An efficient chemoenzymatic approach to (S)-γ-fluoroleucine ethyl ester

An asymmetric synthesis of (S)-gamma-fluoroleucine ethyl ester 1 is described. The key transformation involves a lipase-catalyzed dynamic ring-opening of 2-(3-butenyl)azlactone 7b with EtOH to give amide ester (S)-6b in 84% enantiomeric excess. Removal of the N-pentenoyl group with N,N-dibromodimethylhydantoin in the presence of trifluoroacetic acid afforded the titled compound, which was isolated as its hydrogen sulfate salt in 75% yield and > 97% ee.