Racemic Amines Research Articles

Acylative kinetic resolution of racemic heterocyclic amines using N-phthaloyl-(S)-amino acyl chlorides with alkyl side chains

A comparative study of the acylative kinetic resolution of racemic 2-methyl-1,2,3,4 tetrahydroquinoline and 3,4-dihydro-3-methyl-2H-[1,4]benzoxazine using N-phthaloyl-(S)-amino acyl chlorides with alkyl side chains has been carried out. The influence of steric factors on the stereoselectivity of the acylation was demonstrated. The (S)-enantiomers of the heterocyclic amines (ee >99%) were obtained in good yields via a kinetic resolution protocol using N-phthaloyl-(S)-leucyl chloride.

Enantiomeric Separation of 1-(Benzofuran-2-yl)alkylamines on Chiral Stationary Phases Based on Chiral Crown Ethers

Optically active chiral amines are important as building blocks for pharmaceuticals and as scaffolds for chiral ligands and, consequently, many efforts have been devoted to the development of efficient methods for their preparation. For example, reduction of amine precursors with chiral catalysts, 1 enzymatic kinetic resolution or dynamic kinetic resolution of racemic amines 2 and the direct amination of ketones with transaminases 3 have been developed as the efficient methods for the preparation of optically active chiral amines. During the process of developing or utilizing optically active chiral amines, the methods for the determination of their enantiomeric composition are essential. Among various methods, liquid chromatographic resolution of enantiomers on chiral stationary phases (CSPs) have been known to be one of the most accurate and economic means for the determination of the enantiomeric composition of optically active chiral compounds. 4 Especially, CSPs based on chiral crown ethers have been successfully used for the resolution of racemic primary amines. 5 For example, CSPs based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

Kinetic Resolution of Nitrogen Heterocycles with a Reusable Polymer‐Supported Reagent

Shake it up baby! Simply shaking a polymer-supported reagent and the racemic amine at room temperature kinetically resolves a broad range of N-heterocycles with good selectivity. The polymer-supported reagents are robust, easy to regenerate, and can be reused dozens of times. Cleavable acyl groups can be used to give access to both amine enantiomers in a single resolution.

How the mode of Candida antarctica lipase B immobilization affects the continuous-flow kinetic resolution of racemic amines at various temperatures

Abstract The effect of temperature on enantiomeric ratio (E) and specific reaction rate (rflow) in the continuous-flow mode acetylation of (±)-1-phenylethanamine (rac-1a), (±)-4-phenylbutan-2-amine (rac-1b) and (±)-1,2,3,4-tetrahydro-1-naphthalenamine (rac-1c) by variously immobilized Candida antarctica lipase B biocatalysts was studied in the 0–70 °C range. In the continuous-flow kinetic resolutions with differently immobilized CaLB biocatalysts, the character of temperature effect depended significantly both on the substrate and on the mode of immobilization. Alteration of E in the kinetic resolutions of three differently flexible amines rac-1a–c as a function of temperature was rationalized by the various flexibility of the lipase in its different forms. Our results indicated that the optimal method of immobilization depended both on the nature of the substrate and the reaction conditions.

ChemInform Abstract: Nonenzymatic Acylative Kinetic Resolution of Racemic Amines and Related Compounds.

AbstractReview: ca. 150 refs.

Features and technical applications of ω-transaminases

Chiral amines in enantiopure forms are important chemical building blocks, which are most well recognized in the pharmaceutical industries for imparting desirable biological activity to chemical entities. A number of synthetic strategies to produce chiral amines via biocatalytic as well as chemical transformation have been developed. Recently, ω-transaminase (ω-TA) has attracted growing attention as a promising catalyst which provides an environment-friendly access to production of chiral amines with exquisite stereoselectivity and excellent catalytic turnover. To obtain enantiopure amines using ω-TAs, either kinetic resolution of racemic amines or asymmetric amination of achiral ketones is employed. The latter is usually preferred because of twofold higher yield and no requirement of conversion of a ketone product back to racemic amine. However, the choice of a production process depends on several factors such as reaction equilibrium, substrate reactivity, enzyme inhibition, and commercial availability of substrates. This review summarizes the biochemical features of ω-TA, including reaction chemistry, substrate specificity, and active site structure, and then introduces recent advances in expanding the scope of ω-TA reaction by protein engineering and public database searching. We also address crucial factors to be considered for the development of efficient ω-TA processes.

Diastereoselective Imine-Bond Formation through Complementary Double-Helix Formation

Optically active amidine dimer strands having a variety of chiral and achiral linkers with different stereostructures are synthesized and used as templates for diastereoselective imine-bond formations between two achiral carboxylic acid monomers bearing a terminal aldehyde group and racemic 1,2-cyclohexanediamine, resulting in a preferred-handed double helix stabilized by complementary salt bridges. The diastereoselectivity of the racemic amine is significantly affected by the chirality of the amidine residues along with the rigidity and/or chirality of the linkers in the templates. NMR and kinetic studies reveal that the present imine-bond formation involves a two-step reversible reaction. The second step involves formation of a preferred-handed complementary double helix assisted by the chiral amidine templates and determines the overall reaction rate and diastereoselectivity of the amine.

Preparation of (S)-1-Cyclopropyl-2-methoxyethanamine by a Chemoenzymatic Route Using Leucine Dehydrogenase

(S)-1-Cyclopropyl-2-methoxyethanamine is a key chiral intermediate for the synthesis of a corticotropin-releasing factor-1(CRF-1) receptor antagonist. Resolution of the racemic amine by transaminase from Vibrio fluvalis gave a 38% yield of the S-amine with 53% ee. Resolution by lipase-catalyzed acylation provided the S-amine in 35% yield with 91% ee. With limited success of these resolution approaches, an efficient chemo-enzymatic route to (S)-1-cyclopropyl-2-methoxyethanamine was devised starting from methylcyclopropyl ketone. Permanganate oxidation of the ketone gave cyclopropylglyoxylic acid, which was converted to (S)-cyclopropylglycine by reductive amination using leucine dehydrogenase from Thermoactinomyces intermedius with NADH cofactor recycling by formate dehydrogenase from Pichia pastoris. Both enzymes were cloned and expressed in recombinant E. coli. (S)-Cyclopropylglycine obtained from enzymatic reductive amination was isolated as the N-Boc derivative and converted to the desired amine by redu...

Solvent-free kinetic resolution of primary amines catalyzed by Candida antarctica lipase B: effect of immobilization and recycling stability

Highly enantioselective (E >200) N-acylation of nine racemic primary amines with isopropyl methoxyacetate in the presence of Candida antarctica lipase B (Novozym 435) has been reported to yield the unreacted (S)-amines (ee ⩾98%) and produced the (R)-amides (ee ⩾95%) at 50% conversion under solvent-free conditions. One of the amines and the acyl donor have been used in an equimolar ratio at room temperature (23°C). Under the reaction conditions, the reuse stability of Novozym 435 with 1-phenylethylamine (as a model compound) has been shown to be poor while somewhat improved stability has been observed with an in-house prepared sol–gel CAL-B catalyst.

Nonenzymatic Acylative Kinetic Resolution of Racemic Amines and Related Compounds

AbstractAcylative kinetic resolution of racemic amines occupies a highly important place among the methods for preparation of enantio enriched or pure amines. Nonenzymatic acylative kinetic resolution is usually carried out in the presence of chiral acyl‐transfer catalysts or under the action of chiral enantio‐ or diastereoselective resolving agents. Recently, this line of investigations has been rapidly developed and very interesting results of design and synthetic application of both new catalysts and chiral acylating agents have been obtained. This microreview summarizes the recent advances in this area.

Catalytic Kinetic Resolution of Cyclic Secondary Amines

The catalytic resolution of racemic cyclic amines has been achieved by an enantioselective amidation reaction featuring an achiral N-heterocyclic carbene catalyst and a new chiral hydroxamic acid cocatalyst working in concert. The reactions proceed at room temperature, do not generate nonvolatile byproducts, and provide enantioenriched amines by aqueous extraction.

S N 2-type ring opening of substituted-N-tosylaziridines with zinc (II) halides: Control of racemization by quaternary ammonium salt

Quaternary ammonium salt mediated highly regioselective ring opening of aziridines with zinc(II) halides to racemic and non-racemic β-halo amines in excellent yield and selectivity is described. The reaction proceeds via an SN2-type pathway and the partial racemization of the starting substrate and the product was effectively controlled by using quaternary ammonium salts to afford the enantioenriched products (er up to 95:5). Quaternary ammonium salt mediated highly regioselective ring opening of aziridines with zinc(II) halides to racemic and non-racemic β-halo amines in excellent yield and selectivity is described.

Engineering a Biometallic Whole Cell Catalyst for Enantioselective Deracemization Reactions

The ability of microbial cells to synthesize highly reactive nanoscale functional materials provides the basis for a novel synthetic biology tool for developing the next generation of multifunctional industrial biocatalysts. Here, we demonstrate that aerobic cultures of Escherichia coli, genetically engineered to overproduce a recombinant monoamine oxidase possessing high enantioselectivity against chiral amines, can be augmented with nanoscale Pd(0) precipitated via bioreduction reactions. The result is a novel biometallic catalyst for the deracemization of racemic amines. This deracemization process is normally achieved by discrete sequential oxidation/reduction steps using a separate enantiomer-specific biocatalyst and metal catalyst, respectively. Here, use of E. coli cultures carrying the cloned monoamine oxidase gene and nanoscale bioreduced Pd(0) particles was used successfully for the conversion of racemic 1-methyltetrahydroisoquinoline (MTQ) to (R)-MTQ, via the intermediate 1-methyl-3,4-dihydrois...

A planar chiral non-metallocenic analogue of the most popular N, N-dimethylbenzylaminate palladacycle

A racemic planar chiral tertiary amine pCp-CH 2NMe 2 ( HL 1 , pCp = [2.2]paracyclophane-4-yl) was prepared by aminomethylation of the bromide pCp-Br with Eschenmoser’s salt. Direct cyclopalladation of this new ligand with palladium(II) acetate results in the formation of the racemic CN-dimer rac- 3 in a moderate yield of 64%. The enantiomerically pure dimer ( S pl , S pl )- 3 was obtained by the standard procedure of racemic palladacycle resolution using ( S C )-prolinate as a chiral derivatising agent. The ortho-palladated structure, absolute configuration of the chiral plane and stereochemical peculiarities of the new CN-palladacycle were established by means of NMR spectroscopy and an X-ray diffraction study of its ( S C )-prolinate derivative.

The Lactol Route to Fesoterodine: An Amine-Promoted Friedel–Crafts Alkylation on Commercial Scale

We report the discovery and optimization of an amine-promoted Friedel–Crafts alkylation of cinnamaldehyde with 4-hydroxymethyl phenol. This reaction has been used successfully on commercial scale (200 kg) in the context of the manufacture of fesoterodine, a muscarinic antagonist used for the treatment of overactive bladder. Reductive aminations of diisopropylamine and lactol 4 are also discussed, as well as the resolution of the racemic amine rac-2 into its enantiomerically pure form.

Synthesis and Biological Evaluation of Enantioenriched Phenols Having an Isobornyl Substituent

The resolution of 2-isobornyl-4-methylphenol (±)-1 via diastereomers was carried out. The enantioenriched phenols (+)-1, ( – )-1 and morpholinomethyl derivatives (+)-6, ( – )-6 were tested for their cytotoxic and anti-inflammatory activities in comparison with their racemates. The anti-inflammatory activity comparable to that of indomethacin estimated for the racemic amine (±)-6 is due to the enantiomer (+)-6. The enantiomer ( – )-6 exhibits a moderate cytotoxicity against the human cancer cell line melanoma (MS) while enantiomer ( – )-1 demonstrates the inhibitory activity against the human cancer cell line lung carcinoma (A549). The absolute configuration of the diastereomeric intermediates and separated phenols were established by X-ray diffraction analysis.

Evaluations of poly(methyl methacrylate) bearing glucose pendant or cellobiose pendant in high performance liquid chromatography as polymeric chiral stationary phases

The glucopyranos units consisting of cyclodextrin and polysaccharide derivatives have been extensively used as chromatographic chiral stationary phases. The synthetic polymeric chiral stationary phases based on methyl methacrylate bearing glucose or cellobiose pendants for high performance liquid chromatography (HPLC) were prepared by radical polymerization. Fifteen racemic chiral compounds including alcohols, amines, amides and ketones, were resolved on the two chiral stationary phases using hexane-isopropanol (90/10, v/v), hexane-isopropanol-triethylamine (90/10/0.2, v/v/v) or hexane-isopropanol-trifluoroacetic acid (90/ 10/0.2, v/v/v) as mobile phase, separately. The results showed that both chiral stationary phases possessed good enantioselectivity for most of the racemic alcohols and amines. A few amides and ketones could also be separated. There was some compensation of the chiral discriminating abilities between the two chiral stationary phases. This work indicates that polymers of mono- and disaccharide derivatives can soon become a very attractive new chiral stationary phase for HPLC.

An efficient enzymatic approach to ( S)-1-aryl-allylamines

A range of 1-aryl-allylamines were prepared in moderate to excellent enantioselectivity (ee 63.5%→99.9%) using lipase B from a Candida antarctica catalyzed resolution of racemic amines. This is the first time that CaLB has been used for the resolution of 1-aryl-allylamines. Racemic amines were prepared starting from aromatic aldehydes with a [3,3]-sigmatropic rearrangement of the acyclic imidates as the key step followed by trichloroacetamidate hydrolysis. Aldehydes were converted into acrylic esters using Knoevenagel reaction. After reduction, the corresponding alcohols were used for the preparation of trichloroacetimidates, which were then used in an Overman rearrangement.

ChemInform Abstract: Dynamic Kinetic Resolution of Amines and Amino Acids by Enzyme—Metal Cocatalysis

AbstractReview: ca. 50 refs.

Substituent effect on the stereoselectivity of acylation of racemic heterocyclic amines with N-phthaloyl-3-aryl-( S)-alanyl chlorides

The acylative kinetic resolution of racemic 2-methyl-1,2,3,4-tetrahydroquinoline and 2,3-dihydro-3-methyl-4 H-1,4-benzoxazine using acyl chlorides of N-phthaloyl-( S)-phenylalanine, N-phthaloyl-3-(4-nitrophenyl)-( S)-alanine and N-phthaloyl- O-methyl-( S)-tyrosine as chiral resolving agents has been carried out. It is shown that the effectiveness of an acylative kinetic resolution depends on the electronic effects of substituents in the phenyl fragment of the acylating agent and increases as the electron-donating properties of the para-substituent (OMe > H > NO 2) in phenyl fragment of N-phthaloyl-3-aryl-( S)-alanyl chlorides increase; conducting the process at a reduced temperature also contributes to an enhancement of the kinetic resolution.