Abstract
1,5-Dihydropentalene ( 4) is formed as the main product on treatment of trans-1,2-bis(2,2-dibromocyclo-propyl)ethene 3 with methyllithium at −40°. In addition the reaction affords 1- and 2-propadienylcyclopentadienes ( 5a) and ( 5b), and trans-1,2,4,6,7-octapentaene ( 6), new C 8H 8 isomers. Diels-Alder adducts of 4, 5a and 5b were obt in the reaction with perfluorobut-2-yne. The formation of 1,5-dihydropentalene 4 is explained by a double ring expansion sequence involving consecutive carbene-carbene rearrangements with 1,3-carbon and subsequent 1,2-hydrogen shifts, supported by the reaction of double labelled ( 13C-depleted) 3. From readily available 3 at low temperatures formation and fusion of two 5-membered rings are achieved in one step.
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