Cyclotetramer of a Strained Cyclic Allene

Significance

The authors isolated compound 3 from the Doering-Moore-Skattebøl reaction of 1,1-dibromo-1a,9b-cyclopropa[l]phenanthrene (1) through the strained cyclic allene intermediate 2. The reaction described is formally a [2+2+2+2] ­reaction; however, the authors propose that the product is more likely formed by two sequential dimerizations due to the entropic cost of the concerted mechanism. Such cyclotetramerizations of allenes are rare in the literature and have never been reported with a seven-membered cyclic ­allene.