Cycloadditionen eines intermediären Trimethylendiradikals

Abstract

It has been shown possible to trap the intermediate in cyclopropane thermolysis, the ring-opened form or trimethylene diradical, in an intramolecular formal [2π + 2σ]-cycloaddition reaction. The resulting bishomocyclopentadienes have been characterized by their behaviour towards strong bases. A forbidden thermal disrotatory opening of a cyclopropyl anion is also described. In case of the Diels-Alder adducts of cyclopropenes with fulvenes the [2π + 2σ]-cycloaddition has not been realized. However, when the adducts of cyclopropenes and cyclopentadienone ketals are submitted to this intramolecular valence isomerization, tetracyclooctanones are available in high yields. The mechanistic aspects are discussed, and on the basis of extended Hückel calculations a stepwise cycloaddition has been shown to be the most probable pathway.