1,4- and 1,3-Dipolar Reactivity of α-Alkoxycarbonylcycloimmonium <i>N</i>-Aminides with Dipolarophiles: Synthesis of New Imidazo[2,1-<i>f</i>][1,2,4]triazinium Inner Salts

Abstract

2-Alkoxycarbonylazolium N-aminides are interesting species, as they have the potential to act as efficient 1,4-dipole equivalents when they react with heterocumulenes, such as iso(thio)cyanates and carbodiimides. These reactions give heterobetaines containing the imidazo[2,1-f][1,2,4]triazinium system, in a formal [4 + 2] cyclocondensation process. These N-aminides, however, can also behave as 1,3-dipoles when they react with isocyanates to afford a cycloadduct that, depending on the position of alkoxycarbonyl group, can undergo a reversion process or a ring expansion to the more stable heterobetaine system.