Rearrangement of quinonoid dihydropteridines to 7,8-dihydropteridines

The quinonoid isomer of 6,7-dimethyltetrahydropterin has been prepared in situ from the tetrahydropterin by rapid oxidation with potassium ferricyanide. Measurements of the kinetics of the rearrangement of the quinonoid dihydropterin to 7,8-dihydropterin show that the reaction is buffer-catalyzed. Both the acidic and basic forms of the buffer participate as catalysts, with the relative contribution of each species toward the rate of the reaction being dependent on the pK a value of each buffer. Isotopic experiments have shown that the cleavage of the C—H bond at position 6 is rate-limiting during both the acid- and the base-catalyzed rearrangements.