Electrophilic substitution pyrimidones

As mentioned earlier, electrophilic substitution on unactivated pyrimidines is of little importance. But, as with pyridine, the pyrimidine nucleus can be activated towards electrophilic attack by employing N-oxides or pyrimidones, for the same reasons as were discussed in Chapter 5. 

Mesoionic 3-arylthiazolo(3 2-a]-4-pyrimidones (258 RsH) are extremely susceptible to electrophilic substitution, sometimes... 

Uracils undergo a range of electrophilic substitution reactions such as halogena-tion, phenylsulfenylation, mercuration, and hydroxy- and chloromethylation. Bromination of uracils has been shown to proceed via the bromohydrin adduct, and similarly of 2-pyrimidone, via the bromohydrin-hydrate iodine with tetrabutyl-ammonium peroxydisulfate allows iodination. ... 

Electrophilic substitution at the aromatic ring of chain-fluorinated diazines is rather unfavourable due to their electron-deficient nature. It is possible however when electron-donating substituents are also present in the diazine ring. For example, Shlosser reported successful bromination of pyrimidone 1237 with molecular bromine (Scheme 272) [715]. Halogenation of chain-fluorinated pyrimidine 1239 with S02Cl2-FeCl3 was also reported (Scheme 273) [759]. 

Mesoionic 3-arylthiazolo[3>2-a]-4-pyrimidones (258 RsH) are extremely susceptible to electrophilic substitution, sometimes to the detriment of their stability. It was found that generation of (258 R=H), from corresponding 4-oxo-A -thiazolium perchlorates (259), in the presence of electrophiles readily gives acyl, formyl, and arylazo derivatives (258 R=COalkyl,CH0,N=NAr, respectively)... 

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