Tautomerism and ionisation processes in 6-methylthio-2-oxopurines

6-Methylthio-2-oxopurine (1) is present in aqueous solution mainly as the 3,7-di-NH tautomer. This supports the general assumption that purines avoid 3,9-disubstituted structures. Anion formation in structure (1) takes place first at the 7- and then at the 3-position. Cations bearing a 1-methyl group are unstable, undergoing hydrolysis, even at pH 4·5, to the corresponding xanthines. In the n.m.r. spectra of the 1-methyl derivatives of (1). steric interference causes a marked downfield shift of the 6-SMe signal.