A reductive aminase from Aspergillus oryzae

Enantioselective reduction of sulfur-containing cyclic imines through biocatalysis

The 3-thiazolidine ring represents an important structural motif in life sciences molecules. However, up to now reduction of 3-thiazolines as an attractive approach failed by means of nearly all chemical reduction technologies for imines. Thus, the development of an efficient general and enantioselective synthetic technology giving access to a range of such heterocycles remained a challenge. Here we present a method enabling the reduction of 3-thiazolines with high conversion and high to excellent enantioselectivity (at least 96% and up to 99% enantiomeric excess). This technology is based on the use of imine reductases as catalysts, has a broad substrate range, and is also applied successfully to other sulfur-containing heterocyclic imines such as 2H-1,4-benzothiazines. Moreover the effiency of this biocatalytic technology platform is demonstrated in an initial process development leading to 99% conversion and 99% enantiomeric excess at a substrate loading of 18 g/L in the presence of designer cells.

The 3-thiazolidine ring, a pharmaceutically interesting cyclic structural element found e.g. in some antibiotics, is hard to obtain via currently used approaches. Here, the authors developed a straightforward method to efficiently synthesize a variety of defined, pure 3-thiazolidines.

Transaminases for chiral amine synthesis

Amine transaminases are important biocatalysts for the synthesis of chiral primary amines. Unlike many enzymes that have been employed for the synthesis of optically active amines, amine transaminases are capable of asymmetric synthesis and do not rely on costly cofactors that must be regenerated in situ. However, their application as general catalysts for the preparation of amines is hampered by a limited substrate scope, substrate and (co)product inhibition and difficulties associated with displacing challenging reaction equilibrium. There has been important progress made to overcome these challenges, including the development of enzymes with broader substrate scope and the design of methodology to effectively displace the reaction equilibrium. Amine transaminases are also being applied in an increasing range of (chemo)enzymatic cascades and immobilized for applications in flow.

Biomimetic synthesis of 2-substituted N-heterocycle alkaloids by one-pot hydrolysis, transamination and decarboxylative Mannich reaction

A novel multi-enzymatic approach enabled the facile synthesis of a broad range of biologically active 2-substituted piperidine and pyrrolidine alkaloids.

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A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as tryptorubin A isolated from a Streptomyces species.

Synthesis and Reactivity of Propargylamines in Organic Chemistry

Propargylamines are a versatile class of compounds which find broad application in many fields of chemistry. This review aims to describe the different strategies developed so far for the synthesis of propargylamines and their derivatives as well as to highlight their reactivity and use as building blocks in the synthesis of chemically relevant organic compounds. In the first part of the review, the different synthetic approaches to synthesize propargylamines, such as A³ couplings and C-H functionalization of alkynes, have been described and organized on the basis of the catalysts employed in the syntheses. Both racemic and enantioselective approaches have been reported. In the second part, an overview of the transformations of propargylamines into heterocyclic compounds such as pyrroles, pyridines, thiazoles, and oxazoles, as well as other relevant organic derivatives, is presented.

Biocatalytic Routes to Enantiomerically Enriched Dibenz[c,e]azepines

Biocatalytic retrosynthetic analysis of dibenz[c,e]azepines has highlighted the use of imine reductase (IRED) and ω-transaminase (ω-TA) biocatalysts to establish the key stereocentres of these molecules. Several enantiocomplementary IREDs were identified for the synthesis of (R)- and (S)-5-methyl-6,7-dihydro-5H-dibenz[c,e]azepine with excellent enantioselectivity, by reduction of the parent imines. Crystallographic evidence suggests that IREDs may be able to bind one conformer of the imine substrate such that, upon reduction, the major product conformer is generated directly. ω-TA biocatalysts were also successfully employed for the production of enantiopure 1-(2-bromophenyl)ethan-1-amine, thus enabling an orthogonal route for the installation of chirality into dibenz[c,e]azepine framework.

Photometric Characterization of the Reductive Amination Scope of the Imine Reductases from Streptomyces tsukubaensis and Streptomyces ipomoeae

Imine reductases (IREDs) have emerged as promising enzymes for the asymmetric synthesis of secondary and tertiary amines starting from carbonyl substrates. Screening the substrate specificity of the reductive amination reaction is usually performed by time-consuming GC analytics. We found two highly active IREDs in our enzyme collection, IR-20 from Streptomyces tsukubaensis and IR-Sip from Streptomyces ipomoeae, that allowed a comprehensive substrate screening with a photometric NADPH assay. We screened 39 carbonyl substrates combined with 17 amines as nucleophiles. Activity data from 663 combinations provided a clear picture about substrate specificity and capabilities in the reductive amination of these enzymes. Besides aliphatic aldehydes, the IREDs accepted various cyclic (C₄ -C₈ ) and acyclic ketones, preferentially with methylamine. IR-Sip also accepted a range of primary and secondary amines as nucleophiles. In biocatalytic reactions, IR-Sip converted (R)-3-methylcyclohexanone with dimethylamine or pyrrolidine with high diastereoselectivity (>94-96 % de). The nucleophile acceptor spectrum depended on the carbonyl substrate employed. The conversion of well-accepted substrates could also be detected if crude lysates were employed as the enzyme source.

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.

A biocatalytic cascade for the amination of unfunctionalised cycloalkanes

Here we describe a one-pot, three-enzyme, cascade involving a cytochrome P450 monooxygenase, an alcohol dehydrogenase and a reductive aminase for the synthesis of secondary amines from cycloalkanes.