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Sir: Formycin1) is a representative pyrazolopyri-midine nucleoside that contains the unusual C-riboside linkage and its biological properties2) have been intensely investigated ever since its discovery by H. UMEZAWA and coworkers.1) Formycin is a cytotoxic analog of adenosine and it has been shown that adenosine deami-nase3.4,5,6) and adenosine kinase6,7,8) are two important enzymes in tumors responsible for resistance to formycin. On the other hand, the syn-anti conformations of formycin have become of interest in view of the relationship between substrate conformation and the speci-ficity of enzymes.9) The X-ray crystallographic study10) of formycin hydrobromide revealed that it possesses a syn conformation, whereas unprotonated adenosine possesses an anti con-formation.11) PRUSINER et al.12) have recently reported that formycin monohydrate possesses an intermediate (or high anti) conformation in the solid state, and this fact suggests that the high anti conformation renders it susceptible to the action of adenosine deaminase which is found to be inactive to the systems of syn conformation.13) The high anti conformation of formycin monohydrate has been attributed12) to the electrostatic repulsion between the N-2 (which replaces the C-8 of adenosine) and the sugar O-5' atom. Recently, TOWNSEND et al.14) prepared the N-1 and N-2 methyl derivatives of formyiin and observed that the latter derivative, in which the rotation around the glycosyl (carbon-carbon) bond is hindered by the presence of the N-2 methyl group, exhibited a higher T/C value against leukemia L-1210 than the N-1 methyl derivative, although no further detail was described. In this paper we report the preparation of two anhydroformycins (1, 2) in which the above-mentioned syn and anti conformations are fixed by cyclization. Isopropylidenation of formycin with 2,2-dimethoxypropane in acetone in the presence of 5 equiv. p-toluenesulfonic acid gave 2',3'-O-isopropylideneformycin (3), a colorless powder, in 94% yield; mp 134-138°C; [a]"-88° (c 0 .5, dimethylsulfoxide). Cyclization of 3 was achieved by the use of triphenylphosphine and diethylazodicarboxylate which is a dehydrating reagent between alcohols and active hydrogen compounds reported by WADA and MITSUNOBU.15) The reaction in dry dioxane at room temperature for 2 hours

Cyclonucleoside derivatives of formycin.