Stereocontrolled Double Ring Expansion of Fused Allylidenecyclopropanes. A Novel Route to Hydroazulenes and Other Fused Bicyclic Systems.

A variety of 1-(phenylthio)-1-(trimethylsilyl)cyclopropanes, fused to five-, six-, and seven-membered rings have been prepared by several procedures and reductively lithiated by means of aromatic radical anions. The resulting 1-lithio-1-(trimethylsilyl)cyclopropanes have been treated in most cases with α,β-unsaturated aldehydes, followed by potassium tert-butoxide to yield allylidenecyclopropanes. The latter, upon thermal rearrangement either in a sealed tube or in a flash vacuum pyrolysis apparatus, undergo a double ring expansion to cyclopentenocyclohexenes, -cycloheptenes (hydroazulenes), or -cyclooctenes