Beyond the Divinyl Ketone: Innovations in the Generation and Nazarov Cyclization of Pentadienyl Cation Intermediates

The requirement for new strategies for synthesizing five-membered carbocycles has driven an expansion in study of the Nazarov cyclization. This renewed interest in the reaction has led to the discovery of several interesting new methods for generating the pentadienyl cation intermediates central to the cyclization. Methods reviewed include carbon-heteroatom ionization, double bond functionalization, nucleophilic addition, or electrocyclic ring opening. Additional variations, such as iso- and imino-Nazarov cyclization, employ unconventional substrates to produce new pentacycles. Here we provide an overview of these unconventional, yet highly useful versions of the Nazarov cyclization.