Dihydrobenzimidazole chemistry. Part III. Acid-catalysed cyclisations of o-pyrrolidino anils

Abstract

Anils derived from o-pyrrolidinoaniline and a wide variety of aldehydes are shown to undergo a rapid cyclisation in the cold with acid catalysts to give dihydrobenzimidazoles. Alloxan is shown to react in a similar manner with various anilines to give benzimidazolium salts. The scope and limitations of the reaction are discussed and the mechanism is shown to involve intramolecular proton transfer. The formation of both benzimidazolium salts and spiro-quinoxalines in the alloxan reactions is shown to occur by way of a common dihydrobenzimidazole intermediate. The ring expansion of dihydrobenzimidazoles to quinoxalines constitutes a new type of rearrangement reaction.