The invention concerns 2-oxo-1-pyrrolidine derivatives and a process for preparing them and their uses. The invention also concerns a process for preparing .alpha.-ethyl-2-oxo-1-pyrrolidine acetamide derivatives from unsaturated 2-oxo-1-pyrrolidine derivatives. Particularly the invention concerns novel intermediates and their use in methods for the preparation of S-.alpha.-ethyl-2-oxo-1-pyrrolidine acetamide.