Isopyrimidines

With these radicals, spontaneous C(6)-0 heterolysis is slow ( < 10 s" ). However, if the electron density of the system is increased by OH -induced de-protonation of N(l)-H, 02 elimination is observed [23, 24, 25]. With the peroxyl radical from 5,6-dihydrouracil-6-yl, the heterolysis rate constant is 8.3 X 10 s the reaction leading to the isopyrimidine derivative shown [37]. The reaction is perfectly analogous to the eliminations of the radical anions of nitrobenzenes (Eq. 15) or anthraquinone-2,6-disulfonate (Eq. 18). 

In these oxidation processes, often the kinetically favored rather than the thermodynamically favored product is formed. A case in point is the oxidation of pyrimidine-6-yl radicals by Fe(CN)63 to an isopyrimidine which is only a shortlived intermediate that results in the formation of the pyrimidine and its hydrate (Chap. 10.3). 

Schuchmann MN, von Sonntag C (1988) The rapid hydration of the acetyl radical. A pulse radiolysis study of acetaldehyde in aqueous solution. J Am Chem Soc 110 5698-5701 Schuchmann MN, Al-Sheikhly M, von Sonntag C, Gamer A, Scholes G (1984) The kinetics of the rearrangement of some isopyrimidines to pyrimidines studied by pulse radiolysis. J Chem Soc Perkin Trans 2 1777-1780... 

It has been mentioned above that OH abstracts from Thy also to a minor extent a hydrogen at methyl. Indeed, 5-hydroxymethyluracil has been detected to be formed in about 5% yield (Nishimoto et al. 1983a). It is suggested here that the introduction of the OH group may have occurred via an isopyrimidine-type intermediate as depicted in reactions (92) and (93). 

Table 10.17. Rate constants of isopyrimidine (Schuchmann et al. 1984a) -> pyrimidine rearrangements at 20 °C. ...

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... 

The most detailed information as to the decay of nucleobase peroxyl radicals may be obtained from a study on uracil. The limited space here does not allow for the discussion of details, but some aspects are shown in reactions (41)-(47). With the pyrimidine nucle-obases, isopyrimidines are typical intermediates in these and other oxidation reactions cf. reaction (41). For their chemistry see Ref. 2. 

Details of this kind of superoxide elimination, including die determination of the p Ta value of the peroxyl radical, have been elucidated in a very similar system, glycine anhydride [43]. An isopyrimidine (see below) is formed in reaction (27). 

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 ... 

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