Intramolecular Cyclization of Azides by Iminium Species. A Novel Method for the Construction of Nitrogen Heterocycles under Vilsmeier Conditions

An unprecedented attack of the azide functionality by iminium species, generated in situ under Vilsmeier conditions, provided a novel route for the construction of nitrogen heterocycles. Thus, the treatment of 2-azidoacetophenones with Vilsmeier reagent under reflux conditions gave 5-aryloxazole-4-carboxaldehydes. One-pot synthesis of oxazole carboxaldehydes from 2-bromoacetophenones by dehaloazidation−Vilsmeier cyclization reaction sequence provided better yields. The susceptibility of the carbonyl group to undergo chloroformylation at room temperature without affecting the azide function was exploited to provide an attractive scheme for the synthesis of α-azido-β-chlorovinyl azides from phenacyl azides. The synthesis of a series of N-aryl 5-chloro-2-(dimethylamino)imidazole-4-carboxaldehydes was accomplished by the Vilsmeier cyclization of N-aryl-2-azidoacetamides. The possible mechanisms for the reactions are also discussed.