A pyrimidine ring, especially in the condensed or quaternized state, readily adds strong nucleophilic agents. For instance, the covalent hydration of pyrimidines' is bown, and leads to hydrolytic ring opening under more drastic conditions. Similarly, l-alkylpyridinium salts with aznnonia may suffer covalent addi-tion293, ring opening or even elimination of the alkyl group, the reaction sometimes proceeding through a recyclization stage4 . Hydrasine opens the pyrimidine ring, leading to the corresponding pyrazole5. Anions of strong carbon acids, such as cyanoacetic ester, also may open the pyrimidine ring and cyclization now gives 2-aminopyridine derivatives'. This process occurs with the insertion of some of the atoms of the reagent into the newly formed ring. At the same time, by analogy with the recently discovered recyclizations of pyrimido 1,2_a indoles 7 and pyrazolo 1,5-a_ pyrimidines 8 one might expect pyrimidines to isomerise into pyridines on treatment with powerful nucleophiles through a ring opening and ring closure sequence.
We therefore investigated quaternary salts of type 1, which are converted by alkaline reagents into the anhydronium bases 2, where, owing to resonance, the pyrimidine moiety is electron-deficient, and the methene group carries a substantial negative charge. At the same time, as is well known, nucleophiles may attack position 6 of l-alkylpyrimidinium salts and ring opening followed by recyclization with the participation of the reagent may occur. For instance, liquid ammonia causes dealkylation according to the ANRORC mechanism 4 . In this way the nitrogen atom of ammonia may be incorporated into the pyrimidine ring.
Befluxing the salt la in an aqueous solution of sodium hydroxide lead only to hydrolytic cleavage of the pyrimidine ring with the formation of acetylacetone. Similarly, heating with 30$ aqueous methylamine afforded the imines CH3C(NH2)=CHCOCH3 and CH3C(NHCH3)=CHCOCH3.
However, heating the salts Ia or J_b in a sealed tube for 30 hours with an excess of 25s alcoholic methylamine gave the corresponding P-methylaminopyridines za (3446) or lb (69%) which are isomers of the anhydronium bases za,b. The rearrangement also occurs in aqueous-alcoholic solutions of triethylamine.