Synthesis of [1,3, NH2-(15)N3] (5'S)-8,5'-cyclo-2'-deoxyguanosine.

To facilitate NMR studies and low-level detection in biological samples by mass spectrometry, [1,3, NH2-(15)N3] (5'S)-8,5'-cyclo-2'-deoxyguanosine was synthesized from imidazole-4,5-dicarboxylic acid in 21 steps. The three (15)N isotopes were introduced during the chemo-enzymatic preparation of [1,3, NH2-(15)N3]-2'-deoxyguanosine using an established procedure. The (15)N-labeled 2'-deoxyguanosine was converted to a 5'-phenylthio derivative, which allowed the 8-5' covalent bond formation via photochemical homolytic cleavage of the C-SPh bond. SeO2 oxidation of C-5' followed by sodium borohydride reduction and deprotection gave the desired product in good yield. The isotopic purity of the [1,3, NH2-(15)N3] (5'S)-8,5'-cyclo-2'-deoxyguanosine was in excess of 99.94 atom% based on liquid chromatography-mass spectrometry measurements.