5H-cyclopentapyrazines from 1,2-dialkynylimidazoles

Acyclic C,N-dialkynylimines are a class of aza-enediynes that have been recently shown to undergo Bergman-type cyclization to unreactive 2,5-didehydropyridine intermediates, which collapse to isomeric beta-acrylonitrile products. In an effort to determine the effect of incorporating the aza-enediyne functionality into heterocyclic rings on the thermal rearrangements of these systems, we have prepared a series of 1,2-dialkynylimidazoles, whose thermal rearrangement in 1,4-cyclohexadiene were studied. The major products are spiro[bicyclo[4.1.0]heptane-7,5'-[5H]cyclopentapyrazines] and 5H-cyclopentapyrazines, presumably derived from the corresponding cyclopentapyrazine carbene intermediates. The remarkable molecular rearrangements involved in this process are examined in light of a proposed aza-Bergman-retro-aza-Bergman cascade.