Investigation of the base promoted tandem syn-elimination–Favorskii rearrangement of levoglucosan sulfonates

The reaction of various 2,3,4-trisulfonated 1,6-anhydro-β-D-glucopyranoses with alkoxide was investigated. Upon treatment with methoxide the tritosylate5, tris(chlorosulfate) 20 and tris(trifluoromethanesulfonate) 21 all gave 1,5-anhydro-2,3-dideoxy-3-methoxycarbonyl-β-D-erythro-pentofuranose 6 in 41–57% yield with the yield increasing with improved leaving group ability. The trimesylate19 gave the 3,4-epoxide 23 and no rearranged product. Chloroform was found to be the best solvent for obtaining 6, while alcohol as the solvent only gave epoxide 2. A study of different bases showed that the yield of the Favorskii product was increased 10–20% by using ethoxide as base. Reaction of the corresponding chlorosulfonated 1,6-anhydrogalactopyranose or anhydromannopyranose gave the same product though in lower yield. The Favorskii product 6 rearranged in the presence of traces of acid to a l,5′:1′,5-dianhydride.