Synthetic Access to l‑Guluronic Acid via Fluorine-Directed C‑5 Epimerization

l-Guluronic acid is integral to the structures of alginates and to the pathogenesis of Pseudomonas aeruginosa. 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.