Anionic cyclization of a cross-conjugated enediyne.

Cross-conjugated enediynes cannot follow the Bergman cycloaromatization as it involves a methylenediyne moiety with only five π e-, insufficient for aromatization. Under reductive conditions the cyclization is made feasible by generating a product with a Huckel number of π electrons. We illustrate this principle and demonstrate for the first time an anionic cyclization of a cross-conjugated enediyne that results in formation of a five-membered ring. 9-(3-Phenyl-1-phenylethynylprop-2-ynylidene)-9H-fluorene (3) was reduced by potassium to yield the dianion of 9-(3,4-diphenylcyclopenta-2,4-dienylidene)-9H-fluorene (42-), which contains a cyclopentadienyl fragment, and oxidation with iodine yielded the unstable fulvalene 4.