Asymmetric Synthesis of Primary and Secondary β‐Fluoro‐arylamines using Reductive Aminases from Fungi

Abstract

The synthesis of chiral amines is of central importance to pharmaceutical chemistry, and the inclusion of fluorine atoms in drug molecules can both increase potency and slow metabolism. Optically enriched β-fluoroamines can be obtained by the kinetic resolution of racemic amines using amine transaminases (ATAs), but yields are limited to 50 %, and also secondary amines are not accessible. In order to overcome these limitations, we have applied NADPH-dependent reductive aminase enzymes (RedAms) from fungal species to the reductive amination of α-fluoroacetophenones with ammonia, methylamine and allylamine as donors, to yield β-fluoro primary or secondary amines with > 90 % conversion and between 85 and 99 % ee. In addition, the effect of the progressive introduction of fluorine atoms to the α-position of the acetophenone substrate reveals the effect of mono-, di-and tri-fluorination on the proportion of amine and alcohol in product mixtures, shedding light on the promiscuous ability of imine reductase (IRED)-type dehydrogenases to reduce fluorinated acetophenones to alcohols. Chiral amines are significant functional groups that feature prominently in many pharmaceuticals. There are many asymmetric methods for their synthesis that use transition metal catalysis coupled to chiral ligands, [1,2] but considerations of selectivity, sustainability and green chemistry have dictated that biocatalytic methods have achieved prominence in recent years. [3][4][5] The synthesis of fluoroamines represents a special case of chiral amine synthesis, as the addition of the fluorine atom can improve the efficiency of bioactive molecules through increased potency or slower metabolism. [6,7] β-Fluoroamines are very interesting derivatives as they can be excellent pyridoxal 5'-phosphate (PLP)-dependent enzyme inhibitors. [8] They can be synthesized simply through the ring opening of N-tosyl aziridines with TBAF, [9] but their asymmetric synthesis is rare, and complicated if chirality at both the amine and fluorinebearing carbons is under consideration. In a recent example, Vara and Johnston used Brønsted base catalysts, such as (MeO) 2 PBAM · HNTf, for the enantioselective synthesis of Bocprotected β-amino-α-fluoro-nitroalkanes from α-fluoro arylnitromethane and aldimine precursors. [10] Other examples using organocatalytic catalysts have been provided by Lindsley and co-workers, [11,12] who performed the enantioselective fluorination of N-sulfinyl aldimines, followed by nucleophilic addition using a Grignard reagent, providing the final compounds in high dr (> 20 : 1).More recently, we showed that amine transaminases (ATAs) can be applied to the 100 mg scale asymmetric synthesis of βfluoroamines, with up to 99 % ee., through the kinetic resolution of racemic substrates. [13] A series of racemic β-fluoro arylethylamines including 1 b and 2 b (Scheme 1) was converted by both (S)-and (R)-selective ATAs to give the corresponding enantiopure β-fluoroamines, in addition to acetophenones 3 a and 4 a, co-products that arise through the enanti...

Asymmetric synthesis of primary amines catalyzed by thermotolerant fungal reductive aminases

“…Aromatic ketones could also be aminated under these conditions. As previously described, 29 NfRedAm catalyzed the reductive amination of acetophenone (8) to amine 12f with 96% ee, 4-nitroacetophenone (9), 4-uoroacetophenone (11) or 4-phenyl-2-butanone (5) gave moderate to good conversions (44-76%) yielding the corresponding (R)amine products 5f, 8f, 9f and 11f in moderate to excellent enantiomeric excess (40-96%). Other aromatic substrates such as 1-tetralone (10), 3-acetyl pyridine (12), 2-acetyl pyridine (13), 1-indanone (14) and propiophenone (16), low to poor conversions were obtained (up to 16%) although in high enantiomeric excess (>93%).…”

“…In a recent report, one of our groups described the ability of two of these homologues -NfRedAm from Neosartorya fumigata and NsRedAm from Neosartorya scherifor the reduction of the imine formed between acetophenone and ammonia, albeit at concentrations of 5 mM and 250 mM respectively, to give conversions to the primary amine product which, although modest, were superior to previously described RedAms such as AdRedAm from Ajellomyces dermatitidis. 29 In this study we exploit this initial observation in the characterization of the activity of NfRedAm and NsRedAm and their application to the scalable synthesis of primary and secondary amines using ammonia.…”

Engineering of Thermostable β‐Hydroxyacid Dehydrogenase for the Asymmetric Reduction of Imines

“…[2 – 8] In particular, imine reductases (IREDs) have emerged as a valuable new set of biocatalysts for the asymmetric synthesis of optically active amines [1,9–17] . IREDs and related reductive aminases have been applied in biotransformations and enzyme cascades for the synthesis of chiral amines [18–29] …”

“…[1,[9][10][11][12][13][14][15][16][17] IREDs and related reductive aminases have been applied in biotransformations and enzyme cascades for the synthesis of chiral amines. [18][19][20][21][22][23][24][25][26][27][28][29] The use of sequence-based bioinformatics classification approaches enabled the identification of characteristic sequence motifs and the annotation of putative IREDs in the Imine Reductase Engineering Database (https://ired.biocatnet.de/). [30,31] A deeper analysis of the sequence space of IRED homologues revealed a significant sequence similarity and a similar quaternary structure of β-hydroxyacid dehydrogenases (βHADs).…”

NAD(P)H‐Dependent Enzymes for Reductive Amination: Active Site Description and Carbonyl‐Containing Compound Spectrum

“…Fungal RedAms such as AspRedAm and AtRedAm have more recently been applied to the transformation of fluorokarylketones such as 59 and 60 with ammonia a and small aliphatic amines to give optically active fluoroamine products. [52] Hence AdRedAm from Ajellomyces dermatitidis (Uniprot ID: C5GTJ9) was used for the reductive amination of α-fluoroacetophenone 59 with methylamine b to give the (S)-amine product 59b in 44% isolated yield and with 96% ee. Another fungal RedAm homolog, NfRedAm from Neosartorya fumigatus (Uniprot ID: Q4WDZ8), also displayed exceptional activity in the reductive amination of ketones with ammonia a to give primary amines.…”