threonine derivatives, having tert-butyl or benzyl-based side-chain protection, form isolable 5(4H)-oxazolones on treatment with N-ethyl-N′-3-dimethylaminopropyl carbodiimide. N-chloroacetylated threonine derivatives, on the other hand, do not form oxazolones so readily. The N-acetylated oxazolones are easily epimerized and lead to diastereoisomeric mixtures of threonine derivatives on hydrolysis with dilute