2‐Amino‐8‐(2‐deoxy‐2‐fluoro‐ β ‐ D ‐arabinofuranosyl)imidazo[1,2‐ a ]‐1,3,5‐triazin‐4(8 H )‐one: Synthesis and Conformation of a 5‐Aza‐7‐deazaguanine Fluoronucleoside

Nucleobase‐anion glycosylation of 2‐[(2‐methyl‐1‐oxopropyl)amino]imidazo[1,2‐ a ]‐1,3,5‐triazin‐4(8 H )‐one ( 6 ) with 3,5‐di‐ O ‐benzoyl‐2‐deoxy‐2‐fluoro‐ α ‐ D ‐arabinofuranosyl bromide ( 8 ) furnishes a mixture of the benzoyl‐protected anomeric 2‐amino‐8‐(2‐deoxy‐2‐fluoro‐ D ‐arabinofuranosyl)imidazo[1,2‐ a ]‐1,3,5‐triazin‐4(8 H )‐ones 9 / 10 in a ratio of ca. 1 : 1. After deprotection, the inseparable anomeric mixture 3 / 4 was silylated. The obtained 5‐ O ‐[(1,1‐dimethylethyl)diphenylsilyl] derivatives 11 and 12 were separated and desilylated affording the nucleoside 3 and its α ‐ D anomer 4 . Similar to 2′‐deoxy‐2′‐fluoroarabinoguanosine, the conformation of the sugar moiety is shifted from S towards N by the fluoro substituent in arabino configuration.