Formation of transient intermediates in low-temperature photosensitized oxidation of an 8-(13)C-guanosine derivative.
An 8-(13)C-labeled guanosine derivative, 2',3',5'-O-tert-butyldimethylsilyl-N-tert-butyldimethylsilyl-8-(13)C-guanosine, was synthesized and its photosensitized oxidation with singlet oxygen carried out below -100 degrees C. Two transient intermediates that decompose directly to the final major product 5 and CO(2) were detected by (13)C NMR between -100 and -43 degrees C. The two intermediates are carbamic acids based on (13)C NMR and 2D NMR (HMQC, HMBC) spectra and the formation of final product 5 and of 8-CO(2). No endoperoxide intermediate could be detected by low-temperature NMR spectroscopy even at -100 degrees C. 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 CO(2) 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 (13)C NMR accounts for all the (13)C-containing products in the reaction mixture.
Kang
Ping
P
Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095-1569, USA.
Foote
Christopher S
CS
D002245:Carbon Dioxide / Q000737:chemistry; D002247:Carbon Isotopes; D003080:Cold Temperature; D008401:Gas Chromatography-Mass Spectrometry; D006151:Guanosine / Q000031:analogs & derivatives* / Q000737:chemistry; D019906:Nuclear Magnetic Resonance, Biomolecular; D010084:Oxidation-Reduction; D017319:Photosensitizing Agents / Q000737:chemistry*
C459744:2',3',5'-O-tert-butyldimethylsilyl-N-tert-butyldimethylsilyl-8-(13)C-guanosine; D002247:Carbon Isotopes; D017319:Photosensitizing Agents; D006151:Guanosine; D002245:Carbon Dioxide