The reactivity of 6-aminopyrimidin-4(3H)-ones towards DMAD is successfully explained by theoretical investigation (PM3 semiempirical methods). All the PM3 results (activation energies (AE) for the transition states and the heat of formation (ΔH) for the products) support our previous experimental work [J. Cobo et all. Synlett.1993, 4, 297–299; Tetrahedron1994, 50, 10345–10354]. In those reactions two main products were obtained: the pyridine derivatives 5 as major ones, which are formed by a tandem DA/RDA reaction with extrusion of the methylisocyanate fragment; and 5-ethenylpyrimidin-4(3H)-ones 10 as minor ones, which arised from a Michael addition, being in competition with the above normal DA.