C-5 Epimerisation of D-Mannopyranosyl Fluorides: The Influence of Anomeric Configuration on Radical Reactivity

The fluorine-directing effect has so far been exploited to provide short and efficient synthetic routes to rare L-ido sugars. However, the importance of anomeric configuration to its success has remained experimentally unverified. We now report on the synthesis of alpha- and beta-configured per-O-benzoylated mannopyranosyl fluorides and initially show that their reactivity towards photo-bromination is strongly dependent on the anomeric configuration. The stereochemical basis of the fluorine-directing effect is then validated by revealing the striking difference in stereoselectivity observed for the free-radical reductions of the isolated 5-C-bromo sugars. This work importantly provides a synthetic route to a donor-functionalised derivative of L-gulose and reveals new insights into the behaviour of glycosyl radicals.