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Fused isoindolo1,3benzoor thienooxazepines 8a,band one of their positional isomers aromatic tricyclic N,Oacetals 13bare reported to occur efficiently in a threestep sequence from Nhydroxymethylphthalimide 6. The key step of this methodology is the intramolecular arylation of an endocyclic andor exocyclic Nacyliminium cation. The mechanism leading to these species, in particular to a tricyclic lactam 13b, is discussed.