Photochemical Rearrangement of Some Cyclobutene-1,2-diones in the Presence of Cyclopentadiene: A Mechanistic Study

The irradiation of a variety of substituted cyclobutene-1,2-diones in the presence of cyclopentadiene leads to the formation of 1:1 adducts which contain the basic 5-spirocyclopropyl Δα,β-butenolide unit. The structural similarity of these adducts to those produced originally upon irradiation with benzocyclobutene-1,2-dione in the presence of olefins suggested a possible common mechanism involving the formation of a ring-expanded oxacarbene intermediate. However, mechanistic studies on the nonaromatic cyclobutendione derivatives have demonstrated that it is not the oxacarbene which is the precursor to the butenolide adducts but rather is an α-ketenylcyclobutanone produced by cycloaddition of the photochemically generated bis-ketene to cyclopentadiene. © 1993, American Chemical Society. All rights reserved.