Racemic Amines Research Articles

Asymmetric Amination of Unstrained C(sp3)-C(sp3) Bonds.

The asymmetric functionalization of unstrained C(sp3)-C(sp3) bonds could be a powerful strategy to stereoselectively reconstruct the backbone of an organic compound, but such reactions are rare. Although allylic substitutions have been used frequently to construct C-C bonds by the cleavage of more reactive C-X bonds (X is usually an O atom of an ester) by transition metals, the reverse process that involves the replacement of a C-C bond with a C-heteroatom bond is rare and generally considered thermodynamically unfavorable. We show that an unstrained, inert allylic C-C σ bond can be converted to a C-N bond stereoselectively via a designed solubility-control strategy, which makes the thermodynamically unfavorable process possible. The C-C bond amination occurs with a range of amine nucleophiles and cleaves multiple classes of alkyl C-C bonds in good yields with high enantioselectivity. A novel resolution strategy is also reported that transforms racemic allylic amines to the corresponding optically active allylic amine by the sequential conversion of a C-N bond to a C-C bond and back to a C-N bond. Mechanistic studies show that formation of the C-N bond is the rate-limiting step and is driven by the low solubility of the salt formed from the cleaved alkyl group in a nonpolar solvent.

Preparation of chiral polyether dendrimer and its non-bonding immobilized CALB for high enantioselective synergy resolution of amines

Monodisperse polyethylene glycol monomethyl ether and chiral glycidyl tosylate were utilized as substrates in the synthesis of the chiral monomer unit epoxypropyl polyethylene glycol monomethyl ether (MPEGn-EPO). The initiator propargyl-terminated polyethylene glycol alkyne-PEG3-OH was employed, while the phosphazene base P4-t-Bu facilitated the controlled ROP of MPEGn-EPO, resulting in clickable dendritic P(MPEGn-EPO) with terminal alkyne. The chiral four-arm dendritic polymer 4-Arm-PEG3-Mn was synthesized through a click reaction between 4-Arm-PEG3-N3 and P(MPEGn-EPO) using Cu(PPh3)3Br/CuBr as a catalyst. Candida antarctica lipase B (CALB) was chosen as a model enzyme for the preparation of a non-covalently immobilized biocatalyst (CALB-M) via a simple adsorption process with magnesium silicate and 4-Arm-PEG3-M4-20. The immobilized lipase CALB-M16 demonstrated high enzyme activity in the resolution of racemic amines, yielding chiral amines with enantioselectivity up to 99 % ee. Circular dichroism (CD) analysis indicated the formation of helical conformation in the dendritic polyethers 4-Arm-PEG3-M16-20 in the presence of magnesium silicate at 5 °C. The polyethylene glycol side groups of 4-Arm-PEG3-M16-20 are suggested to potentially form a crown ether-like structure with magnesium ions, which could enhance steric hindrance and induce a right-handed helix. Furthermore, the helical structure of this dendritic polyether is thought to work in synergy with CALB, improving the enantioselective resolution of amines.

Catalytic asymmetric synthesis of planar-chiral dianthranilides via (Dynamic) kinetic resolution

Chirality constitutes an inherent attribute of nature. The catalytic asymmetric synthesis of molecules with central, axial, and helical chirality is a topic of intense interest and is becoming a mature field of research. However, due to the difficulty in synthesis and the lack of a prototype, less attention has been given to planar chirality arising from the destruction of symmetry on a single planar ring. Herein, we report the catalytic asymmetric synthesis of planar-chiral dianthranilides, a unique class of tub-shaped eight-membered cyclic dilactams. This protocol is enabled by cinchona alkaloid-catalyzed (dynamic) kinetic resolution. Under mild conditions, various C2- or C1-symmetric planar-chiral dianthranilides have been readily prepared in high yields with excellent enantioselectivity. These dianthranilides can serve as an addition to the family of planar-chiral molecules. Its synthetic value has been demonstrated by kinetic resolution of racemic amines via acyl transfer, enantiodivergent synthesis of the natural product eupolyphagin, and preliminary antitumor activity studies.

Optimization of Process for Enzymatic Resolution of Racemic Amines using Continuous Flow Bioreactor

The optimization of enzymatic process for the resolution of chemically and pharmaceutically important racemic amines is described using a continuous flow bioreactor. Also described are the isolation and purification methods of the reaction products and the various parameters affecting the optimal reaction conditions for the enzymatic resolution to afford the products in high chemical purity and enantiomeric excess (ee) of over 90%.

Broad Spectrum Enantioselective Amide Bond Synthetase from Streptoalloteichus hindustanus.

The synthesis of amide bonds is one of the most frequently performed reactions in pharmaceutical synthesis, but the requirement for stoichiometric quantities of coupling agents and activated substrates in established methods has prompted interest in biocatalytic alternatives. Amide Bond Synthetases (ABSs) actively catalyze both the ATP-dependent adenylation of carboxylic acid substrates and their subsequent amidation using an amine nucleophile, both within the active site of the enzyme, enabling the use of only a small excess of the amine partner. We have assessed the ability of an ABS from Streptoalloteichus hindustanus (ShABS) to couple a range of carboxylic acid substrates and amines to form amine products. ShABS displayed superior activity to a previously studied ABS, McbA, and a remarkable complementary substrate specificity that included the enantioselective formation of a library of amides from racemic acid and amine coupling partners. The X-ray crystallographic structure of ShABS has permitted mutational mapping of the carboxylic acid and amine binding sites, revealing key roles for L207 and F246 in determining the enantioselectivity of the enzyme with respect to chiral acid and amine substrates. ShABS was applied to the synthesis of pharmaceutical amides, including ilepcimide, lazabemide, trimethobenzamide, and cinepazide, the last with 99% conversion and 95% isolated yield. These findings provide a blueprint for enabling a contemporary pharmaceutical synthesis of one of the most significant classes of small molecule drugs using biocatalysis.

Biotransamination with racemic amines as amine donors: kill three birds with one stone through a dual-enzyme cascade

A novel dual-enzyme cascade process utilizing carbonyl reductase and ω-transaminase enzyme was developed to remove by-products via the asymmetric reduction and increase product formation, making it ideal for both industrial and academic applications.

Catalytic Asymmetric Oxidation of Amines to Hydroxylamines.

Chiral hydroxylamines are increasingly common structural elements in pharmaceuticals and agrochemicals, but their asymmetric synthesis remains challenging. Although enantioselective oxidation is the most straightforward method to prepare chiral oxides with a higher oxidation state, asymmetric and even nonasymmetric amine oxidation to hydroxylamines has been poorly addressed. We report a titanium-catalyzed asymmetric oxidation of racemic amines providing a broad range of structurally diverse chiral hydroxylamines with excellent chemo- and enantioselectivity. Notably, hydroxylamines bearing diverse substituent patterns on the stereocenters, including α,α-ester-alkyl, α,α-amide-alkyl, α,α-aryl-alkyl, α,α-alkynyl-alkyl, and α,α-dialkyl, are well tolerated with good functional group compatibility. Catalyst turnover numbers up to 5000 and selectivity factors up to 278 are observed. This finding offers a democratized platform to chiral hydroxylamines as design elements for drug discovery and provides insights into metal-catalyzed asymmetric oxidation of challenging substrates.

Sugar-Assisted Kinetic Resolutions in Metal/Chiral Amine Co-Catalyzed α-Allylations and [4+2] Cycloadditions: Highly Enantioselective Synthesis of Sugar and Chromane Derivatives.

Functionalized triose-, furanose and chromane-derivatives were synthesized by the titled reactions. The sugar-assisted kinetic resolution/C-C bond-forming cascade processes generate a functionalized sugar derivative with a quaternary stereocenter in a highly enantioselective fashion (up to >99 % ee) by using a simple combination of metal and chiral amine co-catalysts. Notably, the interplay between the chiral sugar substrate and the chiral amino acid derivative allowed for the construction of a functionalized sugar product with high enantioselectivity (up to 99 %) also when using a combination of racemic amine catalyst (0 % ee) and metal catalyst.

Co-Immobilization of a Multi-Enzyme Cascade: (S)-Selective Amine Transaminases, l-Amino Acid Oxidase and Catalase.

An enzyme cascade was established previously consisting of a recycling system with an l-amino acid oxidase (hcLAAO4) and a catalase (hCAT) for different α-keto acid co-substrates of (S)-selective amine transaminases (ATAs) in kinetic resolutions of racemic amines. Only 1 mol % of the co-substrate was required and l-amino acids instead of α-keto acids could be applied. However, soluble enzymes cannot be reused easily. Immobilization of hcLAAO4, hCAT and the (S)-selective ATA from Vibrio fluvialis (ATA-Vfl) was addressed here. Immobilization of the enzymes together rather than on separate beads showed higher reaction rates most likely due to fast co-substrate channeling between ATA-Vfl and hcLAAO4 due to their close proximity. Co-immobilization allowed further reduction of the co-substrate amount to 0.1 mol % most likely due to a more efficient H2 O2 -removal caused by the stabilized hCAT and its proximity to hcLAAO4. Finally, the co-immobilized enzyme cascade was reused in 3 cycles of preparative kinetic resolutions to produce (R)-1-PEA with high enantiomeric purity (97.3 %ee). Further recycling was inefficient due to the instability of ATA-Vfl, while hcLAAO4 and hCAT revealed high stability. An engineered ATA-Vfl-8M was used in the co-immobilized enzyme cascade to produce (R)-1-(3-ethoxy-4-methoxyphenyl)-2-(methylsulfonyl)ethanamine, an apremilast-intermediate, with a 1,000 fold lower input of the co-substrate.

A novel pillar[3]trianglimine macrocycle with a deep cavity used as a chiral selector to prepare a chiral stationary phase by thiol-ene click reaction for enantioseparation in high-performance liquid chromatography.

A chiral pillar[3]trianglimine (C60 H72 N6 O6 ) with a deep cavity has been developed as a chiral selector and bonded to thiolated silica by thiol-ene click reaction to fabricate a novel chiral stationary phase for enantioseparation in high-performance liquid chromatography. The enantioseparation performance of the fabricated chiral stationary phase has been evaluated by separating various racemic compounds, including alcohols, esters, amines, ketones, amino acids, and epoxides, in both normal-phase and reversed-phase elution modes. In total, 14 and 17 racemates have been effectively separated in these two separation modes, respectively. In comparison with two widely used chiral columns (Chiralcel OD-H and Chiralpak AD-H), our novel chiral stationary phase offered good chiral separation complementarity, separating some of the tested racemates that could not be separated or were only partially separated on these two commercial columns. The influences of analyte mass, mobile phase composition, and column temperature on chiral separation have been investigated. Good repeatability, stability, and column-to-column reproducibility of the chiral stationary phase for enantioseparation have been observed. After the fabricated column had been eluted up to 400 times, the relative standard deviations (n=5) of resolution (Rs) and retention time of the separated analytes were<0.39% and<0.20%, respectively. The relative standard deviations (n=3) of Rs and retention time for column-to-column reproducibility were<4.6% and<5.2%, respectively. This study demonstrated that the new chiral stationary phase has great prospects for chiral separation in high-performance liquid chromatography.

Immobilization of Lipase B from Candida antarctica on Magnetic Nanoparticles Enhances Its Selectivity in Kinetic Resolutions of Chiral Amines with Several Acylating Agents.

In lipase-catalyzed kinetic resolutions (KRs), the choice of immobilization support and acylating agents (AAs) is crucial. Lipase B from Candida antarctica immobilized onto magnetic nanoparticles (CaLB-MNPs) has been successfully used for diverse KRs of racemic compounds, but there is a lack of studies of the utilization of this potent biocatalyst in the KR of chiral amines, important pharmaceutical building blocks. Therefore, in this work, several racemic amines (heptane-2-amine, 1-methoxypropan-2-amine, 1-phenylethan-1-amine, and 4-phenylbutan-2-amine, (±)-1a-d, respectively) were studied in batch and continuous-flow mode utilizing different AAs, such as diisopropyl malonate 2A, isopropyl 2-cyanoacetate 2B, and isopropyl 2-ethoxyacetate 2C. The reactions performed with CaLB-MNPs were compared with Novozym 435 (N435) and the results in the literature. CaLB-MNPs were less active than N435, leading to lower conversion, but demonstrated a higher enantiomer selectivity, proving to be a good alternative to the commercial form. Compound 2C resulted in the best balance between conversion and enantiomer selectivity among the acylating agents. CaLB-MNPs proved to be efficient in the KR of chiral amines, having comparable or superior properties to other CaLB forms utilizing porous matrices for immobilization. An additional advantage of using CaLB-MNPs is that the purification and reuse processes are facilitated via magnetic retention/separation. In the continuous-flow mode, the usability and operational stability of CaLB-MNPs were reaffirmed, corroborating with previous studies, and the results overall improve our understanding of this potent biocatalyst and the convenient U-shape reactor used.

Synthesis and asymmetric resolution of substituted 2-aminoindane and 2-aminotetralin derivatives

We performed the racemic synthesis and asymmetric resolution of one 2-aminoindane and three 2-aminotertalins due to their crucial biological roles in the central nervous system (CNS). For this reason, desired (±)-2-aminoindane and (±)-2-aminotetralin derivatives were synthesized starting from appropriate reagents. While highly enantio pure (+)-39, (+)-40, and (−)-40 were obtained from the reaction of racemic amines with (R)-O-acetylmandeloyl chloride followed by crystallization, hydrolysis with KOH and acidification with HCl, (S)-42 and (−)-43 were synthesized via the reaction of racemic amines with (S)-mandelic acid and hydrolysis.

Subtilisin integrated artificial plant cell walls as heterogeneous catalysts for asymmetric synthesis of (S)-amides.

Subtilisin integrated artificial plant-cell walls (APCWs) were fabricated by self-assembly using cellulose or nanocellulose as the main component. The resulting APCW catalysts are excellent heterogeneous catalysts for the asymmetric synthesis of (S)-amides. This was demonstrated by the APCW-catalyzed kinetic resolution of several racemic primary amines to give the corresponding (S)-amides in high yields with excellent enantioselectivity. The APCW catalyst can be recycled for multiple reaction cycles without loss of enantioselectivity. The assembled APCW catalyst was also able to cooperate with a homogeneous organoruthenium complex, which allowed for the co-catalytic dynamic kinetic resolution (DKR) of a racemic primary amine to give the corresponding (S)-amide in high yield. The APCW/Ru co-catalysis constitutes the first examples of DKR of chiral primary amines when subtilisin is used as a co-catalyst.

C1-symmetric aza-crown ethers as chiral shift agents for amines and amino acid derivatives

Reported herein is our recent research progress in the development of macrocyclic chiral recognition studies. Highly enantiopure C1-symmetric aza-crown ethers, which were prepared previously by our lab from catalytic enantioselective symmetry-breaking C (sp3)-H bond transformation of readily available and inexpensive aza-crown ethers, were useful 1H NMR chiral shift reagents to resolve racemic amines and amino acid derivatives, providing an easy and rapid NMR assay of enantiomeric purity of chiral compounds, especially chromophore-devoid ones that are not detected and quantified by colorimetric method.

Deracemization of Racemic Amine Using ω-Transaminase and a Nickel-Based Nanocatalyst

Chiral amines are key building blocks for the development of numerous bioactive compounds. In this study, we developed a concurrent chemoenzymatic cascade approach using ω-transaminase for the isomeric configuration inversion of a racemic amine mixture. One isomer was transaminated using ω-transaminase, generating coproduct ketones and an additional chiral substance. Then, the mixture underwent selective reductive amination of a ketone using a specially designed compatible nickel-based nanocatalyst, which transformed coproduct ketone to racemic amines while leaving the opposite enantiomer unchanged. The combination of the two steps in one reaction system functions as an overall isomeric configuration inversion system. Moreover, the desired chiral amines with an additional chiral substance were formed. The procedure consumed NH3 and generated H2O as the sole byproduct.

Enantioselective Alkylation of Amino Acid Derivatives with ­Unactivated Olefins via C–N Bond Cleavage

AbstractWe reported a nickel-catalyzed enantioconvergent deaminative alkylation of α-amino acid derivatives with unactivated olefins, providing an efficient and convenient access to a range of α-enantioenriched amides. This method represents the first example of enantioselective deaminative functionalization with racemic amine precursors and features in mild conditions and broad substrate scope. New sterically encumbered bis(oxazoline) ligand was developed to improve both reactivity and enantioselectivity, which is key to the success of this reaction.

Enantioselective Synthesis of Pharmaceutically Relevant Bulky Arylbutylamines Using Engineered Transaminases

AbstractATAs engineered for having an enlarged small binding pocket were applied for the synthesis of enantiomerically pure (R)‐benzo[1,3]dioxol‐5‐yl‐butylamine, a chiral component of human leukocyte elastase inhibitor DMP 777 (L‐694,458). Kinetic resolution of the racemic amine was performed by using the L59A variant of the (S)‐selective ATA from Chromobacterium violaceum (Cv‐ATA), providing the residual (R)‐enantiomer in excellent yield and &gt;99% ee. At moderate enzyme loading and absence of co‐solvent, high volumetric productivity of 0.22 mol L−1 h−1 (42.5 g L−1 h−1) was achieved. Complementarily, the (S)‐enantiomer was generated via kinetic resolution using the (R)‐selective ATA‐117‐Rd11 from Arthrobacter sp. with acetone as the amino acceptor. In an alternative approach, we employed ATA‐117‐Rd11 for the asymmetric amination of the prochiral ketone precursor, which at 86% conversion gave the (R)‐benzo[1,3]dioxol‐5‐yl‐butylamine with excellent &gt;99% ee. We further evaluated the utility of Cv‐ATA L59A for the asymmetric synthesis of pharmaceutically relevant (S)‐1‐phenylbutan‐1‐amine, a chiral component of the deubiquitinase inhibitor degrasyn (WP1130). The enzyme showed good tolerance to high concentrations of isopropylamine, producing (S)‐1‐phenylbutan‐1‐amine in enantiomerically pure form (&gt;99% ee).magnified image

Recombinant l-Amino Acid Oxidase with Broad Substrate Spectrum for Co-substrate Recycling in (S)-Selective Transaminase-Catalyzed Kinetic Resolutions.

Chiral and enantiopure amines can be produced by enantioselective transaminases via kinetic resolution of amine racemates. This transamination reaction requires stoichiometric amounts of co‐substrate. A dual‐enzyme recycling system overcomes this limitation: l‐amino acid oxidases (LAAO) recycle the accumulating co‐product of (S)‐selective transaminases in the kinetic resolution of racemic amines to produce pure (R)‐amines. However, availability of suitable LAAOs is limited. Here we use the heterologously produced, highly active fungal hcLAAO4 with broad substrate spectrum. H2O2 as byproduct of hcLAAO4 is detoxified by a catalase. The final system allows using sub‐stoichiometric amounts of 1 mol% of the transaminase co‐substrate as well as the initial application of l‐amino acids instead of α‐keto acids. With an optimized protocol, the synthetic potential of this kinetic resolution cascade was proven at the preparative scale (>90 mg) by the synthesis of highly enantiomerically pure (R)‐methylbenzylamine (>99 %ee) at complete conversion (50 %).

Diisopropyl Malonate as Acylating Agent in Kinetic Resolution of Chiral Amines with Lipase B from Candida antarctica

Activity of diisopropyl malonate (2) as a novel acylating agent was investigated in kinetic resolution (KR) of various racemic amines [(±)-1a-d] catalyzed by lipase B from Candida antarctica. Diisopropyl malonate (2) proved to be effective acylating agent with four racemic amines [(±)-2-aminoheptane, (±)-1-methoxy-2-propylamine, (±)-1-phenylethylamine and (±)-4-phenylbutan-2-amine; (±)-1a-d, respectively] selected for this study. The lipase-catalyzed acylation of the amines (±)-1a-d with 2 proceeded with good conversions (44.9–52.1%) and provided the expected (R)-amides [(R)-3a-d] in moderate to excellent yields (51–98%) with high enantiomeric excess (ee(R)-3a-d 92.0–99.9%) after 4 h reaction time under mild reaction conditions in batch mode. The best conversion (50%) combined with high enantiomeric purity (ee(R)-2d &gt; 99%ee) was achieved in the KR from racemic 2-aminoheptane (±)-1a. The four novel (R)-amides [(R)-3a-d] were isolated and properly characterized.

Synthesis and biological evaluation of conformationally restricted GluN2B ligands derived from eliprodil

N-Methyl-d-aspartate (NMDA) receptors containing one or two GluN2B subunits play a crucial role in a variety of neurodegenerative diseases, such as Alzheimer's and Huntington's disease. In order to increase selectivity for GluN2B NMDA receptors, the piperidine ring of eliprodil (2) was conformationally restricted by introduction of an ethano bridge across C-2 and C-6 resulting in a tropane scaffold. Benzylidenetropanes 15 and 16 and benzyltropanes 17 and 18 were prepared by nucleophilic opening of enantiomerically pure phenyloxiranes 13 and 14 with racemic secondary amines (Z/E)-11 and diastereomeric mixtures (r/s)-12. The diastereomers were separated by preparative HPLC to obtain enantiomerically pure test compounds 15–18. The absolute and relative configuration of the products were determined by X-ray crystal structure analysis. Benzylidenetropanes 15 and 16 as well as benzyltropanes 17 and 18 display very high GluN2B affinity in receptor binding studies. Benzylidinetropanes with the phenyl moiety oriented towards C-5 of the tropane system showed higher GluN2B affinity than their analogs with the phenyl moiety oriented towards C-1. In benzyltropanes endo-configured stereoisomers exhibit higher GluN2B affinity than exo-configured diastereomers. Unfortunately, tropanes 15–18 show also high σ1 and σ2 affinity with the same trends for the stereoisomers as for GluN2B affinity. The high-affinity GluN2B ligand (R,r)-17b was able to inhibit the ion flux in two-electrode voltage clamp experiments using GluN1a/GluN2B expressing oocytes.