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 α- and β-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.
Key words
glycosyl radicals -
l-hexoses - photochemistry - halides - rearrangements
