Trapping the Oxyallyl Cation Intermediate Derived from the Nazarov Cyclization of Allenyl Vinyl Ketones with Nitrogen Heterocycles

Abstract

Abstract The cationic intermediate of the Lewis acid‐initiated Nazarov cyclization of an allenyl vinyl ketone (AVK) was trapped by pyrroles and indoles. The yields ranged from modest to high (up to 93%), and in both cases, only two of the three possible products were produced. Cyclopent‐2‐enones substituted at the 5‐position were predominantly produced, however with increasing alkyl substitution or placement of an electron‐withdrawing group on the nitrogen, an alternative regioisomer could also be formed. The results of this study suggest that a position α to the oxygen of the oxyallyl cation is the electronically preferred trapping site, whilst an exocyclic position is preferred for more sterically encumbered reacting partners.