L-Tryptophan-catalyzed Paal–Knorr pyrrole cyclocondensation: an efficient, clean and recyclable organocatalyst

Abstract

A simple, clean, and efficient solvent-free protocol is introduced for Paal–Knorr pyrrole synthesis catalyzed by l-tryptophan as a thermally resistant, natural primary amino acid. The products were obtained mostly in excellent yields through the reaction of hexane-2,5-dione with aromatic primary amines bearing a variety of substituents. The catalyst could be easily isolated from the reaction mixture and recycled at least six times without significant loss of activity. The procedure has an environmentally benign nature in agreement with the concepts of green chemistry.