Racemic Amines Research Articles

Chalcogen‐Containing Diols: A Novel Chiral Derivatizing Agent for 77Se and 125Te NMR Chiral Recognition of Primary Amines

AbstractIn this work, a novel chiral diol containing a chalcogen was developed for the 77Se and 125Te NMR chiral recognition of racemic primary amines. The enantiopure chiral derivatizing agents were synthesized from a readily available and versatile (S)‐solketal reagent. The synthesis was performed in three steps with satisfactory yields. The chiral derivatizing agents containing selenium or tellurium nuclides were evaluated for effective NMR chiral discrimination of racemic primary amines by a simple three component chiral derivatization protocol through the modified James‐Bull method. The split 77Se and 125Te NMR signals are well‐distinguishable and greater than 1H NMR signals. The Nuclear Overhauser Spectroscopy (NOESY) NMR 2D analysis and semi‐empirical method suggest a conformational preference of diastereoisomers, that provide information of the absolute configuration using selenium and tellurium chiral derivatizing agents.

Artificial Nanometalloenzymes for Cooperative Tandem Catalysis.

Artificial metalloenzymes that combine the advantages of natural enzymes and metal catalysts have been getting more attention in research. As a proof of concept, an artificial nanometalloenzyme (CALB-Shvo@MiMBN) was prepared by co-encapsulation of metallo-organic catalyst and enzyme in a soft nanocomposite consisting of 2-methylimidazole, metal ions, and biosurfactant in mild reaction conditions using a one-pot self-assembly method. The artificial nanometalloenzyme with lipase acted as the core, and the metallo-organic catalyst embedded in micropore exhibited a spherical structure of 30-50 nm in diameter. The artificial nanometalloenzyme showed high catalytic efficiency in the dynamic kinetic resolution of racemic primary amines or secondary alcohols compared to the one-pot catalytic reaction of immobilized lipase and free metallo-organic catalyst. This artificial nanometalloenzyme holds great promise for integrated enzymatic and heterogeneous catalysis.

Deracemization of Racemic Amines to Enantiopure (R)‐ and (S)‐amines by Biocatalytic Cascade Employing ω‐Transaminase and Amine Dehydrogenase

AbstractA one‐pot deracemization strategy for α‐chiral amines is reported involving an enantioselective deamination to the corresponding ketone followed by a stereoselective amination by enantiocomplementary biocatalysts. Notably, this cascade employing a ω‐transaminase and amine dehydrogenase enabled the access to both (R)‐and (S)‐amine products, just by controlling the directions of the reactions catalyzed by them. A wide range of (R)‐and (S)‐amines was obtained with excellent conversions (>80 %) and enantiomeric excess (>99 % ee). Finally, preparative scale syntheses led to obtain enantiopure (R)‐ and (S)‐13 with the isolated yields of 53 and 75 %, respectively.

Diastereoselective Acylation of Racemic Heterocyclic Amines with N-Phthaloyl and N-Naphthaloyl (S)-Amino Acyl Chlorides: Possibility of Parallel Kinetic Resolution

The acylative kinetic resolution of racemic 2-methyl-1,2,3,4-tetrahydroquinoline and 3,4-dihydro-3-methyl-2H-[1,4]benzoxazines with acyl chlorides of N-naphthaloyl-(S)-alanine and N-naphthaloyl-(S)-phenylalanine has been studied. It has been shown that diastereoselective acylation of racemic amines with N-naphthaloyl (S)-amino acyl chlorides results in the predominant formation of (R,S)-amides, whereas acylation of the same amines with N-phthaloyl (S)-amino acyl chlorides proceeds with the opposite diastereoselectivity. The parallel kinetic resolution of racemic 3,4-dihydro-3-methyl-2H-[1,4]benzoxazine using a mixture of acylating agents derived from a single precursor, (S)-phenylalanine, was carried out.

Lipase B from Candida antarctica Immobilized on Epoxy-functionalized Hollow Silica Microspheres: Efficient Biocatalysts for Enantiomer Selective Acylation of Alcohols and Amines

Hollow silica microspheres with promising physical properties (MAT540TM) as support for enzyme immobilization and biocatalyst were investigated in this study. The amine-functionalized MAT540TM was activated by six bisepoxides inclosing different spacers and used as epoxy-functionalized carrier for immobilization of lipase B from Candida antarctica (CaLB). The novel, covalently fixed CaLB biocatalysts were compared in kinetic resolution (KR) of racemic 1-phenyethanol rac-1 and five racemic amines rac-3a-e using shaken flasks and continuous-flow packed-bed microreactors. Mechanic stability, re-usability and the effect of temperature (0–90 °C) on productivity and enantiomer selectivity of the covalently immobilized CaLB were investigated. The best performing CaLB biocatalyst showed good mechanic stability after 24 h operation time in continuous-flow mode at 60 °C and provided in KRs of racemic 1-phenyethanol rac-1 with vinyl acetate and of five racemic amines with isopropyl 2-ethoxyacetate as acylating agent the non-reacted (S)-alcohol [(S)-1] or (S)-amines [(S)-3a-e] and the forming (R)-ester [(R)-2] or (R)-amide [(R)-4a-e] in good yields with high enantiomeric excess (ee > 99 %, for all).

Enantioselective Biocatalyzed Acetylation of Racemic Amines

Enantioselective Biocatalyzed Acetylation of Racemic Amines

Mechanoenzymatic resolution of racemic chiral amines, a green technique for the synthesis of pharmaceutical building blocks

Very recently, several successful biocatalytic processes using mechanical activation to carry out enantioselective reactions have been reported. The present work describes the use of Candida antarctica Lipase B (CALB) in the kinetic resolution of racemic chiral amines by means of High-Speed Ball Milling (HSBM), minimizing the use of solvent and reducing reaction times. This approach provides an efficient and easily scalable strategy that was used for the green synthesis of enantiopure Rasagiline, as an illustrative example.

Enhanced performances of enantioselective permeation through one-handed helical polymer membranes by enantioselective imine exchange reaction with permeants and by partially decomposed reaction of the membrane

Optical resolution by membrane permeation is a very effective continuous method compared with conventional batch methods. We previously reported some optical resolution by simple permeation through chiral poly(substituted acetylene) membranes. In this study, the performances have been improved by the following two methods. One is enantioselective imine exchange reaction of the side imino groups of the polymer membranes with a racemic chiral amine as permeants during permeation. The other is permeation through new composite membranes containing supramolecular polymers by partial in-situ decomposition reaction of the polymer membranes. As a result, permselectivity was enhanced up to 88.5%ee for (RS)-phenylethylamine (PEA).

Cooperative asymmetric reactions combining photocatalysis and enzymatic catalysis.

Living organisms rely on simultaneous reactions catalysed by mutually compatible and selective enzymes to synthesize complex natural products and other metabolites. To combine the advantages of these biological systems with the reactivity of artificial chemical catalysts, chemists have devised sequential, concurrent, and cooperative chemoenzymatic reactions that combine enzymatic and artificial catalysts1-9. Cooperative chemoenzymatic reactions consist of interconnected processes that generate products in yields and selectivities that cannot be obtained when the two reactions are carried out sequentially with their respective substrates2,7. However, such reactions are difficult to develop because chemical and enzymatic catalysts generally operate in different media at different temperatures and can deactivate each other1-9. Owing to these constraints, the vast majority of cooperative chemoenzymatic processes that have been reported over the past 30 years can be divided into just two categories: chemoenzymatic dynamic kinetic resolutions of racemic alcohols and amines, and enzymatic reactions requiring the simultaneous regeneration of a cofactor2,4,5. New approaches to the development of chemoenzymatic reactions are needed to enable valuable chemical transformations beyond this scope. Here we report a class of cooperative chemoenzymatic reaction that combines photocatalysts that isomerize alkenes with ene-reductases that reduce carbon-carbon double bonds to generate valuable enantioenriched products. This method enables the stereoconvergent reduction of E/Z mixtures of alkenes or reduction of the unreactive stereoisomers of alkenes in yields and enantiomeric excesses that match those obtained from the reduction of the pure, more reactive isomers. The system affords a range of enantioenriched precursors to biologically active compounds. More generally, these results show that the compatibility between photocatalysts and enzymes enables chemoenzymatic processes beyond cofactor regeneration and provides a general strategy for converting stereoselective enzymatic reactions into stereoconvergent ones.

Self-sustaining closed-loop multienzyme-mediated conversion of amines into alcohols in continuous reactions

The synthesis of alcohols from amines starting material is an excellent, yet challenging, strategy for the preparation of pharmaceuticals and polymers. Here, we developed a versatile, self-sustaining closed-loop multienzymatic platform for the biocatalytic synthesis of a large range of non-commercially available products in continuous flow with excellent yields (80>99%), reaction times and optical purity of secondary alcohols (>99 e.e.). This process was also extended to the conversion of biogenic amines into high value alcohols, such as the powerful anti-oxidant hydroxytyrosol, and the synthesis of enantiopure 2-arylpropanols via dynamic kinetic resolution of commercially affordable racemic amines. The system exploits the in situ immobilisation of transaminases and redox enzymes which were combined to cater for a fully automated, ultra-efficient synthetic platform with cofactor recycling, in-line recovery of benign by-products and recirculation of the aqueous media containing the recycled cofactors in catalytic amount, which increases the efficiency of the system by over 20-fold.

Optimization of 2-alkoxyacetates as acylating agent for enzymatic kinetic resolution of chiral amines

In this study, the activity of acetic acid esters modified with electron withdrawing 2-alkoxy-groups was investigated as acylating agent in kinetic resolution (KR) of racemic amines. A homologous series of the isopropyl esters of four 2-alkoxyacetic acids (2-methoxy-, 2-ethoxy-, 2-propoxy- and 2-butoxyacetic acids) were prepared and investigated for enantiomer selective N-acylation, catalyzed by lipase B from Candida antarctica, under batch and continuous-flow conditions. In the first set of experiments, isopropyl 2-propoxyacetate showed the highest effectivity with all of the four racemic amines [(±)-1-phenylethylamine, (±)-4-phenylbutan-2-amine, (±)-heptan-2-amine and (±)-1-methoxypropane-2-amine] in the set enabling excellent conversions (≥46%) and enantiomeric excess values (ee ≥ 99%) with each amines in continuous-flow mode KRs under the optimized reaction conditions. In a second set of experiments, KRs of five additional amines – being substituted derivatives of (±)-1-phenylethylamine – further demonstrated the usefulness of isopropyl 2-propoxyacetate – being the best acylating agent in the first set of KRs – in KRs leading to (R)-N-propoxyacetamides with high ee values (≥99.8%).

Resolution of racemic amines via lipase-catalyzed benzoylation: Chemoenzymatic synthesis of the pharmacologically active isomers of labetalol

Lipase-catalyzed benzoylation of amines was shown to be feasible, in some cases with high enantioselectivity, and the best results were obtained using immobilized lipase from Candida antarctica (Novozym 435) and methyl benzoate as acyl donor in the presence of molecular sieves. The procedure was optimized for the resolution of (±)-1-methyl-3-phenylpropylamine, a key intermediate in the synthesis of antihypertensive drug labetalol, and the enantiopure (R)-benzamide was then converted into the pharmacologically active isomers of the drug. In comparison with the reported synthesis of chiral isomers of labetalol, this chemoenzymatic route offers the advantage in the lack of any chiral stoichiometric auxiliary.

Enantioselective biotransformation of sterically hindered amine substrates by the fungus Stemphylium lycopersici.

To screen 20 micro-organisms for ω-transaminase (ω-TA) activity by the kinetic resolution of rac-1-phenylethylamine, followed by testing rac-amines of pharmaceutical interest with bulky substituents and to conduct the asymmetric synthesis of a chiral amine. Stemphylium lycopersici was selected as the best biocatalyst. By the central composite rotatable design (CCRD), it was found that, at lower pH (5·5 and 6·5), the lyophilized micro-organism biocatalysed the kinetic resolution of rac-1-phenylethylamine with 99% enantiomeric excess (e.e.) ((R)-enantiomer) with acetophenone conversions ranged from 41 to 45%. Interestingly, the lyophilized crude enzymatic extract lead to better results at pH from 7·0 to 9·0, with conversions up to 47% and about 99% e.e. We also attested that as much as higher is the pyruvate (amino acceptor) concentration, higher is the acetophenone conversion, corroborating the presence of ω-TA-type enzymes. Among different sterically hindered racemic amines tested, rac-1,2,3,4-tetrahydro-1-naphthylamine and rac-phenylbutylamine were satisfactorily kinetically resolved in up to 91% e.e. (R). The results for the asymmetric synthesis showed excellent conversion (>85%) for the S-1-phenylethylamine, indicating (S)-stereopreference. Stemphylium lycopersici showed to be an important tool for broader substrate scope transaminases and a relevant player on the development of new biocatalysts with ability in asymmetric synthesis reactions. Here in, we contribute to the improvement of the biocatalytic toolbox for chiral amines synthesis. Interestingly, we have found that the crude enzymatic extract of the endophytic fungus S. lycopersici could accept bulky substrates with reasonable activity, compared to the wild-type transaminase already published over literature, and with high enantioselectivity.

Process Development for the Synthesis of Monocyclic β-Lactam Core 17

Process development and multikilogram synthesis of the monocyclic β-lactam core 17 for a novel pyridone-conjugated monobactam antibiotic is described. Starting with commercially available 2-(2,2-diethoxyethyl)isoindoline-1,3-dione, the five-step synthesis features several telescoped operations and direct isolations to provide significant improvement in throughput and reduced solvent usage over initial scale-up campaigns. A particular highlight in this effort includes the development of an efficient Staudinger ketene–imine [2 + 2] cycloaddition reaction of N-Boc-glycine ketene 12 and imine 9 to form racemic β-lactam 13 in good isolated yield (66%) and purity (97%). Another key feature in the synthesis involves a classical resolution of racemic amine 15 to afford single enantiomer salt 17 in excellent isolated yield (45%) with high enantiomeric excess (98%).

Covalently immobilized Trp60Cys mutant of ω-transaminase from Chromobacterium violaceum for kinetic resolution of racemic amines in batch and continuous-flow modes

Covalent immobilization of an engineered ω-transaminase mutant Trp60Cys from Chromobacterium violaceum (CvTAW60C) was performed on bisepoxide-activated aminoalkyl resins. Activity of the various CvTAW60C preparations was evaluated in kinetic resolution of four racemic amines (rac-1a–d). The most active EA-G-CvTAW60C preparation (CvTAW60C attached to polymeric resin with ethylamine function activated with glycerol diglycidyl ether—EA-G) could perform the kinetic resolution of racemic 4-phenylbutan-2-amine (rac-1a) over 49% conversion up to 19 consecutive reaction cycles or in media containing up to 50% v/v DMSO as cosolvent in batch mode reactions. The immobilization process of CvTAW60C onto the EA-G resin filled in stainless steel bioreactors was also tested in flow-through mode. Kinetic resolution of three racemic amines containing aromatic moieties (rac-1a-c) was performed in continuous-flow mode resulting in easy-to-separate mixture of the corresponding ketone (2a–c) and the non-converted (R)-amine in high enantiopurity (ee(R)-1a-c ≥ 96%).

Kinetic Resolution and Deracemization of Racemic Amines Using a Reductive Aminase

AbstractThe NADP(H)‐dependent reductive aminase from Aspergillus oryzae (AspRedAm) was combined with an NADPH oxidase (NOX) to develop a redox system that recycles the co‐factor. The AspRedAm‐NOX system was applied initially for the kinetic resolution of a variety of racemic secondary and primary amines to yield S‐configured amines with enantiomeric excess (ee) values up to 99 %. The addition of ammonia borane to this system enabled the efficient deracemization of racemic amines, including the pharmaceutical drug rasagiline and the natural product salsolidine, with conversions up to >98 % and >99 % ee Furthermore, by using the AspRedAm W210A variant it was possible to generate the opposite R enantiomers with efficiency comparable to, or even better than, the wildtype AspRedAm.

Resolution of Racemic Mixtures by Phase Transition of PEGylated Resolving Agents.

A novel, efficient, and simple method for the resolution of racemic mixtures is presented in which PEGylated resolving agents are subjected to diastereomeric complex formation in alcohols. The resulting complexes then undergo temperature-assisted phase transition, affording a precipitate that is enriched in one enantiomer and separable by filtration. In an aqueous solution, phase transition can be caused by the methods used in the precipitation of poly(ethylene glycol) (e.g., addition of ammonium sulfate). A number of racemic amines have been successfully resolved using this method. The first cycle of resolution affords the amines with an optical purity of 72–85% from their corresponding racemic mixture in good yields (78–90%). An additional cycle improved the optical purity to 87–95%. The PEGylated resolving agents can be recovered and reutilized without the loss of resolution efficiency.

The Kinetic Resolution of Racemic Amines Using a Whole-Cell Biocatalyst Co-Expressing Amine Dehydrogenase and NADH Oxidase

Amine dehydrogenase (AmDH) possesses tremendous potential for the synthesis of chiral amines because AmDH catalyzes the asymmetric reductive amination of ketone with high enatioselectivity. Although a reductive application of AmDH is favored in practice, the oxidative route is interesting as well for the preparation of chiral amines. Here, the kinetic resolution of racemic amines using AmDH was first extensively studied, and the AmDH reaction was combined with an NADH oxidase (Nox) to regenerate NAD+ and to drive the reaction forward. When the kinetic resolution was carried out with 10 mM rac-2-aminoheptane and 5 mM rac-α-methylbenzylamine (α-MBA) using purified enzymes, the enantiomeric excess (ee) values were less than 26% due to the product inhibition of AmDH by ketone and the inhibition of Nox by the substrate amine. The use of a whole-cell biocatalyst co-expressing AmDH and Nox apparently reduces the substrate and product inhibition, and/or it increases the stability of the enzymes. Fifty millimoles (50 mM) rac-2-aminoheptane and 20 mM rac-α-MBA were successfully resolved into the (S)-form with >99% ee using whole cells. The present study demonstrates the potential of a whole-cell biocatalyst co-expressing AmDH and Nox for the kinetic resolution of racemic amines.

Mapping the substrate scope of monoamine oxidase (MAO-N) as a synthetic tool for the enantioselective synthesis of chiral amines

A library of 132 racemic chiral amines (α-substituted methylbenzylamines, benzhydrylamines, 1,2,3,4-tetrahydronaphthylamines (THNs), indanylamines, allylic and homoallylic amines, propargyl amines) was screened against the most versatile monoamine oxidase (MAO-N) variants D5, D9 and D11. MAO-N D9 exhibited the highest activity for most substrates and was applied to the deracemisation of a comprehensive set of selected primary amines. In all cases, excellent enantioselectivity was achieved (e.e. >99%) with moderate to good yields (55–80%). Conditions for the deracemisation of primary amines using a MAO-N/borane system were further optimised using THN as a template addressing substrate load, nature of the enzyme preparation, buffer systems, borane sources, and organic co-solvents.

Exploiting the Biocatalytic Toolbox for the Asymmetric Synthesis of the Heart‐Rate Reducing Agent Ivabradine

AbstractSeveral chemoenzymatic routes have been evaluated for the production of the heart‐rate reducing agent ivabradine. Lipases and ω‐transaminases have been identified as useful biocatalysts for the preparation of key enantiopure precursors. The lipase‐catalysed kinetic resolution by alkoxycarbonylation of a racemic primary amine and subsequent chemical reduction of the resulting carbamate provided an N‐methylated (S)‐amine, one step away from ivabradine. Alternatively, the dynamic kinetic resolution by asymmetric bioamination of an aldehyde precursor enabled, in a four‐step sequence, the preparative scale synthesis of enantiopure ivabradine in 50% overall yield.magnified image