The synthesis of 2-amino-6-methylthio-3-(β-D-ribofuranosyl)pyrimidin-4-one (7a) and its isomer 4-amino-6-methylthio-3-(β-D-ribofuranosyl)pyrimidin-2-one (9a) is described. Dethiation of (7a) gave 2-amino-3-(β-D-ribofuranosyl)pyrimidin-4-one (7c). Nitrosation of 6-amino-3-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)-pyrimidine-2,4-dione (10) followed by catalytic reduction gave a 5,6-diamino-derivative (12), which was converted into 1-(β-D-ribofuranosyl)xanthine (14). Reaction of 6-amino-3-(β-D-ribofuranosyl)pyrimidine-2,4-dione (15) with chloroacetaldehyde afforded 3-(β-D-ribofuranosyl)-7H-pyrrolo[2,3-d]pyrimidine-2,4-dione (16).