Photochemical dimerization of esters of urocanic acid

The irradiation of urocanate esters in the presence of benzophenone in acetonitrile with a mercury lamp led to the formation of dimers. Methyl and ethyl urocanate gave a 2:1 mixture of dimethyl or diethyl c-3,t-4-di-( 1H-imidazol-4-yl)cyclobutane-r-1,t-2-dicarboxylate and dimethyl or diethyl t-3,c-4-di-( 1H-imidazol-4-yl) cyclobutane-r-1,t-2-dicarboxylate. The allyl urocanate gave in high yields only of diallyl c-3,t-4-di-( 1H-imidazol-4-yl) cyclobutane-r-1,t-2-dicarboxylate. The regiochemistry of the reaction can be explained considering the frontier orbital interactions. The stereochemistry of the reaction can be justified on the basis of the Delta H-f of the products. In all the cases, the most stable dimers were obtained. (C) 1998 Elsevier Science S.A.