An 8-C-13-labeled guanosine derivative, 2',3',5'-O-tert-butyldimethylsilyl-N-tert-butyldimethylsilyl-8-C-13-guanosine, was synthesized and its photosensitized oxidation with singlet oxygen carried out below -100 degreesC. Two transient intermediates that decompose directly to the final major product 5 and CO2 were detected by C-13 NMR between -100 and -43 degreesC. The two intermediates are carbamic acids based on C-13 NMR and 2D NMR (HMQC, HMBC) spectra and the formation of final product 5 and of 8-CO2. No endoperoxide intermediate could be detected by low-temperature NMR spectroscopy even at -100 degreesC. A reaction mechanism is proposed involving initial [4 + 2] cycloaddition of singlet oxygen to the imidazole ring to form an unstable endoperoxide, subsequent rearrangement of the endoperoxide to a dioxirane, and decomposition of the dioxirane to the two observed intermediates. Both oxygen atoms Of CO2 are derived from a single oxygen molecule, which strongly supports a dioxirane structure for the precursor of the two observed intermediates. The distribution of products estimated by C-13 NMR accounts for all the C-13-containing products in the reaction mixture.