A CONVENIENT SYNTHESIS OF PYRAZOLO[3,4-B]PYRIDINE NUCLEOSIDES BY CONVENIENT RING-CLOSURE PROCEDURES - X-RAY CRYSTAL AND MOLECULAR-STRUCTURE OF 4-AMINO-1-(BETA-D-RIBOFURANOSYL)-1,7-DIHYDROPYRAZOLO[3,4-B]PYRIDIN-6-ONE

Practical syntheses of 1-(β-D-ribofuranosyl)pyrazolo[3,4-b]pyridines as purine antagonists, related to isoguanosine, guanosine, and xanthosine, are described for the first time. Ethyl 5-amino-1-(2,30-isopropylidene-β-D- ribofuranosyl)-1H-pyrazole-4-carboxylate (10) was converted into 5-amino1-(2,3-0-isopropylidene-β-D-ribofuranosyl)-1H-pyrazole (12), which was in turn converted into 6amino-1-(β-D-ribofuranosyl)-1,7- dihydropyrazolo[3,4-b]pyridin-4-one (4) in four steps. 4-Amino1-(β-D- ribofuranosyl)-1,7-dihydropyrazolo[3,4-b]pyridin-6-one (3) was obtained in satisfactory overall yield from compound (12), prepared in four steps via a ring-closure procedure. The structure of compound (3) was established by a single-crystal X-ray study. 4-Hydroxy-1-(β-D-ribofuranosyl)1,7- dihydropyrazolo[3,4-b] pyridin-6-one (5) was obtained in three steps from the ester (10).