Tautomerism and Ionisation of 6‐Thioxopurin‐8‐Ones. A Contribution to the Behaviour of 6‐Thiopurines

The neutral molecules of 6‐thioxopurin‐8‐ones are present in aqueous solution as lactams‐thiolactams. However, 6‐SH‐tautomers may be present in the anions of the 1‐ and 9‐methyl derivatives. The sequence of dissociation of the NH‐groups is 1 → 9 → 7. Ionisation of the 9‐NH‐group is greatly facilitated in the 3‐methyl derivatives, leading to pK values of about 5. Protonation of the 3‐methyl derivatives takes place predominantly at N‐1 with the formation of “fixed”, amidinium‐like, cations. In all other members of the series, protonation involves mainly the 8‐oxo group. In 3,9‐dimethyl derivatives, the NMR signals of both methyl substituents are markedly deshielded. The behavior of 6‐thioxopurin‐8‐ones resembles that of 6‐thiopurines and 6‐thioxanthines.