Two Methyl Tri-O-benzoyl-hex-enopyranosides Are Amongst the Products of the Reaction of Methyl 2,3,6-Tri-O-benzoyl-β-D-galactopyranoside with Dimethylaminosulfur Trifluoride (DAST)

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Abstract
In the past, we have used deoxyfluoro sugars as probes for studies of ligand-anti-body interactions.1–3 Investigations of the putative role of hydrogen bonding in the binding process involving antibodies that show specificity for epitopes containing β-D-glucopyranose required hitherto unknown methyl 4-deoxy-4-fluoro-β-D-glucopyranoside (5). For the synthesis of 5, we have chosen the readily available4 methyl 2,3,6-tri-O-benzoyl-β-D-galactopyranoside (1) as the starting material, and diethylaminosulfur trifluoride DAST) as the fluorinating reagent. Although DAST has been frequently used to produce fluorinated carbohydrates in high yields, the outcome of the reaction is somewhat unpredictable. Treatment of carbohydrates with the reagent has occasionally resulted in the formation of unexpected side products.5–9 and no specific rules exist regarding the application of DAST for the preparation of fluorosugars. In fact, high yields of the desired products of the substitution with fluoride ion have been sometimes obtained when DAST was applied under conditions generally known to favor elimination over substitution, e.g. in the presence of a base.10–13 Olefinic products resulting from treatments of carbohydrates with DAST have been sporadically isolated and characterized. Here we describe the title reaction which we believe is unique in that two products of elimination have been formed, as a consequence of attacks of the proton-abstracting species at different sites in the molecule.