Anionic cyclization of a cross-conjugated enediyne

Area of Science:

• Organic Chemistry
• Synthetic Chemistry

Background:

• Cross-conjugated enediynes typically do not undergo Bergman cycloaromatization due to insufficient pi electrons.
• Reductive conditions can enable cyclization by forming products with a Hückel number of pi electrons.

Purpose of the Study:

• To illustrate and demonstrate an anionic cyclization of cross-conjugated enediynes.
• To achieve the formation of a five-membered ring through this novel method.

Main Methods:

• Reduction of 9-(3-Phenyl-1-phenylethynylprop-2-ynylidene)-9H-fluorene (3) using potassium.
• Generation of the dianion of 9-(3,4-diphenylcyclopenta-2,4-dienylidene)-9H-fluorene (4(2-)).
• Oxidation of the dianion with iodine.

Main Results:

• Successfully demonstrated an anionic cyclization of a cross-conjugated enediyne.
• Formation of a five-membered ring containing a cyclopentadienyl fragment.
• Isolation of an unstable fulvalene product (4).

Conclusions:

• Anionic cyclization under reductive conditions is a viable strategy for cross-conjugated enediynes.
• This approach overcomes the electronic limitations of Bergman cycloaromatization.
• The study presents the first anionic cyclization of a cross-conjugated enediyne yielding a five-membered ring.