New vinylogous mesomeric betaines: synthesis and tautomerism of pyridiniopyrimidine appended 5-iminopenta-1, 3-dienolates

The pyrimidine-pyridinium salts 4-15 were regioselectively synthesized by n ucleophilic substitution on the 4-amino-2,5,6-trichloropyrimidines 1-3. The structure of 7 was established by X-ray crystallography. The cross-conjuga ted mesomeric betaines 16 and 17 were formed smoothly on treatment of 6 and 9 in aqueous ethanol with the anion exchange resin Amberlite(R) IRA-400 in its hydroxy form. Under similar conditions, pericyclic ring-cleavage of th e bispyridinium salts 10-15 yielded the title compounds 18-23 as a mixed po pulation of tautomers in rapid equilibrium.