ELECTROPHILIC AND NUCLEOPHILIC SUBSTITUTION REACTIONS OF 8-PYRIDYL- AND 8-(1-METHYLPYRIDINIO)-PURINES

When 6-methylthio-8-(3- or 4-pyridyl)purines are treated with methyl iodide in dimethylformamide, they are first quaternised at the pyridyl nitrogen. The quaternary 8-(1-methylpyridinio)-substituent then prevents further alkylation in the purine ring, with the exception of position 3. In contrast, in the 8-(2-pyridyl) series, alkylation proceeds only at the purine nitrogens, the only exception being the 3-methyl-6-methylthio-derivative, which undergoes quaternisation. In the isomeric 3-methyl-6-methylthio-8-pyridylpurines, the rates of quaternisation follow the order 4-pyridyl > 3-pyridyl > 2-pyridyl. A quaternary 8-pyridyl substituent enhances thiohydrolysis of the 6-methylthio-group. The synthesis and physical properties of 8-pyridylhypoxanthines, 6-mercapto-8-pyridylpurines, and their N- and S-methyl derivatives are described.