Development of Synthetic Methods for N-Labeled Nucleosides to Elucidate the Structural Recognition Ability of Nucleic Acids

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NMR studies employing oligonucleotides regio-selectively labeled with ^<15>N atom provide valuable information regarding nucleic acid structures, nucleic acids binding with drugs, and nucleotide-protein interactions. The potential utility of the ^<15>N-labeled oligonucleotides has led to the considerable interest in the development of synthetic routes to the required ^<15>N-labeled nucleosides. The N_1-and N^6-positions of the adenine ring or N_3-and N^4-positions of the cytosine ring are good candidates for the ^<15>N-labeling, because they can form bydrogen bonds with suitable donors or acceptors in the nucleic acids, drugs, and proteins. On this line, we have developed a newly devised method for the debenzylation of N^6-benzyladenosine and N^4-benzylcytidine which involves oxidation with ammonium peroxydisulfate under mild conditions. The present method is based on the chemical reactivities of cation radical species of the nucleosides and provides simple and high-yield procedures for the [1-and 6-^<15>N] -labeled adenosines or [3-and 4-^<15>N] -labeled cytidines from inosines or uridines without protection of hydroxyl groups in the sugar moiety.