Selective synthesis of pyrrolo[1,2-a]azepines or 4,6-dicarbonyl indoles via tandem reactions of alkynones with pyrrole derivatives

Abstract

Novel methodologies for the selective synthesis of pyrrolo[1,2-a]azepines or 4,6-dicarbonyl indoles starting from pyrrole derivatives and alkynones are described. When reactions were carried out with 1,2,4-trisubstituted N-propargyl pyrroles using a ZnI₂ catalyst, pyrrolo[1,2-a]azepines were obtained. Whereas 4,6-dicarbonyl indoles were produced selectively with 1,2-disubstituted pyrroles in the presence of silica gel. The reaction outcomes depend on the substituent pattern of the substrates and the nature of the catalysts chosen. Control reactions suggested that the formation of a conjugated enamine intermediate was crucial for both the processes. With easily accessible starting materials, inexpensive catalysts and an easy-to-handle procedure, this reaction has the potential to become a general protocol for the synthesis of pyrrolo[1,2-a]azepines or indoles.