SELECTIVITIES IN THE FORMATION OF PYRIDINES AND PYRIMIDINES BY AMMONIA-INDUCED CYCLOCONDENSATIONS OF VINAMIDINIUMS

Arylvinamidines (2-, 3- or -aryl-4-(N,N-dimethyl)amino-1-azabuta-1,3-d ienes), generated from 1,1,5,5-tetramethyl-2- or -3-phenyl-1,5-diazape ntadienium salts, cyclocondense orientation-specifically under two reg ioselections forming 1-4' + 4-3' and 1-2' + 4-1' bonds on exposure to ammonia. The initial cyclates aromatise eliminatively to give mixtures of diarylpyridines and arylpyrimidines. The 2-arylvinamidines do not participate as 2-centre reactants and their 4-aryl isomers not as 4-ce ntre reactants in the cyclocondensations which appear to be stepwise a nd not concerted. Reasons for the selective participation appear to be that the required eliminations from tile initial cyclates are disfavo ured in the first case and that a geometric factor prevents cyclate-fo rmation in the second.