β-Phenylalanine Ester Synthesis from Stable β-Keto Ester Substrate Using Engineered ω-Transaminases

Abstract

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