Tautomerism 2-oxopyrimidine

The equilibrium between the amino and imino forms of cytosine has also been studied by Cieplak et al.9 Their results agree with those of Colominas et al. In this study, the related 2-oxopyridine and 2-oxopyrimidine molecules were also treated. In both these molecules the amine group of cytosine is not present, and in oxopyridine only one ring nitrogen is present. This enabled the keto-imino tautomerism to be studied in isolation. In both cases the imino form dominates in the gas phase, but the keto form is stabilised by solvation, and dominates in solution, in agreement with experiment. 

Cieplak, P., and Geller, M. (1985). J. Mol. Struct. (THEOCHEM) 121, 247—Monte Carlo simulation of water effects on tautomeric equilibrium of 2-oxopyrimidine. 

The intramolecular Diels-Alder reactions of simple, 4-hydroxy- or 4-alkyl-6-oxopyrimidines ° bearing olefinic, acetylenic, or nitrile dienophiles have been investigated (Table 10-VI). In many instances, the initial intramolecular pyrimidine Diels-Alder products are thermally stable due to the formation of a bicyclic amide. The elimination of cyanic acid (HNCO) apparently requires thermal tautomerization of the amide to the hydroxyimine and subsequent retro-Diels-Alder reaction. The application of these observations in the total synthesis of ( )-acetinidine has been detailed [Eqs. (20) and (21). ... 

As a typical pi-deficient system, hydroxyl groups on the pyrimidine are involved in tautomeric equilibrium with the 0x0 form, and it is this form that greatly dominates in the equilibrium. Nevertheless, electrophilic substitution is easily effected on the oxopyrimidine stmcture. As an example, the diketo pyrimidine uracil (9.120) reacts readily with nitric acid to give the 5-nitro product 9.121. This suggests that the reactive species may be the hydroxyl form, albeit in a low concentration (Scheme 9.60). 

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