Stereocontrolled double ring expansion of fused allylidenecyclopropanes. A novel route to hydroazulenes and other fused bicyclic systems

Article abstract of DOI:10.1021/ja00076a037

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. When the distal double bond of the allylidenecyclopropane possesses a terminal trans substituent, that substituent in the hydroazulene pyrolysis product is predominantly in the exo position (cis to the hydrogen atom at the ring junction carbon atom); cis substituents end up mainly in the endo position. Thus, the ring closure occurs mainly in a conrotatory sense. When the distal unsaturation is incorporated into a ring, a tricyclic system results. Carbonyl functionality can be introduced into either position of the five-membered rings and either of the two carbon atoms adjacent to the ring junction of the seven- or eight-membered ring of the ring expansion products by using appropriately substituted rearrangement substrates. Complete regioselectivity is observed when the original six- or seven-membered ring has a ketone group adjacent to the ring junction; the product is a completely conjugated dienone. Complete regioselectivity in the opposite sense is observed upon rearrangement of the corresponding kinetic silyl enol ether. This methodology is demonstrated by the synthesis of (+)-9-epiledene, containing the gross structural features of the aromadendrene sesquiterpenes, and of (±)-α-bulnesol, a guiazulene sesquiterpene.