Synthetic Access to <scp>l</scp> -Guluronic Acid via Fluorine-Directed C-5 Epimerization

l-Guluronic acid is integral to the structures of alginates and to the pathogenesis of <i>Pseudomonas aeruginosa</i>. The exploitation of this hexose in both existing and new contexts is, however, limited by its prohibitively high commercial cost. We report on a short and efficient synthetic route to an l-GulA building block from a simple d-mannose thioglycoside. In this synthesis, the fluorine-directing effect is exploited to achieve a stereoselective C-5 epimerization. DFT calculations illuminate the substituent effects, which operate to confer this selectivity.