Synthesis of 3-substituted-4-hydroxyquinoline N-oxides from the Baylis–Hillman adducts of o-nitrobenzaldehydes

The reaction of the Baylis–Hillman adducts derived from o-nitrobenzaldehydes in trifluoroacetic acid in the presence of triflic acid (0.2 equiv.) afforded 3-substituted-4-hydroxyquinoline N-oxides and in good to moderate yields. The reaction mechanism was evidenced by the experiment with , the Baylis–Hillman adduct of 2-nitrobenzaldehyde N-tosylimine, as the one involving N-hydroxyisoxazoline as the key intermediate.