Solution photodimerization of 1,3-dimethyluracil

Abstract

The photodimerization of 1.3-dimethyluracil in a variety of organic solvents is described. The reactions lead to the formation of the four possible cyclobutane-type dimers, the relative amounts depending on the solvent employed. The structures and stereochemistry of the dimers are described and a triplet state is proposed as their precursor.

The Photochemistry of Uracil: A Reinvestigation

“…The chloroform was removed on a water aspirator driven apparatus at room temperature and then drying was continued at 60°C on the Büchi Rotovapor; this latter step removes most of the DMF remaining after the water extraction of the CHCl 3 . (The procedure used here omits a step included in the previous protocols [22–24], namely treatment with aqueous sodium cyanide; we found the end results were the same without it.) During the rotatory evaporation, the Sarstedt tube was fitted to the taper seal 24/40 joint of an antiflash rotatory evaporator trap using a Fisherbrand Sav‐It 16 mm tube closure pierced by several holes.…”

“…Elad and coworkers (24) found that methylation of the c,s CBD of Ura to form the corresponding DMU dimer went smoothly when it was treated with silver oxide and methyl iodide in DMF. In our work we used the modified procedure, outlined above for C‐methylation of DMU CBDs, to carry out a similar reaction on fractions containing isolated Ura dimers, or dimer mixtures, to form the corresponding DMU, DMU‐DMT and DMT dimers.…”

“…After methylation of the “ice dimer” of Ura, Elad et al. (24) noted that the end product was contaminated with the c,a dimer of DMU. Varghese (21) indicated, in his study of the photoreaction of Ura in the frozen state, that about 10% of the dimer formed was the c,a isomer, the remainder being the c,s form.…”

The Paternò–Büchi Reaction of 1,3‐Dimethyluracil and 1,3‐Dimethylthymine with 4,4′‐Disubstituted Benzophenones

“…In addition, if an exciplex intermediate [1d] formed via charge transfer prior to the formation of the diradicals occurred in the PB reaction, a high efficiency for the formation of the exciplex would also be an important factor for the high efficiency of the DMT system due to a lower oxidative potential for DMT. In contrast with the PB reaction, the role of the 5-methyl group should be steric hindrance in the photochemical [2+2] cycloaddition between two DMT molecules to form a cyclobutane dimer via a triplet pathway, because the efficiency for DMT [30] is lower than DMU, [31] using acetone as photosensitizer. Thus, the 5-methyl group of DMT plays very different roles in the two-step [2+2] cycloaddition (the PB reaction) and in the concerted [2+2] cycloaddition (the photodimerization).…”

Luminescence Detection and Photodimerization of 1,3-Dimethyluracil Coordinated to Copper(I)

“…In m odel sys tem s it has been shown that the cycloaddition of dim ethylthym ine [7] and dim ethyluracil [8] can be also photosensitized by suitable ketones. We suspected that this photodim erization could be induced by transition m etals, too.…”