Two recently identified (S)-selective ω-transaminases (ω-TAs) that originate from Paracoccus denitrificans (Strep-PD-ωTA, cloned with an N-terminal Strep-tag II) and Pseudomonas fluorescens (PF-ωTA) were employed for the asymmetric amination of selected prochiral ketones. The substrates tested were transformed into optically pure amines (Ͼ99 % ee) with high conversion (up to Ͼ99 %). The ω-TAs led to higher conversion in the absence of dimethyl sulfoxide as a cosolvent

The asymmetric bioreduction of α,β-unsaturated γ-keto esters using ene-reductases from the Old Yellow Enzyme family proceeds with excellent stereoselectivity and high conversion levels, covering a broad range of acyclic and cyclic derivatives. Various strategies were employed to provide access to both enantiomers, which are versatile precursors of bioactive molecules. The regioselectivity of hydride addition on di-activated alkenes was elucidated by isotopic labeling experiments and showed strong preference for the keto moiety as activating/binding group as opposed to the ester. Finally, chemoenzymatic synthesis of (R)-2-(2-oxocyclohexyl)acetic acid was achieved in high ee on a preparative scale combining enzymatic reduction followed by ester hydrogenolysis. Scheme 1 γ-Oxo acids and esters employed as precursors of biologically active molecules. 1 † Electronic supplementary information (ESI) available: Synthetic procedures and NMR data. See Scheme 4 Access to (R)-2-(2-oxocyclohexyl)acetic acid [(R)-7b] via chemoenzymatic bioreduction-deprotection using NCR. Scheme 5 Elucidation of the anchor group of di-activated substrates (exemplified for 3a) in ene-reductases via incorporation of deuterium.Scheme 6 Baker's yeast catalyzed bioreduction of rac-8b for determination of absolute configuration. 14 Green Chemistry PaperThis journal is

A case of hydride‐independent reaction catalyzed by flavin‐dependent ene‐reductases from the Old Yellow Enzyme (OYE) family was identified. α‐Angelica lactone was isomerized to the conjugated β‐isomer in a nicotinamide‐free and hydride‐independent process. The catalytic cycle of C=C bond isomerization appears to be flavin‐independent and to rely solely on a deprotonation–reprotonation sequence through acid–base catalysis. Key residues in the enzyme active site were mutated and provided insight on important mechanistic features. The isomerization of α‐angelica lactone by OYE2 in aqueous buffer furnished 6.3 mm β‐isomer in 15 min at 30 °C. In presence of nicotinamide adenine dinucleotide (NADH), the latter could be further reduced to γ‐valerolactone. This enzymatic tool was successfully applied on semi‐preparative scale and constitutes a sustainable process for the valorization of platform chemicals from renewable resources.

In contrast to the widely studied asymmetric bioreduction of α,β‐unsaturated carboxylic acid esters catalyzed by ene‐reductases, the reaction applied to lactones remains unexplored. A broad set of ene‐reductases was found to reduce various α‐, β‐ and γ‐substituted α,β‐unsaturated butyrolactones to yield the corresponding saturated non‐racemic lactones. Substitution patterns greatly influenced activities and stereoselectivities and lactone products were obtained in moderate to excellent yields; importantly, enzyme‐based stereocontrol allowed access to both enantiomers in up to >99% ee. Chiral recognition of a distant γ‐center led to kinetic resolution with remarkable enantioselectivities (E values up to 49). An unprecedented case of dynamic kinetic resolution was observed with 3‐methyl‐5‐phenylfuran‐2(5H)‐one, whereby spontaneous racemization of the substrate furnished the product in up to 73% conversion and >99% ee and 96% de.

A series of functionalized α‐chiral phosphonates was obtained by enzymatic reduction of the corresponding β‐activated vinylphosphonate derivatives employing several ene‐reductases of the Old Yellow Enzyme family as biocatalysts. The insufficient activation of the phosphonate group alone was compensated by introduction of an electron‐withdrawing group at the β‐position, which resulted in high levels of conversion (up to >99%). All active enzymes consistently furnished (R)‐configurated products with exquisite stereoselectivity, thereby providing a stereo‐complementary approach to current asymmetric hydrogenation protocols on similar compounds. Preparative‐scale syntheses performed in aqueous buffer using formate/formate dehydrogenase for recycling of the nicotinamide cofactor granted access to β‐keto‐, cyano‐ and ester phosphonates from the (E)‐isomers of α,β‐unsaturated phosphonates in up to 72% isolated yield and >99% ee.magnified image