Deracemization of Secondary Alcohols through a Concurrent Tandem Biocatalytic Oxidation and Reduction

Enantioconvergent synthesis of chiral fluorenols from racemic secondary alcohols via Pd(ii)/chiral norbornene cooperative catalysis

An efficient protocol for the asymmetric synthesis of fluorenols has been developed through an enantioconvergent process enabled by Pd(ii)/chiral norbornene cooperative catalysis. This approach allows facile access to diverse functionalized chiral fluorenols with constantly excellent enantioselectivities, applying readily available racemic secondary ortho-bromobenzyl alcohols and aryl iodides as the starting materials.

Catalytic Photochemical Deracemization via Short-Lived Intermediates

Upon irradiation in the presence of a suitable chiral catalyst, racemic compound mixtures can be converted into enantiomerically pure compounds with the same constitution. The process is called photochemical deracemization and involves the formation of short-lived intermediates. By opening different reaction channels for the forward reaction to the intermediate and for the re-constitution of the chiral molecule, the entropically disfavored process becomes feasible. Since the discovery of the first photochemical deracemization in 2018, the field has been growing rapidly. This review comprehensively covers the research performed in the area and discusses current developments. It is subdivided according to the mode of action and the respective substrate classes. The focus of this review is on the scope of the individual reactions and on a discussion of the mechanistic details underlying the presented reaction.

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.

Chemoenzymatic Synthesis of Tenofovir

We report on novel chemoenzymatic routes toward tenofovir using low-cost starting materials and commercial or homemade enzyme preparations as biocatalysts. The biocatalytic key step was accomplished either via stereoselective reduction using an alcohol dehydrogenase or via kinetic resolution using a lipase. By employing a suspension of immobilized lipase from Burkholderia cepacia (Amano PS-IM) in a mixture of vinyl acetate and toluene, the desired (R)-ester (99% ee) was obtained on a 500 mg scale (60 mM) in 47% yield. Alternatively, stereoselective reduction of 1-(6-chloro-9H-purin-9-yl) propan-2-one (84 mg, 100 mM) catalyzed by lyophilized E. coli cells harboring recombinant alcohol dehydrogenase (ADH) from Lactobacillus kefir (E. coli/Lk-ADH Prince) allowed one to reach quantitative conversion, 86% yield and excellent optical purity (>99% ee) of the corresponding (R)-alcohol. The key (R)-intermediate was transformed into tenofovir through "one-pot" aminolysis-hydrolysis of (R)-acetate in NH3-saturated methanol, alkylation of the resulting (R)-alcohol with tosylated diethyl(hydroxymethyl) phosphonate, and bromotrimethylsilane (TMSBr)-mediated cleavage of the formed phosphonate ester into the free phosphonic acid. The elaborated enzymatic strategy could be applicable in the asymmetric synthesis of tenofovir prodrug derivatives, including 5'-disoproxil fumarate (TDF, Viread) and 5'-alafenamide (TAF, Vemlidy). The molecular basis of the stereoselectivity of the employed ADHs was revealed by molecular docking studies.

Catalytic Deracemization Reactions

Deracemization, which converts a racemate into its single enantiomer without separation of the intermediate, has gained renewed interest in asymmetric synthesis with its inherent atomic economy and high efficiency. However, this ideal process requires selective energy input and delicate reaction design to surmount the thermodynamical and kinetical constraints. With the rapid development of asymmetric catalysis, many catalytic strategies in concert with exogenous energy input have been exploited to facilitate this nonspontaneous enantioenrichment. In this perspective, we will discuss the basic ideas to accomplish catalytic deracemization, categorized by the three major exogenous energy sources including chemical (redox)-, photo- and mechanical energy from attrition. Emphasis will be given to the catalytic features and the underlying deracemization mechanism together with perspectives on future development.

Chemoenzymatic deracemization of lisofylline catalyzed by a (laccase/TEMPO)-alcohol dehydrogenase system

This article reports on a novel biocatalytic method for the synthesis of both enantiomers of lisofylline based on Trametes versicolor laccase, TEMPO as a redox mediator and stereocomplementary recombinant alcohol dehydrogenases as biocatalysts.

Shortening Synthetic Routes to Small Molecule Active Pharmaceutical Ingredients Employing Biocatalytic Methods

Biocatalysis, using enzymes for organic synthesis, has emerged as powerful tool for the synthesis of active pharmaceutical ingredients (APIs). The first industrial biocatalytic processes launched in the first half of the last century exploited whole-cell microorganisms where the specific enzyme at work was not known. In the meantime, novel molecular biology methods, such as efficient gene sequencing and synthesis, triggered breakthroughs in directed evolution for the rapid development of process-stable enzymes with broad substrate scope and good selectivities tailored for specific substrates. To date, enzymes are employed to enable shorter, more efficient, and more sustainable alternative routes toward (established) small molecule APIs, and are additionally used to perform standard reactions in API synthesis more efficiently. Herein, large-scale synthetic routes containing biocatalytic key steps toward >130 APIs of approved drugs and drug candidates are compared with the corresponding chemical protocols (if available) regarding the steps, reaction conditions, and scale. The review is structured according to the functional group formed in the reaction.

Visible-Light-Driven Catalytic Deracemization of Secondary Alcohols

Deracemization of racemic chiral compounds is an attractive approach in asymmetric synthesis, but its development has been hindered by energetic and kinetic challenges. Here we describe a catalytic deracemization method for secondary benzylic alcohols which are important synthetic intermediates and end products for many industries. Driven by visible light only, this method is based on sequential photochemical dehydrogenation followed by enantioselective thermal hydrogenation. The combination of a heterogeneous dehydrogenation photocatalyst and a chiral molecular hydrogenation catalyst is essential to ensure two distinct pathways for the forward and reverse reactions. These reactions convert a large number of racemic aryl alkyl alcohols into their enantiomerically enriched forms in good yields and enantioselectivities.

Biocatalytic Reduction Reactions from a Chemist's Perspective

Reductions play a key role in organic synthesis, producing chiral products with new functionalities. Enzymes can catalyse such reactions with exquisite stereo-, regio- and chemoselectivity, leading the way to alternative shorter classical synthetic routes towards not only high-added-value compounds but also bulk chemicals. In this review we describe the synthetic state-of-the-art and potential of enzymes that catalyse reductions, ranging from carbonyl, enone and aromatic reductions to reductive aminations.

Biocatalytic Oxidation of Alcohols

Enzymatic methods for the oxidation of alcohols are critically reviewed. Dehydrogenases and oxidases are the most prominent biocatalysts, enabling the selective oxidation of primary alcohols into aldehydes or acids. In the case of secondary alcohols, region and/or enantioselective oxidation is possible. In this contribution, we outline the current state-of-the-art and discuss current limitations and promising solutions.

Redox deracemization of β,γ-alkynyl α-amino esters

The first non-enzymatic redox deracemization method using molecular oxygen as the terminal oxidant has been described. The one-pot deracemization of β,γ-alkynyl α-amino esters consisted of a copper-catalyzed aerobic oxidation and chiral phosphoric acid-catalyzed asymmetric transfer hydrogenation with excellent functional group compatibility. By using benzothiazoline as the reducing reagent, an exclusive chemoselectivity at the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N bond over the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond was achieved, allowing for efficient deracemization of a series of α-amino esters bearing diverse α-alkynyl substituent patterns. The origins of chemo- and enantio-selectivities were elucidated by experimental and computational mechanistic investigation. The generality of the strategy is further demonstrated by efficient deracemization of β,γ-alkenyl α-amino esters.

A one-pot deracemization of β,γ-alkynyl α-amino esters consisting of an aerobic oxidation and chiral phosphoric acid-catalyzed asymmetric transfer hydrogenation has been described.

Redox deracemization of diarylmethyl alkynes

A highly effective one-pot redox deracemization of diarylmethyl alkynes has been disclosed.

Rhodococcus as A Versatile Biocatalyst in Organic Synthesis

The application of purified enzymes as well as whole-cell biocatalysts in synthetic organic chemistry is becoming more and more popular, and both academia and industry are keen on finding and developing novel enzymes capable of performing otherwise impossible or challenging reactions. The diverse genus Rhodococcus offers a multitude of promising enzymes, which therefore makes it one of the key bacterial hosts in many areas of research. This review focused on the broad utilization potential of the genus Rhodococcus in organic chemistry, thereby particularly highlighting the specific enzyme classes exploited and the reactions they catalyze. Additionally, close attention was paid to the substrate scope that each enzyme class covers. Overall, a comprehensive overview of the applicability of the genus Rhodococcus is provided, which puts this versatile microorganism in the spotlight of further research.

Synthesis of enantiomerically pure alcohols and amines via biocatalytic deracemisation methods

Deracemisation via chemo-enzymatic or multi-enzymatic approaches is the optimum substitute for kinetic resolution, which suffers from the limitation of a theoretical maximum 50% yield albeit high enantiomeric excess is attainable. This review covers the recent progress in various deracemisation approaches applied to the synthesis of enantiomerically pure alcohols and amines, such as (1) dynamic kinetic resolution, (2) cyclic deracemisation, (3) linear deracemisation (including stereoinversion) and (4) enantioconvergent methods.

Switching the substrate preference of fungal aryl-alcohol oxidase: towards stereoselective oxidation of secondary benzyl alcohols

Using PELE computational simulations the ability to deracemize secondary benzylic alcohols was introduced (by I500M/F501W double mutation) in stereoselective AAO.

Manipulating the stereoselectivity of a thermostable alcohol dehydrogenase by directed evolution for efficient asymmetric synthesis of arylpropanols

Chiral arylpropanols are valuable components in important pharmaceuticals and fragrances, which is the motivation for previous attempts to prepare these building blocks enantioselectively in asymmetric processes using either enzymes or transition metal catalysts. Thus far, enzymes used in kinetic resolution proved to be best, but several problems prevented ecologically and economically viable processes from being developed. In the present study, directed evolution was applied to the thermostable alcohol dehydrogenase TbSADH in the successful quest to obtain mutants that are effective in the dynamic reductive kinetic resolution (DYRKR) of racemic arylpropanals. Using rac-2-phenyl-1-propanal in a model reaction, (S)- and (R)-selective mutants were evolved which catalyzed DYRKR of this racemic substrate with formation of the respective (S)- and (R)-alcohols in essentially enantiomerically pure form. This was achieved on the basis of an unconventional form of iterative saturation mutagenesis (ISM) at randomization sites lining the binding pocket using a reduced amino acid alphabet. The best mutants were also effective in the DYRKR of several other structurally related racemic aldehydes.

Chemoenzymatic Deracemization of Chiral Sulfoxides

The highly enantioselective enzyme methionine sulfoxide reductase A was combined with an oxaziridine-type oxidant in a biphasic setup for the deracemization of chiral sulfoxides. Remarkably, high ee values were observed with a wide range of substrates, thus providing a practical route for the synthesis of enantiomerically pure sulfoxides.

Redox deracemization of 1,3,4,9-tetrahydropyrano[3,4-b]indoles

The existing asymmetric synthesis of enantiopure α-substituted cyclic ethers predominantly relies on the enantioselective C-C bond formation involving a prochiral oxocarbenium ion intermediate. In such a strategy, enantioselectivity and efficiency are typically susceptible to the electronic and substituent effects of either nucleophile or electrophile partners. Here, we describe a strategically different redox deracemization of α-substituted 1,3,4,9-tetrahydropyrano[3,4-b]indoles. This method exhibits good compatibility with the regional variation of the electronic or substituent effect of substrates, thus providing a practical and efficient supplement to the traditional strategy.

Enzymatic cascades for the stereo-complementary epimerisation of in situ generated epoxy alcohols

The synthesis of optically pure secondary epoxy alcohols from racemic allylic alcohols using a single whole-cell biocatalyst of recombinant Escherichia coli coexpressing three oxidoreductases is described. The cascade involves the concurrent action of a styrene monooxygenase that catalyzes the formation of the chiral epoxy group, and two alcohol dehydrogenases that fulfil the epimerisation of the hydroxy group. Two sets of alcohol dehydrogenases were each applied to couple with styrene monooxygenase in order to realize the epimerisation in a stereo-complementary manner. Excellent enantio- and diastereo-selectivities were achieved for most of the 12 substrates.

Recycling in Asymmetric Catalysis

Cyclic reaction networks consisting of an enantioselective product-forming step and a reverse reaction of the undesired enantiomer back to starting reactant are important for the generation of compounds with high enantiomeric purity. In order to avoid an equilibrium racemic state, a unidirectional cyclic process where product formation and regeneration of starting reactant proceed through different mechanistic pathways is required. Such processes must necessarily include a thermodynamically unfavorable step, since the product of the forward reaction is the reactant of the reverse reaction and vice versa. Thermodynamically uphill processes are ubiquitous to the function of living systems. Such systems gain the required energy by coupling to thermodynamically downhill reactions. In the same way, artificial cyclic reaction networks can be realized in systems open to mass or energy flow, and an out-of equilibrium nonracemic steady state can be maintained as long as the system is supplied with energy. In contrast to a kinetic resolution, a recycling process where the minor enantiomer is converted to starting reactant can result in a quantitative yield, but the enantiomeric purity of the product is limited by the selectivity of the catalysts used for the reactions. On the other hand, in a kinetic resolution, the slowly reacting enantiomer can always be obtained in an enantiomerically pure state, although the yield will suffer. In cyclic reaction systems which use chiral catalysts for both the forward and the reverse processes, a reinforcing effect results, and selectivities higher than those achieved by a single chiral catalyst are observed. A dynamic kinetic resolution can in principle also lead to a quantitative yield, but lacks the reinforcing effect of two chiral catalysts. Most examples of cyclic reaction networks reported in the literature are deracemizations of racemic mixtures, which proceed via oxidation of one enantiomer followed by reduction to the opposite enantiomer. We have developed cyclic reaction networks comprising a carbon-carbon bond formation. In these processes, the product is generated by the addition of a cyanide reagent to a prochiral aldehyde. This is followed by hydrolysis of the minor enantiomer of the product to generate the starting aldehyde. A unidirectional cycle is maintained by coupling to the exergonic transformation of the high potential cyanide reagent to a low potential compound, either a carboxylate or carbon dioxide. The products, which are obtained with high enantiomeric purity, serve as valuable starting materials for a variety of biologically and pharmaceutically active compounds.

Enantioselective cascade biocatalysis for deracemization of 2-hydroxy acids using a three-enzyme system

Background

Enantiopure 2-hydroxy acids are key intermediates for the synthesis of pharmaceuticals and fine chemicals. We present an enantioselective cascade biocatalysis using recombinant microbial cells for deracemization of racemic 2-hydroxy acids that allows for efficient production of enantiopure 2-hydroxy acids.

Results

The method was realized by a single recombinant Escherichia coli strain coexpressing three enzymes: (S)-2-hydroxy acid dehydrogenase, (R)-2-keto acid reductase and glucose dehydrogenase. One enantiomer [(S)-2-hydroxy acid] is firstly oxidized to the keto acid with (S)-2-hydroxy acid dehydrogenase, while the other enantiomer [(R)-2-hydroxy acid] remains unchanged. Then, the keto acid obtained reduced to the opposite enantiomer with (R)-2-keto acid reductase plus cofactor regeneration enzyme glucose dehydrogenase subsequently. The recombinant E. coli strain coexpressing the three enzymes was proven to be a promising biocatalyst for the cascade bioconversion of a structurally diverse range of racemic 2-hydroxy acids, giving the corresponding (R)-2-hydroxy acids in up to 98.5 % conversion and >99 % enantiomeric excess.

Conclusions

In summary, a cascade biocatalysis was successfully developed to prepare valuable (R)-2-hydroxy acids with an efficient three-enzyme system. The developed elegant cascade biocatalysis possesses high atom efficiency and represents a promising strategy for production of highly valued (R)-2-hydroxy acids.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0560-1) contains supplementary material, which is available to authorized users.

Concise Redox Deracemization of Secondary and Tertiary Amines with a Tetrahydroisoquinoline Core via a Nonenzymatic Process

A concise deracemization of racemic secondary and tertiary amines with a tetrahydroisoquinoline core has been successfully realized by orchestrating a redox process consisted of N-bromosuccinimide oxidation and iridum-catalyzed asymmetric hydrogenation. This compatible redox combination enables one-pot, single-operation deracemization to generate chiral 1-substituted 1,2,3,4-tetrahydroisoquinolines with up to 98% ee in 93% yield, offering a simple and scalable synthetic technique for chiral amines directly from racemic starting materials.