N-Substituted α-amino β-oxo esters have been obtained by amination of β-enamino esters with ethyl N-[(4-nitrobenzenesulphonyl) oxy] carbamate (NsONHCO2Et), in the absence of added bases. The use of optically active pyrrolidines with C2 symmetry as chiral auxiliaries induces diastereoselectivities up to 80%.