The reactivity of 4-methylthiopteridine

6,7-Dimethyl-4-methylthiopteridine (IV) resembles 6-methylthiopurine (I) in its dipole moment, and both compounds undergo alkylation in the pyrimidine ring, para to the thioether group. However, in contrast to the purine (I), the pteridine (IV) is readily amenable to thiohydrolysis, presumably under the influence of the π-deficient pyrazine ring.