Parallel mechanisms for the cycloaromatization of enyne allenes†

Abstract

The Myers–Saito cycloaromatization of enyne allenes is proposed to consist of two parallel mechanisms, one involving a biradical and the other with dipolar character. MCSCF calculations suggest that a nonplanar cyclic allene could be fairly close in enthalpy to the biradical, while the planar zwitterion originally proposed as a possible second intermediate is in fact a transition state for the interconversion of the two enantiomeric cyclic allenes. Competitive trapping experiments rule out the presence of a single intermediate and are consistent with the participation of parallel pathways. The reaction of (Z)-hepta-1,2,4-trien-6-yne in cyclopentadiene gave an inseparable mixture of two tetracyclic products whose structures were elucidated with 2-D NMR.