N-alkoxy-N-acylnitrenium ions as possible intermediates in intramolecular aromatic substitution: novel formation of N-acyl-3,4-dihydro-1H-2,1-benzoxazines and N-acyl-4,5-dihydro-1H,3H-2.1-benzoxazepine

N-Halogeno-N-alkoxyamides undergo intramolecular aromatic substitution by thermal- or Lewis acid-catalysed heterolysis of the nitrogen–halogen bond. N-Acyl-N-alkoxynitrenium ions are likely intermediates. N-Chloro-N-methoxybiphenyl-2-carboxamide (4) yields N-methoxyphenanthridone (11) quantitatively with AgBF4, while O-2-phenylethyl-N-chlorobenzohydroxamate (19) is converted in good yield into N-benzoyl-3,4-dihydro-1H-2,1-benzoxazine (21) with AgBF4, AgClO4, HgO, and Hg(OAc)2. N-Acetyl-3,4-dihydro-1H-2,1-benzoxazine (22) is formed similarly. O-3-Phenylpropyl-N-chlorobenzohydroxamate (23) cyclises to N-benzoyl-4,5-dihydro-1H,3H-2,1-benzoxazepine (24) with AgBF4.