Abstract: The N-acyliminium cations are powerful intermediates in the construction of nitrogen-containing heterocycles. They are much more reactive than iminium cations in both intramolecular cyclizations and intermolecular cycloadditions. In this paper, a new and efficient route for the synthesis of quino[1,2-c][1,3]benzoxazin-6-ones (3a—3h) and quino[1,2-c][1,3]naphthoxazin-6-ones (3i—3j) was developed via Diels-Alder reaction of N-acyliminium cations with olefins (2a—2d) in moderate and good yields at an ambient temperature. The N-acyliminium cations, N-phenyl[1,3]benzoxazin-2-onium ion and N-phenyl[1,3]naphthoxazin-2-onium ion, are easily prepared by BF₃·OEt₂ catalyzed dehydroxylation of 4-hydroxy-3-phenyl[1,3]benzoxazin-2-ones (1a—1c) and 4-hydroxy-3-phenyl[1,3]naphthoxazin-2-one(1d).

Key words: Quino[1,2-c][1,3]benzoxazin-6-one, 4-Hydroxy-N-phenyl[1,3]benzoxazin-2-one, N-acyliminium cation