Anionic activation in polymer-supported reactions. Part 2. Stereochemical studies on the introduction of fluorine at chiral centres and in biologically significant molecules

(F–) Anion exchange resins used as a polymer-supported reagent are a source of ‘naked’ fluoride and give use to clean SN2 reactions as shown by the preparation of epimerically pure 3-fluoro-steroids and of both diastereoisomers of ethyl 2-fluoropropionate. Fluoro-sugars are also prepared from the corresponding toluene-p-sulphonates. The remaining water of the resin enhances the basic behaviour of F–, as in phase-transfer catalyst techniques, but careful dehydration drives the reaction to clean substitution.