Molecular structures of the photodimers of 5-phenyluracil and 6-phenyluracil

Two new head-to-head cyclobutane photodimers have been obtained in single stereoisomeric form by irradiation of 5-phenyl- and 6-phenyluracils in acetone solution. While the latter substrate gives a cis–anti–cis adduct, the former provides, for the first time in a solution phase reaction, a cis–syn–cis photodimer as the only product.