Biocatalysis is gaining increasing attention in the academic and industrial sector due to the possibility of developing highly stereoselective transformations in a sustainable manner. The creation of stereogenic centers in organic synthesis is not trivial and multiple approaches have been disclosed based on 2 organometallic and organocatalytic methods with the use of day by day more complex catalysts to induce asymmetry in selected transformations. The intrinsic chirality of enzymes makes them powerful tools for the development of stereoselective transformations, catalysing a wide range of chemical reactions due to the high abundance and diversity of enzymes in nature. In addition, the enormous advances in rational design and molecular biology methods have opened up the possibility to create more robust and versatile biocatalysts, which have improved the initial activities displayed by wild-type enzymes. Therefore, their applicability has been widely increased in terms of reaction conditions, substrate specificity, activity and selectivity among others. All these properties have attracted the industrial sector, which has taken advantage of the enzyme selectivities in multiple scenarios. Herein, the focus has been put in recent developments of stereoselective transformations for the synthesis of valuable building blocks towards the production of pharmaceuticals and biologically active natural products.

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...

Abstract:The applicability of deep eutectic solvents has been demonstrated for the first time in promiscuous lipase-catalysed aldol reactions. The model reaction between 4-nitrobenzaldehyde and acetone was examined in depth, an excellent compatibility being found between porcine pancreas lipase and choline chloride: glycerol mixtures for the formation of the aldol product in high yields. The system was compatible with a series of aromatic aldehydes and ketones including acetone, cyclopentanone and cyclohexanone. In some cases the corresponding ,β-unsaturated carbonyl compounds were found as minor products. Control experiments demonstrate that the enzymatic preparation was also responsible of the collateral dehydration reaction once the aldol product is formed.

Abstract:A one-pot chemoenzymatic method has been described for the synthesis of -butyrolactones starting from the corresponding ketones through a Baeyer-Villiger reaction.The approach is based on a lipase-catalyzed perhydrolysis for the formation of peracetic acid, which is the responsible for the ketone oxidation. Optimization studies have been performed in the oxidation of cyclobutanone, finding Candida antarctica lipase type B, ethyl acetate and urea-hydrogen peroxide complex as the best system. The relative ratio of these reagents has also been analyzed in depth. This synthetic approach has been successfully extended to a family of 3-substituted cyclobutanones in high substrate concentration, yielding the corresponding lactones with excellent isolated yields and purities, under mild reaction conditions and after a simple extraction protocol.

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.

ABSTRACT. A set of transaminases has been investigated for the biocatalytic amination of 1-(4-chloropyridin-2-yl)alkan-1-ones. The influence of the chain length of the n-1-alkanone at the C-2 position of the pyridine has been studied in the reaction with different (R)-and (S)-selective transaminases. Thus, enantiopure amines were isolated with high purity starting from a wide selection of prochiral ketones. On the one hand excellent yields (97->99% conversion, up to 93% isolated yield) and stereoselectivity values (>99% ee for both amine enantiomers) were found for n-1-alkanone linear short chain substituents such as ethanone or propanone. On the other hand, more hindered substrates were accepted only when using evolved enzymes such as an evolved variant of (R)-Arthrobacter (ArRmut11-TA). An initial common structural feature was the presence of a chlorine atom on the C-4 position of the pyridine core, which was found to increase the reactivity of the starting ketone, giving extra versatility for the introduction of other chemical functionalities towards more complex and applicable organic molecules. In order to gain a deeper understanding about the substrate specificity of different transaminases, additional structural features were considered by variation of the acetyl group position on the pyridine ring and the use of related acetophenone derivatives.

Herein, a sequential and selective chemoenzymatic approach is described involving the metalcatalysed Wacker-Tsuji oxidation of allylbenzenes followed by the amine transaminase-catalysed biotransamination of the resulting 1-arylpropan-2-ones. Thus, a series of nine optically active 1-arylpropan-2-amines were obtained with good to very high conversions (74-92%) and excellent selectivities (> 99% enantiomeric excess) in aqueous medium. The Wacker-Tsuji reaction has been exhaustively optimised searching for compatible conditions with the biotransamination experiments, using palladium(II) complexes as catalysts and iron(III) salts as terminal oxidants in aqueous media. The compatibility of palladium/iron systems for the chemical oxidation with commercially available and made in house amine transaminases was analysed, finding ideal conditions for the development of a general and stereoselective cascade sequence. Depending on the selectivity displayed by selected amine transaminase, it was possible to produce both 1-arylpropan-2-amines enantiomers under mild reaction conditions, compounds that present therapeutic properties or can be employed as synthetic intermediates of chiral drugs from the amphetamine family.

The stereoselective synthesis of chiral amines is an appealing task nowadays. In this context, biocatalysis plays a crucial role due to the straightforward conversion of prochiral and racemic ketones into enantiopure amines by means of a series of enzyme classes such as amine dehydrogenases, imine reductases, reductive aminases and amine transaminases. In particular, the stereoselective synthesis of 1,5-benzoxathiepin-3-amines have attracted particular attention since they possess remarkable biological profiles; however, their access through biocatalytic methods is unexplored. Amine transaminases are applied herein in the biotransamination of 3,4-dihydro-2H-1,5-benzoxathiepin-3-one, finding suitable enzymes for accessing both target amine enantiomers in high conversion and enantiomeric excess values. Biotransamination experiments have been analysed, trying to optimise the reaction conditions in terms of enzyme loading, temperature and reaction times.