Reaction of ketene-S, N-acetals (1-6), which are useful synthetic intermediates for heterocycles, with aryl isocyanates (7) is described. Annulation of 1-3 and 4-6 with 7 in boiling toluene gave bicyclic (8-10) and monocyclic (11-13) uracil derivatives, respectively. Addition of 2, 3, and 4 to 7 under mild conditions afforded the 1 : 1 adducts (18, 19, and 21), respectively. Compounds 18 and 19 also reacted with 7 to give bicyclic uracil derivatives.