Asymmetric Biocatalytic Synthesis of Fluorinated Pyridines through Transesterification or Transamination: Computational Insights into the Reactivity of Transaminases

Abstract

Abstract. The synthesis of a family of pyridines bearing a fluorinated substituent on the aromatic ring has been carried out through two independent and highly stereoselective chemoenzymatic strategies. Short chemical synthetic routes toward fluorinated racemic amines and prochiral ketones have been developed, which served as substrates to explore the suitability of lipases and transaminases in asymmetric biotransformations. The lipase-catalyzed kinetic resolution via acylation of racemic amines proceeded smoothly giving conversions close to 50% and excellent enantioselectivities. Alternatively, the biotransamination of the corresponding prochiral ketones was investigated giving access to both optically pure amine enantiomers using transaminases with complementary selectivity. High to quantitative conversion values were achieved, which allowed the isolation of the amines in moderate to high yields (40-88%). A deeper understanding of the latter process was enabled by performing theoretical calculations on thermodynamic and mechanistic aspects. Calculations showed that the biotransamination reactions are highly favoured by the presence of fluorine atoms and the pyridine ring.

Development of Biotransamination Reactions towards the 3,4-Dihydro-2H-1,5-benzoxathiepin-3-amine Enantiomers

“…This prompted us to study the biocatalytic process in depth. The biotransamination of 3,4-dihydro-2H-1,5-benzoxathiepin-3-one (6, 20 mM) was then studied in standard conditions previously employed in our research group [46,51]. These settings include the use of a large excess of isopropylamine as amine donor (1 M), pyridoxal 5'-phosphate (PLP, 1 mM) as cofactor, a 100 mM phosphate buffer pH 7.5 with acetonitrile (5% v/v) as cosolvent to favour the ketone solubility, at 30 °C and 250 rpm for 20 h (Scheme 2).…”

“…These settings include the use of a large excess of isopropylamine as amine donor (1 M), pyridoxal 5'-phosphate (PLP, 1 mM) as cofactor, a 100 mM phosphate buffer pH 7.5 with acetonitrile (5% v/v) as cosolvent to favour the ketone solubility, at 30 °C and 250 rpm for 20 h (Scheme 2). Three different types of enzymes were Due to the amine transaminases catalytic mechanism, which involves two pairs of ketones and amines in equilibrium, the reductive amination of the substrate must be thermodynamically favoured in order to obtain high yields of the desired product [45,46]. In order to displace the equilibrium towards amine formation, the removal of the generated co-products by coupling different multienzyme networks is often required [20], but also worth noting is the use of sacrificial substrates, which normally range from the use of a large excess of a commercially available amine donor, typically isopropylamine [47], to "smart cosubstrates", mainly diamines, in a stoichiometric amount that are able to drive equilibrium by spontaneous cyclisation or aromatisation reactions [31,[48][49][50].…”

Stereoselective Synthesis of 1‐Arylpropan‐2‐amines from Allylbenzenes through a Wacker‐Tsuji Oxidation‐Biotransamination Sequential Process

“…(ArR-TA and ArRmut11-TA) were provided by Prof. Wolfgang Kroutil (University of Graz) and were overexpressed in E. coli and used as lyophilised cells. [32] All other reagents were obtained from commercial sources (Sigma-Aldrich, Acros, and Fluka) and used as received except dry methanol that was previously distilled under nitrogen using calcium hydride as desiccant. Thin-layer chromatography (TLC) analyses were conducted using Merck Silica Gel 60 F254 precoated plates and visualised with UV and potassium permanganate stain.…”

“…[31] Herein, a total of 31 ATAs were screened in the biotransamination of 1-phenylpropan-2-one (2 a, 20 mM) for 5 h at 30°C. Some of them derived from Chromobacterium violaceum, Arthobacter citreus and Arthrobacter species were overexpressed in E. coli, [32] and others were used as received from commercial sources (Table S12). A large excess of isopropylamine ( i PrNH 2 , 50 equiv.)…”

Asymmetric Amination of α‐Chiral Aliphatic Aldehydes via Dynamic Kinetic Resolution to Access Stereocomplementary Brivaracetam and Pregabalin Precursors

“…[5] UCB pharma has developed several synthetic routes for the preparation of the 2-oxo-pyrrolidine derivative, [6][7][8] however, these exclusively chemical methods come along with poor stereoselectivity and hence elaborate chromatography steps in order to obtain the far more active (2S,4R)-stereoisomer in pure form. Interestingly, despite of their popularity for the synthesis of chiral amines used for APIs and their successful application also on industrial scale, [24][25][26][27][28][29][30][31] transaminases (TA) [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] have not yet been considered as the biocatalyst of choice to prepare key intermediates of Pregabalin and Brivaracetam by deracemisation of the corresponding aldehydes. This process has been improved in terms of yield and economical aspects by replacing chemical resolution by a lipasecatalysed one.…”