Three-bond formation, pyrimidine ring

Synthetic methods for the preparation of six-membered heterocyclic systems which proceed via the formation of three or four bonds are virtually restricted in application to the monocyclic heterocycles and have been most widely applied to pyridine and pyrimidine derivatives. In principle, reactions which proceed with the formation of three ring bonds can be sub-classified into three groups, namely, those involving [4 + 1 + 1] atom fragments, [3 + 2 + 1] atom fragments and [2 + 2 + 2] atom fragments. 

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