Photochemistry of 4-Thiothymine Derivatives in the Presence of N-9-Substituted-Adenine Derivatives: Formation of N-6-Formamidopyrimidines.

UV irradiation of aqueous solutions containing either 4-thiothymin-1-ylacetic acid (1b) and adenosine (2a), 4-thiothymidine (1a) and adenin-9-ylacetic acid (2b), or 1b and 2b led to 4,5-diamino-6-formamidopyrimidine (N-6-Fapy-Ade) derivatives as observed after irradiation of a mixture of 1a and 2a (J. Am. Chem. Soc. 1996, 118, 8142-8143). These new observations demonstrate that the replacement of one or both nucleoside sugar residues by a carboxymethyl group does not affect the regioselective course of the photochemical reaction. The thermal decomposition of 3a that resulted from irradiation of 1a in the presence of 2a, was examined along with its behavior under mild alkaline conditions. Finally, irradiation of N-3-methyl-4-thiothymidine (6a) in the presence of adenosine gave the N-3-methylcytidine derivative 7.