Pyrimidines mesomeric structures

With respect to (75) the possible mesomeric structures are given in Scheme 5. A few remarks will be placed here regarding possible mesomeric or tautomeric structures for the other species depicted in Scheme 3. For all one- or two-electron reduced species various tautomeric forms can exist, e.g. iminol tautomers. Such tautomers do not exist in flavoquinone. In addition, in (76) and (79) the negative charge could also be placed on the other hetero atoms in the pyrimidine subnucleus... 

Structure of pyrimidine resonance contribntors (mesomeric structures)... 

The structure of the reaction product of 2-aminopyridine and diethyl malonate, described by Chichibabin as 2,4-dioxo-3,4-dihydro-2//-pyrido-[l,2-<7]pyrimidine,96 was first questioned by Snyder and Robison253 on the basis of the high melting point and poor solubility of the compound. They suggested the tautomeric 2-hydroxy-4-oxo-4H-pyrido[l,2-a]pyrimidine structure. The problem was solved by Katritzky and Waring273 who compared the UV spectrum of the product with that of fixed tautomers and found that the product may best be described as anhydro- 2-hydroxy-4-oxo-4/f-pyrido[l,2- ]pyrimidinium)hydroxide (63). Because of the chemical behavior of these compounds, however, the contribution of other mesomeric forms to the structure has also been considered.122 Thus, PPP-SCF quantum chemical calculations suggest that 1,4-dipolar cycloadditions to the C-3 and C-9a atoms are to be expected.352 This type of reaction does in fact occur (see Section III,C,10). Katritzky and Waring273 estimated the ratio of the mesomeric betaine (63 R = H) and the 2-hydroxy-4-oxo tautomers to be about 20 1. 

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