Synthesis of bitriazolyl nucleosides and unexpectedly different reactivity of azidotriazole nucleoside isomers in the Huisgen reaction

Novel bitriazolyl nucleosides were synthesized via the Huisgen reaction, starting with 3-azidotriazole nucleoside (1). Surprisingly, its isomer, 5-azidotriazole nucleoside (1') did not yield the corresponding Huisgen reaction products efficiently because it was rapidly reduced to amine in the presence of Cu(II)-ascorbate. The significant differences between the reactivity of these two isomers in Cu(II)-ascorbate mediated reactions are mainly due to differences in their electronic properties and steric congestion as a result of different relative positions of the azido and the ribosyl moieties.