Development of an in situ-Product Crystallization (ISPC)-Concept to Shift the Reaction Equilibria of Selected Amine Transaminase-Catalyzed Reactions

Crystallization-Assisted Asymmetric Synthesis of Enantiopure Amines Using Membrane-Immobilized Transaminase

The production of active pharmaceutical ingredients (APIs) requires enantiopure chiral amines, for which greener synthesis processes are needed. Transaminases (TAs) are enzymes that catalyze the enantioselective production of chiral amines from prochiral ketones through transamination under mild conditions. Yet, industrial applications of biocatalytic transamination remain currently hindered by the limited stability of soluble enzymes and by the unfavorable thermodynamic equilibrium of targeted asymmetric reactions. Enzyme immobilization can be applied to address stability, recoverability, and reusability issues. In the perspective of process intensification, we chose to immobilize TAs on polymeric (polypropylene) membranes. In the asymmetric synthesis of (R)-2-fluoro-α-methylbenzylamine ((R)-FMBA), such membrane-immobilized TAs exhibited superior specific activity and stability compared with soluble TAs; they also outperformed TAs immobilized on resins. The reaction yield remained, however, limited by thermodynamics. To further enhance the synthesis yield, the reaction was coupled with the in situ crystallization of (R)-FMBA with 3,3-diphenylpropionic acid (DPPA). By doing so, the theoretical equilibrium conversion was pushed from ∼44% to ∼83%. In fact, a 72% overall recovery yield of crystallized (R)-FMBA was demonstrated. The enantioselectivity of the reaction mixture was preserved. Importantly, purification was greatly facilitated since the target enantiopure amine was readily recovered as high-purity (R)-FMBA:DPPA crystals. The biocatalytic membranes were found to be fully reusable, performing successive high-yield asymmetric syntheses with only minor deactivation. Overall, the crystallization-assisted strategy proposed herein offers a greener path for the biocatalytic production of valuable chiral targets.

Challenges and good practices on transaminase-catalysed synthesis of optically pure amines

Transaminase-catalysed synthesis of chiral amines has been highlighted as an important strategy to get access to optically pure primary amines with high selectivity. However, their application is still hindered from several factors. The co-factor instability leads to instability of the protein itself, while the thermodynamics typically do not favor the desired amination reaction. Thus, several strategies have been suggested to tackle the thermodynamic issue, while parameters that initially may seem trivial, such as selection of buffer salt, pH and temperature, recently were studied more thoroughly. In this chapter we provide a review on the suggested strategies with specific commentaries on their application, as well as protocol for the synthesis of optically pure amines with the two most commonly used amine donors, namely alanine and isopropylamine.

Crystallization Assisted Dynamic Kinetic Resolution for the Synthesis of (R)-β-Methylphenethylamine

This study explores a combination of the concept of enantioselective enzymatic synthesis of β-chiral amines through transamination with in situ product crystallization (ISPC) to overcome product inhibition. Using 2-phenylpropanal as a readily available and easily racemizing substrate of choice, (R)-β-methylphenethylamine ((R)-2-phenylpropan-1-amine) concentrations of up to 250 mM and enantiomeric excesses of up to 99 % are achieved when using a commercially available transaminase from Ruegeria pomeroyi in a fed-batch based dynamic kinetic resolution reaction on preparative scale. The source of substrate decomposition during the reaction is also investigated and the resulting unwanted byproduct formation is successfully reduced to insignificant levels.

Crystallization-based downstream processing of ω-transaminase- and amine dehydrogenase-catalyzed reactions

This study highlights the use of selective crystallization as a downstream-processing concept for amine products from biocatalytic reactions.

Applied biocatalysis beyond just buffers - from aqueous to unconventional media. Options and guidelines

In nature, enzymes conventionally operate under aqueous conditions. Because of this, aqueous buffers are often the choice for reaction media when enzymes are applied in chemical synthesis. However, to meet the demands of an industrial application, due to the poor water solubility of many industrially relevant compounds, an aqueous reaction system will often not be able to provide sufficient substrate loadings. A switch to a non-aqueous solvent system can provide a solution, which is already common for lipases, but more challenging for biocatalysts from other enzyme classes. The choices in solvent types and systems, however, can be overwhelming. Furthermore, some engineering of the protein structure of biocatalyst formulation is required. In this review, a guide for those working with biocatalysts, who look for a way to increase their reaction productivity, is presented. Examples reported clearly show that bulk water is not necessarily required for biocatalytic reactions and that clever solvent systems design can support increased product concentrations thereby decreasing waste formation. Additionally, under these conditions, enzymes can also be combined in cascades with other, water-sensitive, chemical catalysts. Finally, we show that the application of non-aqueous solvents in biocatalysis can actually lead to more sustainable processes. At the hand of flowcharts, following simple questions, one can quickly find what solvent systems are viable.

Concept Study for an Integrated Reactor-Crystallizer Process for the Continuous Biocatalytic Synthesis of (S)-1-(3-Methoxyphenyl)ethylamine

An integrated biocatalysis-crystallization concept was developed for the continuous amine transaminase-catalyzed synthesis of (S)-1-(3-methoxyphenyl)ethylamine, which is a valuable intermediate for the synthesis of rivastigmine, a highly potent drug for the treatment of early stage Alzheimer’s disease. The three-part vessel system developed for this purpose consists of a membrane reactor for the continuous synthesis of the product amine, a saturator vessel for the continuous supply of the amine donor isopropylammonium and the precipitating reagent 3,3-diphenylpropionate and a crystallizer in which the product amine can continuously precipitate as (S)-1-(3-methoxyphenyl)ethylammonium-3,3-diphenylpropionate.

The challenge of using isopropylamine as an amine donor in transaminase catalysed reactions

Amine transaminases (ATAs) propose an appealing alternative to transition metal catalysts as they can provide chiral amines of high purity from pro-chiral compounds by asymmetric synthesis. Industrial interest on ATAs arises from the fact that chiral amines are present in a wide spectrum of pharmaceutical and other high value-added chiral compounds and building blocks. Despite their potential as useful synthetic tools, several drawbacks such as challenges associated with the thermodynamic equilibrium can still impede their utilization. Several methods have been developed to displace the equilibrium, such as the use of alanine as an amine donor and the subsequent removal of pyruvate with a two-enzyme system, or the use of o-xylylene diamine. To date, the preferred amine donor remains isopropylamine (IPA), as the produced acetone can be removed easily under low pressure or slight heating, without complicating the process with other enzymes. Despite its small size, IPA is not widely accepted from wild-type ATAs, and this fact compromises its wide applicability. Herein, we index the reported biocatalytic aminations with IPA, comparing the sequences, while we discuss significant parameters of the process, such as the effect of temperature and pH, as well as the protein engineering and process development advances in the field. This information is expected to provide an insight for potential designs of tailor-made ATAs and IPA processes.