Generation of [5.5.n] Tricyclic Ring Systems by Radical-Promoted Inter- and Intramolecular [3 + 2] Cycloadditions

Abstract

A new method for the synthesis of [5.5.n] tricyclic ring systems via radical fragmentation and double cyclization is described. The general process (Scheme 1) involves addition of an alkylthio radical to an alkenylcyclopropane 1 to generate the cyclopropylcarbinyl radical which opens to the homoallylic radical; this radical then adds to an alkene or alkyne radical trap to give a new alkyl radical which then adds back to the thioalkyl allylic system to generate, after loss of the alkylthio radical, the bi- or tricyclic product, 2, thus making the process analogous to a [3 + 2] cycloaddition. Thus addition of the butylthio radical (generated by photolysis of dibutyl disulfide) to the bicyclo[3.1.0]hex-2-en-6-yl carboxylate 3 in the presence of an alkene or alkyneeither an acyclic radical trap, e.g., ethyl vinyl ether, isopropenyl acetate, methyl acrylate, or methyl propiolate, or a cyclic one, e.g., cyclopentenone, dihydrofuran, cyclopentenyl acetate, or cyclopenteneaffords the desired bi- or tricyclic prod...