Two molecules of an isocyanate react with one molecule of a 1,3-diene in the presence of a nickel(0) catalyst to give a 6-substituted dihydropyrimidine-2,4-dione. A five-membered azanickelacyclic intermediate is initially generated through oxidative cyclization of a hetero-pair of the isocyanate and the 1,3-diene onto nickel(0). Another molecule of the isocyanate is subsequently incorporated therein and reductive elimination follows.