(2 E,3Z)-2-(1-Methyl-2,5-dioxoimidazolidin-4-ylidene)-3-(arylamino- or heteroarylamino)methylenesuccinate 5 obtained by 2+2 cycloaddition of (5 Z)-5-(dimethylamino)methylene-3-methylimidazolidine-2,4-dione ( 1) and dimethyl acetylenedicarboxylate ( 2) followed by substitution of the dimethylamino group with aromatic or heteroaromatic amines, afforded by heating in ethanol in the presence of potassium hydroxide, potassium salts 6. Acidification of 6 with hydrochloric acid afforded mixtures of ( E)- and ( Z)-isomers of methyl 4-(2-hydroxy-1-methyl-5-oxo-1 H-imidazol-4(5 H)-ylidene)-5-oxo-1-phenyl-4,5-dihydro-1 H-pyrrole-3-carboxylates. On the other hand, alkylation of compounds 6 with methyl iodide or benzyl bromide produced the corresponding methyl ( E)-4-(2-methoxy- or 2-benzyloxy-1-methyl-5-oxo-1 H-imidazol-4(5 H)-ylidene)-5-oxo-1-phenyl-4,5-dihydro-1 H-pyrrole-3-carboxylates 9, derivatives of a new triazafulvalene system. Display omitted