Distinct Solvent-dependence in the Photoreactions of Purine Nucleosides with Pyrimido[5,4-g]pteridinetetrone N-Oxide : Possible Generation of Hydroxyl Radical from the Excited N-Oxide in Alcohols.

Photoreaction of 2',3',5'-tri-O-acetyladenosine (5) with pyrimido[5,4-g]pteridinetetrone N-oxide (1) in acetonitrile gave N 6 -cyanomethyl-2',3',5'-tri-O-acetyladenosine (6) as a major detectable product via coupling of adenosyl radical with cyanomethyl radical generated by the mediation of 1. Under the analogous conditions, N 2 -benzoyl-2',3',5'-tri-O-acetylguanosine (8) underwent oxidative degradation of the guanine skeleton by 1. In sharp contrast, photoreactions of 5 and 8 with 1 in tert-butanol resulted in the formation of the corresponding 8-hydroxypurine nucleosides (7) and (9), respectively. These facts and other observations suggest that 1 could generate hydroxyl radical upon irradiation in alcohols