Synthesis of 2′‐Deoxyribofuranosides of 8‐Aza‐7‐deazaguanine and Related Pyrazolo[3,4‐d]pyrimidines

The N(1)‐ and N(2)‐(2′‐deoxyribofuranosides) 1 and 2, respectively, of 8‐aza‐7‐deazaguanine were prepared via phase‐transfer glycosylation in the presence or absence of Bu4NHSO4 as catalyst of 6‐amino‐4‐methoxy‐lH‐pyrazolo[3,4‐d]pyrimidine (7c) with 2‐deoxy‐3,5‐di‐O‐(p‐toluoyl)‐α‐D‐erythro‐pentofuranosyl chloride (10). On a similar route, but without catalyst and employing THF as organic phase, the 6‐amino‐4‐chloronucleosides 11b and 12b were synthesized from 7a and converted into the N(1)‐and N(2)‐substituted 4‐thioxo analogues 17a and 18a, respectively. The ratio of N(1)‐ to N(2)‐glycosylation was 2:1 for 7c and 1:1 for 7a, viz. depending on the nucleobase structure. The rate of the H+‐catalyzed N‐glycosyl hydrolysis was strongly decreased for the N(2)‐(β‐D‐2′‐deoxyribofuranosides) as compared to the N(1)‐compounds. However, the N(1)‐nucleoside 1, which is an isostere of 2′‐deoxyguanosine, is sufficiently stable to be employed later in solid‐phase oligonucleotide synthesis.