Sigma-complexes in the pyrimidine series. 11. 5-arylazo-5-nitro-2,5- and -4,5-dihydro-pyrimidines from anionic sigma complexes of 5-nitropyrimidine

We have studied the reaction of acetonyl anionic sigma complexes of 5-nitropyrimidines with aryldiazonium salts. We have established that, independently of the position of the geminal unit in the sigma complex, electrophilic attack by the diazonium ion occurs at the C(5) atom with formation of 5-arylazo-5-nitro-2,5- and 4,5-dihydropyrimidines. The latter are unstable and upon isolation are converted to N-methoxycarbonyl-N′-(1-acetonyl-2-nitro-2-arylhydrazono)ethyl-O-methylisourea. The structure of one of these compounds has been proven by x-ray diffraction. Under alkaline conditions, aromatization of 2,5-dihydropyrimidines to 5-arylazo-2-acetonylpyrimidines occurs.