An organic-soluble guanosine derivative, 2',3',5'-O- (tert-butyldimethylsilyl)guanosine (1), was prepared and its photosensitized oxidation was carried out in several solvents at various temperatures. Singlet oxygen is the reactive oxidizing agent responsible for this reaction. Neither an endoperoxide nor a dioxetane intermediate was detected by low-temperature NMR even at -78degreesC. A product (A) with an oxidized imidazole ring was the only major product detected at room temperature; this compound could be isolated by low-temperature column chromatography and was characterized by H-1 and C-13 and mass spectroscopy. CO2 was the other major product. A small amount of the corresponding 8-oxo-7,8-dihydroguanosine derivative B was detected during the initial stage of the photooxidation and was shown to be intermediate in the formation of two products of extensive degradation, C and D. Reaction of 1 with the singlet oxygen analogues 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) and 4-phenyl-1,2,4-triazoline3,5-dione (PTAD) gave products consistent with a proposed mechanism involving the rearrangement of an initially formed endoperoxide to give A and B from reaction of 1 with singlet oxygen.