Protonation of the reactive intermediates produced in the reaction between alkyl isocyanides and dialkyl acetylenedicarboxylates by carboxylic acids leads to vinylnitrilium cations which then undergo nucleophilic addition of the conjugate bases of the carboxylic acids. This intermediate rearranges to produce dialkyl (E)2-{[(aryl) acetyl(alkylamino)]carbonyl}-2-butenedioate derivatives.