Blocked isocytosine isocyanates are conveniently obtained by the reaction of 1,1'-carbonyldiimidazole (CDI) with isocytosines. The resulting precursors for quadruple hydrogen-bonded structures can be isolated and stored for further use. Reaction with either aliph. or arom. amines gives the corresponding mono-, bi-, or trifunctional ureidopyrimidinone derivs. in good to excellent isolated yields.