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The chemical synthesis of certain 4-substituted pyrazolo[3,4-d]pyrimidine nucleosides is described. Using 1-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl)pyrazolo[3,4-d]pyrimidin-4-one (1) as the starting material, the reactive intermediate 4-chloro-1-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl)pyrazolo[3,4-d]pyrimidine (2) was prepared in excellent yield. Compound 2 served as a versatile precursor for the synthesis of a number of 4-substituted pyrazolo[3,4-d]pyrimidine nucleosides. In antitumor studies of these nucleosides, in vitro and in vivo, it was found that any alteration of the 4-amino substituent of 4-amino-1-beta-D-ribofuranosylpyrazolo[3,4-d]pyrimidine (3) was accompanied by a significant decrease or loss of antitumor activity. On the other hand, introduction of certain substituents at the 3 position of 3 (synthesis reported previously) led to a dramatic increase in antitumor activity in comparison to the parent compound.