Lewis Acid-Catalyzed Synthesis of Functionalized Pyrroles

The synthesis of highly functionalized pyrroles is described. The sequence involves the preliminary preparation of α-aminohydrazones by Michael addition of primary amines to 1,2-diaza-1,3-butadienes. The treatment of these compounds with dialkyl acetylenedicarboxylates produces α-(N-enamino)-hydrazones that were converted into the corresponding pyrroles by Lewis acid-catalyzed ring closure. A screening of several Lewis/Bronsted acid catalysts was also performed.