Abstract: Our research group has focused its investigations in the study of the reactivity of heteroculumenes. We have recently reported that N-aryl ketenimines and carbodiimides substituted at the ortho position by an azidomethyl function undergo, under mild thermal treatment, an intramolecular [3 + 2] cycloaddition reactions affording indolo[1,2-a]quinazolines and tetrazolo[5,1-b]quinazolines.   Within this frame we reasoned that an aziridine function could act as the three-atom component, by replacing the azido group, in analogous pericyclic reactions of related heterocumulenic compounds. With this aim, we designed N-aryl heterocumulenes bearing a 2-(ethoxycarbonyl)aziridine function at ortho position. These new ketenimines and carbodiimides should be easily accessible from phosphazene derivatives by aza-Wittig reaction with ketenes and isocyanates, respectively.   In this communication we disclose the synthetic route designed to obtain these heterocumulenes and the results of our attempts to thermally promote their intramolecular [3 + 2] cycloaddition reactions. Moreover, we also show the unexpected results reached when the phosphine used for preparing the phosphazene precursors of those azacumulenes is other than the usual triphenylphosphine.