A convenient synthesis of functionalized 2-iminothiazolidin-4-ones and thiadiazoloquinazolinones has been achieved by the reaction of dialkyl acetylenedicarboxylates (DAAD) with 1-pheny-3-pyridin-2-yl-thiourea (1) and 3-amino-2-mercapto-3H-quinazolin-4-one (8), respectively. The starting materials for these processes were prepared from phenyl isothiocyanate, 2-aminopyridine for (1) and anthranilic acid, carbon disulfide, hydrazine for (8). Treatment of these compounds with DAAD resulted in the formation of an intermediate that readily underwent intramolecular cyclization to afford functionalized five membered heterocyclic rings was formed.