Application of Threonine Aldolases for the Asymmetric Synthesis of α‐Quaternary α‐Amino Acids

We report the synthesis of diverse β‐hydroxy‐α,α‐dialkyl‐α‐amino acids with perfect stereoselectivity for the α‐quaternary center through the action of l‐ and d‐specific threonine aldolases. A wide variety of aliphatic and aromatic aldehydes were accepted by the enzymes and conversions up to >80 % were obtained. In the case of d‐selective threonine aldolase from Pseudomonas sp., generally higher diastereoselectivities were observed. The applicability of the protocol was demonstrated by performing enzymatic reactions on preparative scale. Using the d‐threonine aldolase from Pseudomonas sp., (2R,3S)‐2‐amino‐3‐(2‐fluorophenyl)‐3‐hydroxy‐2‐methylpropanoic acid was generated in preparative amounts in one step with a diastereomeric ratio >100 favoring the syn‐product. A Birch‐type reduction enabled the reductive removal of the β‐hydroxy group from (2S)‐2‐amino‐3‐hydroxy‐2‐methyl‐3‐phenylpropanoic acid to generate enantiopure l‐α‐methyl‐phenylalanine via a two‐step chemo‐enzymatic transformation.