Photorearrangement of the ortho-cycloadduct of 6-chloro-1,3-dimethyluracil to benzene through [pi 4s+pi 2a] photocycloaddition

Reaction pathway for the formation of the hydrogen chloride adducts of pyrimidosemibullvalene-2,4-dione derived from the photoreaction of 6-chloro-1,3-dimethyluracil in frozen benzene is interpreted by the mechanism involving the initial ortho-cycloaddition, not meta-cycloaddition, followed by the photochemical disrotatory cleavage of the cyclobutene moiety, and the successive intramolecular photo-Diels-Alder reaction of the resulting cyclooctatetraene ring.