The amino blocking reagent 1-isopropyl-3-ethoxy-4-nitro-2-oxo-3-pyrroline and the N-hydroxysuccinimide esters of N-(1-cyclohexyl- and 1-isopropyl-4-nitro-2-oxo-3-pyrrolin-3-yl)glycine. Reagents for the introduction of N-glycyl residues