Amine oxidation. Part V. Reactions of some N-oxides, including heterocyclic-ring formation, with sulphur dioxide, acetic anhydride, and trifluoroacetic anhydride

Abstract

The reactions of some N-oxides with sulphur dioxide, acetic anhydride, or trifluoroacetic anhydride are reported. With NN-dimethylbenzylamine N-oxide and its p-NO₂ and p-OMe derivatives, the study centred on the discrimination shown by the reagent between the N-methyl and N-methylene groups in the dealkylation to the secondary amine; sulphur dioxide differs markedly from the other two reagents both in this respect and in effecting reduction of the N-oxide to the parent tertiary amine. Sulphur dioxide effects the dehydrative cyclisation of 3,4-dimethoxy-NN-dimethylphenethylamine N-oxide to give 61% of 1,2,3,4-tetrahydro-6,7-dimethoxy-2-methylisoquinoline, with the tertiary amine as a minor product, when formic acid is the solvent, whereas only the secondary and/or tertiary amine is formed in water or acetic acid; the courses of these reactions are interpreted in terms of the formation of an iminium ion, when the ionising power of the solvent is great enough, which can undergo either intramolecular aromatic substitution or solvolysis, depending on the nucleophilicity of the solvent. The use of the sulphur dioxide–formic acid reagent in inducing the dehydrative cyclisation of three other N-oxides is described. Trifluoroacetic anhydride also induces these reactions but is far less effective and prone to give polymers.