Beta-Phenylalanine Ester Synthesis From Stable -Keto Ester Substrate Using Engineered -Transaminases

The successful synthesis of chiral amines from ketones using -transaminases has been shown in many cases in the last two decades. In contrast, the amination of -keto acids is a special and relatively new challenge, as they decompose easily in aqueous solution. To avoid this, transamination of the more stable -keto esters would be an interesting alternative. For this reason, -transaminases were tested in this study, which enabled the transamination of the -keto ester substrate ethyl benzoylacetate. Therefore, a -transaminase library was screened using a coloring o-xylylenediamine assay. The -transaminase mutants 3FCR_4M and ATA117 11Rd show great potential for further engineering experiments aiming at the synthesis of chiral (S)- and (R)--phenylalanine esters. This alternative approach resulted in the conversion of 32% and 13% for the (S)- and (R)-enantiomer, respectively. Furthermore, the (S)--phenylalanine ethyl ester was isolated by performing a semi-preparative synthesis.