New and Efficient Synthesis of Pyrrolo[3,2-b]pyrrole-2,5-diones by Double-Anion-Capture Reactions of Ester Carbanions with Bis(imidoyl)chlorides of Oxalic Acid

A variety of pyrrolo[3,2- b ]pyrrole-2,5-diones were efficiently prepared by a new domino-reaction of ester carbanions with oxalic acid−bis(imidoyl)chlorides. This reaction proceeded by condensation of 2 equiv of the ester with the bis(imidoyl)chloride and subsequent intramolecular attack of the nitrogen atoms of the bis-enamine intermediate onto the ester groups. The products, which can be regarded as dilactams of pentalene, represent useful synthetic pigments due to their optical features, their stability, and low solubility. The UV−vis properties of the pyrrolo[3,2- b ]pyrrole-2,5-diones could be efficiently controlled by the substituents attached to the heterocyclic core. The scope and the limitations of the new cyclization reaction were investigated.