An enzymatic cascade for high-yield and stereoselective synthesis of 4-fluoro-L-threonine

The critical role of fluorine in bioactive molecule design requires selective fluorination methods for synthesizing novel building blocks, such as fluorinated amino acids. Here, we focused on L-threonine aldolases (LTAs), enzymes that mediate reversible aldol additions to the α carbon of glycine. Their C–C bond formation ability and substrate flexibility make these enzymes ideal catalysts for fluorine biocatalysis. We harnessed the promiscuous activity of the LTAs isolated from either Escherichia coli or Pseudomonas putida on 2-fluoroacetaldehyde in a two-step enzymatic cascade for efficient 4-fluoro-L-threonine synthesis. By implementing 2-fluoroethanol as the primary fluorodonor in these cascades, we demonstrated that the LTA enzyme isolated from P. putida mediates a high 4-fluoro-L-threonine yield (>90%) while displaying stereoselectivity for the L-syn form.