The acid catalyzed condensation reactions of N-substituted ureas with enolether ester (I) or nitrile (II) were carried out, and the condensation products were isolated and those structures were confirmed. Alkylation of ethyl 2-oxo-1, 2, 3, 4-tetrahydro-5-pyrimidinecarboxylate (XXI) afforded N-substituted tetrahydropyrimidines. Dehydrogenation of these N-substituted tetrahydropyrimidines gave N-substituted dihydropyrimidines readily.