AbstractSeveral imidazo[4,5‐d]pyridazine nucleosides which are structurally similar to inosine were synthesized. Anhydrous stannic chloride‐catalyzed condensation of persilylated imidazo[4,5‐d]‐pyridazin‐4(5H)one (1) and imidazo[4,5‐d]pyridazine‐4,7(5H,6H)dione (16) with 1‐O‐acetyl‐2,3,5‐tri‐O‐benzoyl‐β‐D‐ribofuranose (3) provided (after sodium methoxide deblocking) 6‐β‐D‐ribo furanosylimidazo[4,5‐d]pyridazin‐4(5H)one (5) and 3,6‐di‐(β‐D‐ribofuranosyI)imidazo[4,5‐d]pyridazin‐4‐one (7); and 1‐(β‐D‐ribofuranosyl)imidazo[4,5‐d]pyridazine‐4,7(5H,6H)dione (19) and 1,5 or 6‐di‐(β‐D‐ribofuranosyl)imidazo[4,5‐d]pyridazine‐4,7(5Hor 6H)dione (21), respeeitvely. 4,7‐Diehloro‐1‐β‐D‐ribofuranosylimidazo[4,5‐d]pyridazine (12) and dimethyl 1‐β‐D‐ribofuranosylimidazole‐4,5‐dicarboxylate (26), both prepared from stannic chloride‐catalyzed ribosylations of the corresponding heterocycles, were converted in several steps to 3‐β‐D‐ribo‐furanosy limidazo[4,5‐d]pyridazin‐4(5H)one (14) and nucleosidc19, respectively. Acid‐catalyzed isopropylidenation of mesomeric betaine7or nuclcoside14provided 3‐(2,3‐isopropylidene‐β‐D‐ribofuranosyl)imidazo[4,5‐d]pyrizin‐4(5H)one (31). 1‐β‐D‐Ribofuranosylimidazo[4,5‐d]‐pyr