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

Abstract

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