1,3-dipolar cycloaddition reaction of imidazolium N-ylide with electron deficient alkenes carrying 1,2,4-oxadiazole moiety

The 1,3-dipolar cycloaddition is one of the most utilized reactions between dipoles and dipolarophiles for the formation of five-membered heterocycles. There have been various examples of these heterocycles in the literature; however, nitrogen-containing ones are of great interest. In consideration of nitrogen-based heterocycles, pyrrole [1,2-a] imidazoles have drawn our attention due to not only their diverse biological activity but also being a part of many naturally occurring products. On the other hand, 1,2,4-oxadiazoles are of great importance among the five-membered heterocycles bearing an N-O atom. They have many applications, especially in the field of pharmacology, exhibiting remarkable biological activity. Therefore, in order to construct new pyrrole [1,2-a] imidazoles containing a 1,2,4-oxadiazole ring, 1,3-dipolar cycloaddition of imidazolinium N-ylide with p-substituted phenyl vinyl oxadiazoles has been investigated throughout this MSc dissertation study. The outcomes of the study were discussed in three parts. In the first part, imidazolium N-ylide was synthesized via the reaction of 2-bromoacetophenone and 1-vinylimidazole in one step without any purification and characterized. Secondly, as a dipolarophilic reagent, 3-substituted phenyl-5-vinyl-1,2,4-oxadiazoles were obtained by reacting p-substituted benzamidoxime with acryloyl chloride according to a reliable literature method. In the last part, the 1,3-dipolar cycloaddition reaction of imidazolium N-ylide with p-substituted phenyl-5-vinyl-1,2,4-oxadiazole derivatives unexpectedly gave rise to 7 new pyrrole[1,2-a] imidazole by the extrusion of ethene gas. The plausible mechanism was proposed for the formation of cycloaddition products. The structure of the products was identified based on data obtained from IR, 1H NMR, 13C NMR, and LC-MS measurements.