Formation and Properties of Isopyrimidines

Isopyrimidines are short-lived intermediates formed in the reactions of C(6)-radicals of pyrimidines which carry an H-atom at A (l). Such radicals can be oxidized via the peroxyl [cf. reactions (26) and (27)] but also by oxidants such Fe(CN)6 directly to the radical cation [reactions (37) and (41)]. The isopyrimidines undergo rearrangement and hydration reactions. Some rate constants are compiled in Table 6. 

Isopyrimidine Neutral spont. S- Neutral H -catal. dm mof s Neutral OH -catal. dm mol s- Anion spont. s Anion OH -catal. dm mol s  

Isopyrimidine Neutral spont. s 1 Neutral H+- catal.dm3 mol-1 sJ Neutral OH -catal.dm3 mol-1 sJ Anion spont. s-1 Anion OH -catal. dm3 moh1 s-1 

Deprotonation of the carbocation at N( 1) [reaction (137)] is kinetically favored over the deprotonation at C(5) [reaction (136)]. Thus, isouracil is an important intermediate in these reactions. It is sufficiently long-lived to follow its reactions by pulse radiolysis. Isouracil is in equilibrium with its anion [equilibrium (138)]. This allows reactions (139) and (140) to proceed as well. However, when N(3) is substituted such as in 3MeUra, this additional pathway is no longer possible. For a compilation of rate constants see Table 10.17. 

---