Formation of 2'-deoxyoxanosine from 2'-deoxyguanosine and nitrous acid: mechanism and intermediates

The reaction mechanism for the formation of 2′-deoxyoxanosine from 2′-deoxyguanosine by nitrous acid was explored using methyl derivatives of guanosine and an isolated intermediate of the reaction. When 1-methylguanosine was incubated with NaNO₂ under acidic conditions, N⁵-methyloxanosine and 1-methylxanthosine were generated, whereas the same treatment of N²,N²-dimethylguanosine generated no product. In a similar experiment without NO₂^–, participation of a Dimroth rearrangement was ruled out. In the guanosine–HNO₂ reaction system, an intermediate with a half-life of 5.6 min (pH 7.0, 20°C) was isolated and tentatively identified as a diazoate derivative of guanosine. The diazoate intermediate was converted into oxanosine and xanthosine at a molar ratio (oxanosine:xanthosine) of 0.26 at pH 7.0 and 20°C. The ratio was not affected by the incubation pH between 2 and 10, but increased linearly with temperature from 0.22 (0°C) to 0.32 (50°C). The addition of acetone also increased the ratio up to 0.85 (98% acetone). Based on these results, a con-ceivable pathway for the formation of 2′-deoxyoxanosine from 2′-deoxyguanosine by HNO₂ is proposed.