The reaction of 6-chloropurine-2-amine (I) with methanol and NaH gives 6-O-methylguanine (II), which is mixed with uracil arabinoside (III), purine nucleoside phosphorylase and uridine phosphorylase and submitted to incubation in a potassium phosphate solution at 37 C. After 26 days of incubation, the target compound is isolated.