6-Methylthiopurin-8-ones. a study of their tautomerism and their reactions with electrophilic reagents

6-Methylthiopurin-8-one (1) is present in aqueous solution as the 7-NH,9-NH-tautomer. Monoanion formation involves predominantly the 9-NH group (ca. 80%), and in protonation positions 1 and 3 participate about equally. In compound (1) and its N-methyl derivatives, as in other series of purines, formation of 'resonance-stabilised' ions is preferred over that of 'fixed' ions. The course of methylation of the neutral molecules or of the anions by electrophilic methylating reagents parallels in general the direction of proton attachment. However, methylation of the neutral molecule of (1) takes place exclusively at N-3, owing to steric interference of the 6-methylthiosubstituent with alkylation at N-1.