Pyrazolo[3,4-d]pyrimidineribonucleosides related to 2-aminoadenosine and isoguanosine: synthesis, deamination and tautomerism

Abstract

The syntheses and properties of 8-aza-7-deazapurine (pyrazolo[3,4-d]pyrimidine) ribonucleosides related to 2-aminoadenosine and isoguanosine are described. Glycosylation of 8-aza-7-deazapurine-2,6-diamine 5 with 1-O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose (12) in the presence of BF₃·Et₂O as a catalyst gave the N⁸ isomer 14 (73%) with a trace amount of the N⁹ isomer 13a (4.8%). Under the same reaction conditions, the 7-halogenated 8-aza-7-deazapurine-2,6-diamines 6–8 afforded the thermodynamically more stable N⁹ nucleosides 13b–d as the only products (53–70%). Thus, a halogen in position 7 shifts the glycosylation from N⁸ to N⁹. The 8-aza-7-deazapurine-4,6-diamine ribonucleosides 1a–d were converted to the isoguanosine derivatives 3a–d by diazotization of the 2-amino group. Although compounds 1a,b do not contain a nitrogen at position 7 (the enzyme binding site), they were deaminated by adenosine deaminase; however, their deamination occurred with a much slower velocity than that of the related purines. The pK_a values indicate that the 7-non-functionalized nucleosides 1a (pK_a 5.8) and 15 (pK_a 6.4) are possibly protonated in neutral conditions when incorporated into RNA. The nucleosides 3a–d exist predominantly in the keto (lactam) form with K_TAUT (keto/enol) values of 400–1200 compared to 10³–10⁴ for pyrrolo[2,3-d]pyrimidine isoguanosine derivatives 4a–c and 10 for isoguanosine itself, which will reduce RNA mispairing with U.