Configuration of the C4-photodimers of 2- chloro-1 ̂ -naphthoquinone

Irradiation of crystalline 2-chloro-1,4-naphthoquinone by sunlight, filtered through window glass, led to the formation of a head-to-head cyclobutane dimer with syn-configuration. Pyrexfiltered sunlight irradiation of an acetic anhydride solution of 2chforo-1,4-naphthoquinone produced a head-to-head antidimer. An isomeric head-to-tail antf-dimer was synthesized. The structure and configuration of the C4-dimers of 2-chloro1,4-naphthoquinone were elucidated from an extensive i.r. and Ή and C n.m.r. study. The importance of the sign of proton-proton and C Ή coupling constants in determining the structures of these dimers, is discussed. S. Air, J, Chem , 1981, 34. 76—80