Substituted pyrazolo[3,4-d]pyrimidines were prepared by reaction of methyl- or phenylhydrazine with pyrimidine derivatives containing a methylthio or chlorine substituent as nucleofuge. Using a methylthio-6-imino-1,3-thiazine as starting material instead of a methylthiopyrimidine the conversion with phenylhydrazine could already be achieved under mild conditions thus leading first to the formation of a hydrazinopyrimidine. The affinity of the products against the human adenosine A2A receptor was determined.