Evaluation of N ‐hydroxymethylphthalimide in alkaline medium: Novel entry to the tricyclic [1,3]oxazepine core via an intramolecular π and O ‐cationic cyclization

Fused isoindolo[1,3]benzo(or thieno)oxazepines 8a,b and one of their positional isomers aromatic tricyclic N,O ‐acetals 13b are reported to occur efficiently in a three‐step sequence from N ‐hydroxy‐methylphthalimide (6) . The key step of this methodology is the intramolecular arylation of an endocyclic and/or exocyclic N ‐acyliminium cation. The mechanism leading to these species, in particular to a tricyclic lactam 13b , is discussed.