A convenient synthesis of 1-(β-D-ribofuranosyl)imidazo[4,5-d]pyridazin-4(5H)-one (2-aza-3-deazainosine) and its 2′-deoxy counterpart by ring closure of imidazole nucleosides

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Abstract
Cyclization of methyl 5-formyl-1-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)imidazole-4-carboxylate (5) with 97% hydrazine in ethanol furnished 1-(β-D-ribofuranosyl)imidazo[4,5-d]pyridazin-4(5H)-one (8) in good yield. Likewise, ring closure of methyl 5-formyl-1-(2-deoxy-3,5-di-O-p-toluoyl-β-D-erythro-pentofuranosyl)Imidazole-4-carboxylate (19) using the same conditions provided 1-(2-deoxy-β-D-erythro-pentofuranosyl)imidazo[4,5-d]pyridazin-4(5H)-one (16). This 2′-deoxyribonucleoside (16) was also obtained by deoxygenation of (8).