Isolation of a cyclopropane-containing product from the rearrangement of a 3-aza-3-ene-1,5-diyne under acid catalysis

A 3-aza-enediyne that undergoes rapid aza-Bergman rearrangement was treated with trifluoromethanesulfonic acid in the presence of 1,4-cyclohexadiene in an attempt to trap the putative 2,5-didehydropyridinium aza-Bergman intermediate. No pyridine products were detected; rather, a cyclopropane derivative of 1,4-cyclohexadiene derived from a 5-oxazolylcarbene was isolated.