Recyclization of isopropyl 2-(1-aryl-4,5-dioxo-2-phenyl-4,5-dihydro-1H- pyrrol-3-yl)-2-oxoacetates by the action of thiourea

Reactions of monocyclic 1H-pyrrole-2,3-diones with thiourea were not reported previously. We examined the reaction of isopropyl 2-(1-aryl-4,5-dioxo-2phenyl-4,5-dihydro-1H-pyrrol-3-yl)-2-oxoacetates Ia and Ib [1] with thiourea at a reactant ratio of 1 : 1 in a mixture of anhydrous chloroform and 1,4-dioxane on heating under reflux and obtained in 0.5–2 h (TLC monitoring) 2-amino-6-aryl-9-[phenyl(arylamino)methylidene]-1-thia-3,6-diazaspiro[4.4]non-2-ene4,7,8-triones IIa and IIb. The product structure was proved by the X-ray diffraction data for compound IIa. Presumably, compounds IIa and IIb were formed via initial addition of thiourea as nitrogen-centered nucleophile at the carbonyl carbon atom in position 5 of the pyrrole ring, cleavage of the pyrrole ring at the N–C bond, and subsequent closure of thiazole ring as a result of intramolecular nucleophilic addition of the SH group in intermediate A at the ketone carbonyl carbon atom (former C). The hemiacetal hydroxy group in intermediate B thus formed attacks the ester carbonyl carbon atom with elimination of isopropyl alcohol to build up furan ring (compounds III). Analogous recyclization scheme was observed by us previously in the reaction of pyrrolediones Ia and Ib with 1,3-N,C-binucleophiles (imines derived from dimedone) [2]. In the final step, compound III undergoes amination with aromatic amine which is likely to be liberated in the side reaction involving cleavage of the