Commercial transaminases for the asymmetric synthesis of bulky amines.

Transaminase-catalysed asymmetric amination is a valuable transformation for the synthesis of chiral amines. However, its use in synthetic chemistry has been curtailed by a narrow substrate scope and limited information on commercially available catalysts. In this work we have explored the substrate scope of selected commercially available transaminases, focusing on prochiral ketones bearing two bulky substituents and their application in both enzyme-catalysed and enzyme-triggered reactions. (R)- and/or (S)-selective enzymes converted methyl-, isopropyl-, n-butyl-, and cyclohexyl-phenone substrates to the corresponding amines in the range of 6 - >99% ee. In some cases, a stereochemical switch was detected, in which (R)-enantioselectivity was observed with enzymes typically assigned as (S)-selective. Upscaling of selected biotransformations provided chiral amines, in >99% ee and up to 40% isolated yield. Furthermore, transaminase-triggered reactions, which were previously limited to methyl- and ethyl-derivatives, were expanded to phenyl-derivatives for the formation of a cis-2,6-disubstituted piperidine and 2,4-diphenyl pyrroline, and isolated in up to 67% yield.