Nucleosides. 117. Synthesis of 4‐oxo‐8‐(β‐D‐ribofuranosyl)‐3H‐pyrazolo[1,5‐a]‐1,3,5‐triazine (OPTR) via 3‐amino‐2N‐carbamoyl‐4‐(β‐D‐ribofuranosyl)pyrazole (ACPR) derivatives

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Abstract
Reaction of 2‐formyl‐2‐(2,3‐O‐isopropylidene‐5‐O‐trityl‐D‐ribofuranosyl)acetonitrile (VII) with semicarbazide hydrochloride followed by sodium ethoxide treatment afforded an α,β‐mixture of 3‐amino‐2N‐carbamoyl‐4‐(2,3‐O‐isopropylidene‐5‐O‐trityl‐D‐ribofuranosyl)pyrazole (IX). Conversion of IX to 4‐oxo‐8‐(2,3‐O‐isopropylidene‐5‐O‐trityl‐D‐ribofuranosyl)‐3H‐pyrazolo[1,5‐a]‐1,3,5‐triazine (XIII) was achieved by treatment of IX with ethylorthoformate. The β‐isomer IXb gave only the β‐isomer XIIIb, and the α‐isomer IXa was converted exclusively into the α‐isomer XIIIa. Upon deprotection with 3% n‐butanolic hydrogen chloride, both IXa and IXb gave the same mixture of the α‐ and β‐isomers of 3‐amino‐2N‐carbamoyl‐4‐(D‐ribosyl)pyrazole, which were separated by chromatography.The syntheses of the hitherto unknown compounds, 3‐amino‐2N‐carbamoylpyrazole (IVa) and its 4‐methyl analog (IVb) are also reported. Experimental details of the synthesis of 3‐amino‐4‐(2,3‐O‐isopropylidene‐5‐O‐trityl‐β‐D‐ribofuranosyl)pyrazole (XIIb), an important intermediate for “purine‐like” C‐nucleosides, are also described.