Anionic cyclization of a cross-conjugated enediyne.
Cross-conjugated enediynes cannot follow the Bergman cycloaromatization as it involves a methylenediyne moiety with only five pi e(-), insufficient for aromatization. Under reductive conditions the cyclization is made feasible by generating a product with a Hückel number of pi 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 (4(2-)), which contains a cyclopentadienyl fragment, and oxidation with iodine yielded the unstable fulvalene 4.
Eshdat
Lior
L
Department of Organic Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
Berger
Harald
H
Hopf
Henning
H
Rabinovitz
Mordecai
M
D000480:Alkynes / Q000737:chemistry*; D000903:Antibiotics, Antineoplastic / Q000737:chemistry / Q000494:pharmacology; D006844:Hydrocarbons, Cyclic / Q000138:chemical synthesis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy
D000480:Alkynes; D000903:Antibiotics, Antineoplastic; D006844:Hydrocarbons, Cyclic