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-y lacetic acid (Ib) and adenosine (2a), 4-thiothymidine (1a) and adenin- 9-ylacetic acid (2b), or 1b and 2b led to 4,5-diamino-6-formamidopyrim idine (N-6-Fapy-Ade) derivatives as observed after irradiation of a mi xture of la and 2a (J. Am. Chem. Sec. 1996, 118, 8142-8143). These new observations demonstrate that the replacement of one or both nucleosi de sugar residues by a carboxymethyl group does not affect the regiose lective course of the photochemical reaction. The thermal decompositio n of 3a that resulted from irradiation of la in the presence of 2a, wa s examined along with its behavior under mild alkaline conditions. Fin ally, irradiation of N-3-methyl-4-thiothymidine (6a) in the presence o f adenosine gave the N-3-methylcytidine derivative 7.