Syntheses of 6‐Amino‐1‐(β‐D‐ribofuranosyl)‐1 H ‐pyrazolo[3,4‐ d ]‐1,3‐oxazin‐4‐one, an isostere of oxanosine, and the guanosine analog 6‐amino‐1‐(β‐d‐ribofuranosyl)‐1 H ‐pyrazolo[3,4‐ d ]pyrimidin‐4(5 H )‐one via ring closure of pyrazole‐5‐thioureido intermediates

6‐Amino‐1‐(β‐D‐ribofuranosyl)‐1 H ‐pyrazolo[3,4‐ d ]‐1,3‐oxazin‐4‐one ( 4 ), an isostere of the nucleoside antibiotic oxanosine has been synthesized from ethyl 5‐amino‐1‐(2,3‐ O ‐isopropylidene‐β‐D‐ribofuranosyl)pyrazole‐4‐carboxylate ( 6 ). Treatment of 6 with ethoxycarbonyl isothiocyanate in acetone gave the 5‐thioureido derivative 7 , which on methylation with methyl iodide afforded ethyl 1‐(2,3‐ O ‐isopropylidene‐β‐D‐ribofuranosyl)‐5‐[( N' ‐ethoxycarbonyl‐ S ‐methylisothiocarbamoyl)amino]pyrazole‐4‐carboxylate ( 8 ). Ring closure of 8 under alkaline media furnished 6‐amino‐1‐(2,3‐ O ‐isopropylidene‐β‐D‐ribofuranosyl)‐1 H ‐pyrazolo[3,4‐ d ]‐1,3‐oxazin‐4‐one ( 10 ), which on deisopropylidenation afforded 4 in good yield. 6‐Amino‐1‐(β‐D‐ribofuranosyl)‐1H‐pyrazolo[3,4‐ d ]pyrimidin‐4(5 H )‐one ( 5 ) has also been synthesized from the AICA riboside congener 5‐amino‐1‐(2,3‐ O ‐isopropylidene‐β‐D‐ribofuranosyl)pyrazole‐4‐carboxamide ( 12 ). Treatment of 12 with benzoyl isothiocyanate, and subsequent methylation of the reaction product with methyl iodide gave 1‐(2,3‐ O ‐isopropylidene‐β‐D‐ribofuranosyl)‐5‐[( N' ‐benzoyl‐ S ‐methylisothiocarbamoyl)amino]pyrazole‐4‐carboxamide ( 15 ). Base mediated cyclization of 15 gave 6‐amino‐1‐(2,3‐ O ‐isopropylidene‐β‐D‐ribofuranosyl)‐1 H ‐pyrazolo[3,4‐ d ]pyrimidin‐4(5 H )‐one ( 14 ). Deisopropylidenation of 14 with aqueous trifluoroacetic acid afforded 5 in good yield. Compound 4 was devoid of any significant antiviral or antitumor activity in culture.