A synthesis of 7-amino-3-(ß-D-arabinofuranosyl)pyrazolo[4,3-d]pyrimidine (13), the D-arabino analogue of formycin (1), from D-mannose is described. 1-(2,4-Dinitrophenyl)-3-(penta-O-acetyl-D-manno-pentahydroxypentyl)pyrazolewas converted into the 4-nitro-derivative 18 in 90% yield using trifluoroacetyl nitrate. In four steps 18 was transformed into the 3-ß-D-arabinofuranosylpyrazole 12 which gave the N-nitro derivative 24 in two further stages. Cine-substitution of the N-nitro group in 24 by cyanide ion was followed by elaboration of the pyrimidine ring to give 13. All the reactions proceeded with complete regio and stereoselectivity.