Pyrimidine ring contributors

The substitution of pteridines at positions adjacent to the pyridine-like nitrogen atoms in either the pyrimidine or the pyrazine is a well-established synthetic procedure and remains an important contributor to the synthesis of complex substituted pteridines. Significant extensions of these methods have been described at both the pyrimidine and pyrazine rings. 

Both purines and pyrimidines contain nitrogens in their rings glutamate and aspartate are heavy contributors of these nitrogens (fig. 5.4). 

The structures of the diazines (six-membered systems with two nitrogen atoms in the ring) are analogous, but now there are two nitrogen atoms and a corresponding two lone pairs as an illustration, the main contributors (11-18) to pyrimidine are shown below. 

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