7‐Iodo‐5‐aza‐7‐deazaguanine: Syntheses of Anomeric D ‐ and L ‐Configured 2‐Deoxyribonucleosides

Iodination of N 2 ‐isobutyryl‐5‐aza‐7‐deazaguanine ( 7 ) with N ‐iodosuccinimide (NIS) gave 7‐iodo‐ N 2 ‐isobutyryl‐5‐aza‐7‐deazaguanine ( 8 ) in a regioselective reaction ( Scheme 1 ). Nucleobase‐anion glycosylation of 8 with 2‐deoxy‐3,5‐di‐ O ‐toluoyl‐ α ‐ D ‐ or α ‐ L ‐ erythro ‐pentofuranosyl chloride furnished anomeric mixtures of D ‐ and L ‐nucleosides. The anomeric D ‐nucleosides were separated by crystallization to give the α ‐ D ‐anomer and β ‐ D ‐anomer with excellent optical purity. Deprotection gave the 7‐iodo‐5‐aza‐7‐deazaguanine 2′‐deoxyribonucleosides 3 ( β ‐ D ; ≥99% de) and 4 ( α ‐ D ; ≥99% de). The reaction sequence performed with the D ‐series was also applied to L ‐nucleosides to furnish compounds 5 ( β ‐ L ; ≥99% de) and 6 ( α ‐ L ; ≥95% de).