Replacement of the NCN or CNC Fragment

Intermolecular NCN or CNC transfragment reactions have been extensively studied and many examples of these transformations are reported in the literature. 

Besides quaternary pyrimidinium salts, pyrimidines with a strong electron-withdrawing substituent on one of the carbons of the pyrimidine ring show enhanced reactivity of the heterocyclic ring toward nucleophiles. In light of the results mentioned previously with pyrimidinium salts, it is not too surprising that reflux of an ethanolic solution of 5-nitropyrimidine with benzamidine hydrochloride or pivalamidine hydrochloride in the presence of triethylamine afforded 5-nitro-2-phenylpyrimidine (108, 84% 

The challenging question can be raised as to whether the formation of 109 can be explained in an alternative way, i.e., by a Diels-Alder reaction with an inverse electron demand, in which the C = N group of the electron-rich benzamidine forms a regiospecific cycloadduct across the 1,4-position 

Another supporting piece of evidence was obtained in a study of the reaction of 5-nitropyrimidine with benzamidine hydrochloride in the absence of triethyl amine (which usually was added to liberate the free benzamidine). In amidinium salts the C-N bonds are about equal in length [1.314 A 

Other interesting examples of intermolecular N-C-N transfragment replacement are the ones being found when 1,3-dimethyluracil (113, R = R = H) and several of its C-5/C-6 mono-substituted or C-5,6 di-substituted derivatives react with different 1,3-ambident nucleophiles (77JHC537 84H(2)89). Reaction of (113, R = R = H) with guanidine gives isocytosine 115 (R = R = H) in reasonable-to-good yields. 

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