Amine transaminases are frequently used for the production of chiral amines starting from prochiral ketones. These amines can be applied as active pharmaceutical ingredients or drug precursors. However, there are still limitations to the use of amine transaminases when it comes to bulky ketone substrates, such as biaryl ketones. Using data mining, an (R)-selective amine transaminase from Exophiala xenobiotica was identified which naturally converts biaryl ketone substrates to the corresponding amines with up to 85% conversion and excellent enantioselectivity (>99% ee). Its protein crystal structure was obtained with a resolution of 1.52 Å, which enabled us to explain this interesting substrate acceptance. Structure-guided protein engineering resulted in a quintuple variant with increased stability. Moreover, the amino acid exchange T273S increased the activity and broadened the substrate scope enabling conversions of various biaryl ketones with up to >99%. A preparative biotransformation of 1-(4-(pyridin-3-yl)phenyl)ethenone at 75 mM (15 g/L) resulted in 96% of isolated yield of the respective amine.
The first application of Deep Eutectic Solvents (DESs) in asymmetric bioamination of ketones has been accomplished. The amine transaminases (ATAs) turned out to be particularly stable in DES-buffer mixtures at a percentage of up to 75% (w/w) neoteric solvent. Moreover, this reaction medium was used to perform a chemoenzymatic cascade toward biaryl amines by coupling a Suzuki reaction sequentially with an enantioselective bioamination catalyzed by the recently discovered ATA from Exophiala xenobiotica (EX-TA). The solubilizing properties of DESs enabled the metal-catalyzed step at 200 mM loading of substrate and the subsequent biotransformation at 25 mM.
One of the pioneering examples of chemoenzymatic cascades in water such as the palladium-catalysed Suzuki-cross coupling followed by an enzymatic reduction has been revisited by the employment of a medium containing Deep Eutectic Solvents (DESs) for the catalytic performance. Thus, the unique properties of these neoteric solvents enabled to reach high substrate concentration for the overall process. Moreover, both isolated enzymes and whole cells exhibited excellent activities which allowed to obtain a set of chiral biaryl alcohols in good yields and very high enantiomeric excess (> 99 %). Scheme 1. Devised chemoenzymatic cascade towards chiral biaryl alcohols in DES-buffer medium.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.