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

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 a 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.