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

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