Base-catalyzed transformations of iminium phenylhydrazones : A new convenient synthesis of imidazo[1,2a]azacycloalkanes

β-Enamine esters react with phenyldiazonium fluoroborate to give iminium phenylhydrazone salts which, without isolation, can be converted by a base-catalysed reaction to imidazole derivatives. The latter reaction is critically dependent upon the nature of the base-component of the enamine ester. The mechanism of imidazole formation involves the following sequence of steps: (a) deprotonation of the iminium salt, (b) a symmetry-allowed 1,5-dipolar cyclization of the betaine intermediate and (c) subsequent elimination of aniline, with concomitant aromatization. The scope of the reaction as a facile approach to the synthesis of imidazole derivatives is discussed.