Photocycloadditions of tetrachloro-1,4-benzoquinone (chloranil) onto cyclobutene and cyclopropene. Expected and unexpected products

Solutions of chloranil (CA) in chlorobenzene were irradiated in the presence of cyclobutene and cyclopropene. Cyclobutene gave rise to two conventional 1:2 cycloadducts onto the dichloroethene subunits of CA and an alpha,beta-unsaturated alpha,gamma-dichloro-gamma-lactone. Heating of the crude product in methanol converted the lactone into an alpha,beta-unsaturated methyl gamma-oxocarboxylate (25% yield) and a large amount of the major 1:2 cycloadduct, which contains chlorocyclobutane entities, into a cyclopropylcarbinyl chloride derivative (24% yield). An entirely new product type was the result in the case of cyclopropene. After treatment of the crude product with methanol a tetracyclic acetal containing a cyclopentanone and a dihydropyran subunit was isolated in 36% yield. Apparently, CA had taken up two molecules of cyclopropene. One of the resulting cyclopropane entities must have undergone a rearrangement en route to the final product.