7-substituted 8-aza-7-deazapurines and 2,8-diaza-7-deaza-purines: synthesis of nucleosides and oligonucleotides

7-Substituted 8-aza- 7-deazaadenosines 1a-e were synthesized by Sonogashira cross coupling from the corresponding 7-iodo nucleoside in 36-79% yields. Starting from 7-bromo (or 7-iodo)-8-aza-7-deazaadenine, 2a,b were obtained by acid-catalyzed glycosylation followed by deprotection in 53 and 35% yields, repectively. Compounds 2b was applied to cross coupling reaction to give 2c-d in 34-95% yield. Compounds 2a and 4b were further transformed to the phosphoramidites 5 and 6b in 9 and 49% overall yields, which were incorporated into oligonucleotides.