Biocatalytic synthesis of chiral amine building blocks

Many commercially important molecules, such as agrochemicals and active pharmaceutical ingredients (APIs), contain chiral amines. However, the synthesis of chiral amines by chemical means is often challenging and in particular in the case of aliphatic amines only low enantiomeric excesses (ees) are achieved. By using enzymes, high pressure hydrogen, high temperatures, precious metals, and organic solvents can often be avoided. In the production of chiral amines, lipases are the most important class commercially, catalysing the enantioselective acylation of chiral amines, resulting in a kinetic resolution. However, while their enantio-selectivity is usually excellent when substituents on the α-carbon are sufficiently different in size, it tends to be poor if substituents are similar (e.g. 2-aminobutane). Additionally, while it is possible to recover both enantiomers and racemizeand recycle the unwanted enantiomer, a synthesis of just the desired enantiomer from a prochiral precursor, such as a ketone, has advantages. Here, amine transaminases are the most developed class of enzymes, with several examples of enzyme engineering and scale-up in the literature. While most research with transaminases is focussed on bulky-bulky ketones, in this work the use of transaminases for the synthesis small chiral amines is being explored. Here, a significant limitation of wild-type transaminases proved to be an advantage: the small pocket that typically does not accept substituents significantly larger than a methyl-group allows for excellent enantioselectivity (> 99.5% ee) even for very small chiral amines, such as 2-aminobutane, for which a multi-gram scale synthesis in continuous flow is described.However, attempts at engineering a transaminase for the synthesis of 2,2-dimethylhexan-3-amine were less successful, with only traces of activity being observed. With the cyclic pro-chiral ketone, the enantiomeric outcome of the reaction depended on the reaction conditions (ionic strength and concentration of organic molecules), with ees ranging from 70% (S) to 19% (R). The discovery of the tetrameric quaternary structure of two (R)-selective transaminases (RTA) (from Aspergillus terreus and Thermomyces stellatus) is also described. Using this information, a rational mutation stabilizing the tetramer was introduced, which resulted in an overall more stable catalyst that could be used at higher substrate concentrations compared to the wild-type. Finally, a sequential cascade involving transaminases followed by a Buchwald-Hartwig amination (BHA) is described, which allows access to chiral N-arylamines without the need for purification of the intermediate. Employing a biphasic water-toluene system and using a 3rd-generation Buchwald precatalyst, the BHA showed excellent formation of the desired amine also in the presence of excess amine donor (in particular with alanine), allowing for the quick generation of diverse libraries of compounds which may be of use during drug discovery