Tautomerism, ionisation, and methylation of 2-(methylthio)- and 2,8-bis(methylthio)-hypoxanthines

Anion formation in 2-(methylthio)- and 2,8-bis(methylthio)-hypoxanthine occurs first at the 1-NH group and then at 7(9)-NH. Protonation involves the imidazole ring, with the exception of the 3-methyl derivatives, which attach the proton to N-1. 3-Methylhypoxanthine is methylated to form the 1,3-dimethylhypoxanthinium cation, but in the 3-methyl derivatives of 2-(methylthio)-and 2,8-bis(methylthio)-hypoxanthine attack at N-7 is predominant overthat at N-1. Formation of the cations [RN-^⋯C(SMe)- ^⋯NMe]⁺ is followed by S-demethylation.