA-CILs was investigated using (rac)-mandelic acid and its derivatives as substrates. The results showed that the choice of the anion was crucial for the chiral discrimination of these DAA-CILs. In general, bistrifluoromethanesulfonimide anion (NTf2-) resulted in the improvement and a broad spectrum of enantioselectivity of the CILs. The alkyl chain attached to the DAA group and quaternary ammonium cation was also responsible for the chiral discrimination, and the C3 or C4 carbon chain was favorable for the chiral molecule recognition ability of the CILs. A molecular simulation of the diastereomeric interactions between the selected [DAAPrTMA]NTf2 CIL and racemic α-methoxy mandelic acid (α-MPAA) achieved a well understanding of the chiral recognition mechanism. Additionally, the optical purity of α-MPAA could be detected via NMR analysis easily and quickly. This work leads us to develop new CILs with good molecular recognition ability, DAA-based quaternary ammonium salts.

Simultaneous Enantiodivergent Synthesis of Diverse Lactones and Lactams via Sequential One-Pot Enzymatic Kinetic Resolution-Ring-Closing Metathesis Reactions.

One of the goals of diversity-oriented synthesis is to achieve the structural diversity of obtained compounds. As most biologically active compounds are chiral, it is important to develop enantioselective methods of their synthesis. The application of kinetic resolution in DOS is problematic because of low efficiency (max. 50% yield) and many purification steps. The further derivatization of kinetic resolution products in DOS leads to the formation of a narrow library of compounds of the same stereochemistry. To overcome these limitations, we present a new concept in which the kinetic resolution is combined, the subsequent reaction of which in a one-pot protocol leads to the simultaneous formation of two skeletally and enantiomerically diverse platform molecules for DOS. Their further derivatization can gain access to a double-sized library of products in respect to a classical approach. The validity of our concept was evidenced in enzymatic kinetic resolution followed by a ring-closing metathesis cascade. From racemic carboxylic acid ester, a simultaneous formation of enantiopure lactones and lactams was achieved. These compounds are important building blocks in organic and medicinal chemistry and until now were synthesized in separate procedures.

Non-Enzymatic Kinetic Resolution and Desymmetrization of α-Quaternary Carboxylic Acids via Chiral Bifunctional Sulfide-Catalyzed Bromolactonization

Abstract Kinetic resolution of racemic carboxylic acids is a reliable method to enantioselectively prepare chiral carboxylic acids. Although efficient catalytic kinetic resolutions of chiral α-tertiary carboxylic acids have been reported, the kinetic resolution of α-quaternary carboxylic acids bearing an all-carbon quaternary stereocenter has remained a formidable challenge. Herein, we report a precious example of a kinetic resolution of α-quaternary carboxylic acids via a chiral bifunctional sulfide-catalyzed bromolactonization of alkynes. The ability of chiral sulfides to recognize α-quaternary carboxylic acids was evaluated via the catalytic enantioselective desymmetrizing bromolactonization of achiral α,α-dipropargyl carboxylic acids, which is a reaction that is related to target kinetic resolution. The optimum chiral sulfide was successfully applied to the efficient kinetic resolution of α-propargyl carboxylic acids that bear an all-carbon quaternary stereocenter.

Systematic comparison of racemic and enantiopure multicomponent crystals of phenylsuccinic acid—the role of chirality

Comparison of binary cocrystals of chiral and racemic carboxylic acids showed that the introduction of chiral building blocks may lead to the formation of subclasses of multicomponent crystals with unique Z′′/Zr values combined with complex protonation stages of the molecules.

Novel chiral Oxa-bridged Calix[2]arene[2]triazine derivatives as NMR chiral reagents for enantioselective recognition of α-racemic carboxylic acids

A new family of chiral tetraoxacalix[2]arene[2]triazine receptors (2a–2c) have been synthesized efficiently in 69–73% yields. By 1H NMR spectroscopy, the chiral enantiodiscrimination ability of novel hosts against a series racemic carboxylic acids were examined. The chiral calix[2]arene[2]triazine-based compounds were found to have good enantiomeric recognition ability to racemic carboxylic acids. We also observed that the tetraoxacalix[2]arene[2]triazine derivative 2c is a more versatile chiral solvating agents than 2a and 2b, as 2c shows a better performance for the determination of the enantiomeric excess. The molar ratio of the each enantiomers of guest molecules with receptors were examined by using Job plots, respectively. The graphics show that the hosts form 1:1 complexes with all of the guests.

Candida rugosa lipase encapsulated with magnetic sporopollenin: design and enantioselective hydrolysis of racemic arylpropanoic acid esters

The aim of this study was to prepare the encapsulation of Candida rugosa lipase (CRL) with magnetic sporopollenin. The sporopollenin was covalent immobilized onto magnetic nanoparticles (Fe3O4), grafted amino (APTES), or epoxy groups (EPPTMS). CRL was sol-gel encapsulated in the presence of magnetic sporopollenin/Fe3O4 nanoparticles. The influence of activation agents ([3-(2,3-epoxypropoxy) propyl] trimethoxysilane (EPPTMS), (3-Aminopropyl)triethoxysilane (APTES) and pH and thermal stabilities of the biocatalyst were assessed. Experimental data showed the improved catalytic activity at different pH and temperature values. At 60 °C, free lipase lost its initial activity within 80 min of time, although the encapsulated lipases retained their initial activities of about 65% by APTES and 60% by EPPTMS after 120 min of heat treatment at 60 °C. The catalytic properties of the encapsulated lipases were utilized to hydrolysis of racemic aromatic carboxylic acid methyl esters (Naproxen and 2-phenoxypropionic acid). The results show that the sporopollenin-based encapsulated lipase (Fe-A-Spo-E) has greater enantioselectivity and conversion in comparison with the encapsulated lipase without supports (lipase-enc).

An Enantioselective Hydrogenation of an Alkenoic Acid as a Key Step in the Synthesis of AZD2716

A classical resolution of a racemic carboxylic acid through salt formation and an asymmetric hydrogenation of an α,β-unsaturated carboxylic acid were investigated in parallel to prepare an enantiomerically pure alkanoic acid used as a key intermediate in the synthesis of an antiplaque candidate drug. After an extensive screening of rhodium- and ruthenium-based catalysts, we developed a rhodium-catalyzed hydrogenation that gave the alkanoic acid with 90% ee, and after a subsequent crystallization with (R)-1-phenylethanamine, the ee was enriched to 97%. The chiral acid was then used in sequential Negishi and Suzuki couplings followed by basic hydrolysis of a nitrile to an amide to give the active pharmaceutical ingredient in 22% overall yield.

Chemo-enzymatic Baeyer-Villiger oxidation of 4-methylcyclohexanone via kinetic resolution of racemic carboxylic acids: direct access to enantioenriched lactone.

A new method for the asymmetric chemo-enzymatic Baeyer-Villiger oxidation of prochiral 4-methylcyclohexanone to (R)-4-methylcaprolactone in the presence of (±)-4-methyloctanoic acid, Candida Antarctica lipase B and 30% aq. H2O2 has been developed. A mechanism for the asymmetric induction based on kinetic resolution of racemic carboxylic acids is proposed.

Chirality- and sequence-selective successive self-sorting via specific homo- and complementary-duplex formations.

Self-recognition and self-discrimination within complex mixtures are of fundamental importance in biological systems, which entirely rely on the preprogrammed monomer sequences and homochirality of biological macromolecules. Here we report artificial chirality- and sequence-selective successive self-sorting of chiral dimeric strands bearing carboxylic acid or amidine groups joined by chiral amide linkers with different sequences through homo- and complementary-duplex formations. A mixture of carboxylic acid dimers linked by racemic-1,2-cyclohexane bis-amides with different amide sequences (NHCO or CONH) self-associate to form homoduplexes in a completely sequence-selective way, the structures of which are different from each other depending on the linker amide sequences. The further addition of an enantiopure amide-linked amidine dimer to a mixture of the racemic carboxylic acid dimers resulted in the formation of a single optically pure complementary duplex with a 100% diastereoselectivity and complete sequence specificity stabilized by the amidinium–carboxylate salt bridges, leading to the perfect chirality- and sequence-selective duplex formation.

ChemInform Abstract: Preparative Resolution of Bromocyclopropylcarboxylic Acids.

AbstractThe first crystallization of racemic carboxylic acids (V) with cinchonine followed by an acid‐base extraction of crystalline crop leads to enantiomerically pure acids ((+)‐V), whilst the acid‐base work‐up of the mother liquor affords crude acids ((‐)‐V).

Scalable Synthesis of a Nonracemic α-Arylpropionic Acid via Ketene Desymmetrization for a Glucokinase Activator

Process research and development for a synthesis of the chiral carboxylic acid (R)-2 as a key intermediate of the glucokinase activator (R)-1 is described. The construction of the stereocenter at the α-carbon is a key point for the synthesis of (R)-2. The proposed process utilizes desymmetrization of a ketene in situ generated from the corresponding racemic carboxylic acid Rac-2 with (R)-pantolactone as a chiral auxiliary followed by hydrolysis of the resulting ester. This key step has been successfully scaled up to 20 kg, which demonstrates that this synthetic approach is comparable with a previously reported approach via enantioselective hydrogenation.

脱水縮合を伴う速度論的光学分割反応の開発

脱水縮合を伴う速度論的光学分割反応の開発

Synthesis of crambescin B carboxylic acid, a potent inhibitor of voltage-gated sodium channels

The stereocontrolled synthesis of a racemic carboxylic acid of crambescin B, a marine alkaloid, is described. The synthesis features two highly stereoselective reactions: (I) palladium-catalyzed hydroxymethylation of an alkynyl aziridine having an N-guanidino group and (II) cascade bromocyclization providing a spiro-hemiaminal structure. The cell-based colorimetric assay showed that the synthesized carboxylic acid exhibited a potent inhibitory activity on voltage-gated sodium channels.

Calixarene based chiral solvating agents for α-hydroxy carboxylic acids

Novel chiral calix[4]arene derivatives functionalized at the lower rim have been prepared from the reaction of p-tert-butylcalix[4]arene diamine derivative with N-Phthaloyl-L-phenylalanine or (2S)-2-((benzyloxy)carbonyl)amino)-3-hydroxypropanoic acid or (2S,3R)-2-((benzyloxy)carbonyl)amino-3-hydroxybutanoic acid in 63–81% yield. The structures of these receptors were characterized by FTIR, 1H, 13C and 2D COSY NMR spectroscopy. The enantioselective recognition of these receptors towards the enantiomers of racemic carboxylic acids was studied by 1H NMR spectroscopy. The molar ratios of the chiral compounds with each of the enantiomers of guests were determined by using Job plots. The Job plots indicate that the hosts form 1:2 instantaneous complexes with all guests. The receptors exhibited different chiral recognition abilities toward the enantiomers of racemic guests. NMR studies demonstrated that the receptors function as highly effectivechiral shift reagents for determining the enantiomeric purity of a series of carboxylic acids.

Kinetic Resolution of Racemic Carboxylic Acids through Asymmetric Protolactonization Promoted by Chiral Phosphonous Acid Diester

Chiral phosphonium salts induce the kinetic resolution of racemic α-substituted unsaturated carboxylic acids through asymmetric protolactonization. Both the lactones and the recovered carboxylic acids are obtained with high enantioselectivities and high S (= kfast/kslow) values. Asymmetric protolactonization also leads to the desymmetrization of achiral carboxylic acids. Notably, chiral phosphonous acid diester not only induced the enantioselectivity but also promoted protolactonization.