Photochemical dimerization of esters of urocanic acid

Abstract

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(1 H-imidazol-4-yl)cyclobutane-r-l, t-2-dicarboxylate and dimethyl or diethyl t-3, c-4-di-(1 H-imidazol-4-yl)cyclobutane-r-1, t-2-dicarboxylate. The allyl urocanate gave in high yields only of diallyl c-3, t-4-di(1 H-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 Δ H f of the products. In all the cases, the most stable dimers were obtained.