Gold(I)-Catalyzed Benz[c]azepin-4-ol Synthesis by Intermolecular [5 + 2] Cycloaddition

A gold(I)-catalyzed intermolecular formal [2 + 5] cycloaddition for the preparation of benzofused N-heterocyclic azepine products is presented. A number of benz[c]azepin-4-ol products were readily prepared in one step from easily accessible phenylpropargyl acetals and benzaldimine substrates in the presence of a gold(I) catalyst. A direct one-pot procedure from the propargyl and the respective aldehyde and amine substrates was successful as well. The reaction to access the benzofused azepines could be rationalized by a cascade reaction, including a nucleophilic benzaldimine N-attack at a highly reactive phenylpropargyl-gold(I) carbenoid complex, generated from propargyl acetal. A subsequent deauration step promotes ring closure by 1,7-electrocyclization through an intramolecular Pictet-Spengler-type reaction with the aldiminium moiety.