Hepatitis A virus (HAV) 3C enzyme is a cysteine proteinase essential for viral replication and infectivity and represents a target for the development of antiviral drugs. A number of serine and threonine β-lactones were synthesized and tested against HAV 3C proteinase. The dN- Cbz-serine β-lactone 5a displays competitive reversible inhibition with a K i value of 1.50× 10-6 M. Its enantiomer, lN-Cbz-serine β-lactone 5b is an irreversible inactivator with k ...