Protonic reactivity of sucrose in anhydrous hydrogen fluoride

Abstract

Sucrose reacts quantitatively, when dissolved at high concentration in anhydrous hydrogen fluoride, to afford a complex mixture of difructose dianhydrides and their glucosylated derivatives. Oligo- and small poly-saccharides up to dp 14 were detected by FABMS. Oligosaccharides up to dp 4, representing ≈50% of the total mixture, have been isolated and characterized by mass spectrometry, 13C NMR spectroscopy, and comparison with reference oligosaccharides previously obtained by unambiguous synthesis, α- d-Fructofuranose β- d-fructopyranose 1,2′:2,1′-dianhydride is the main spirodioxanyl pseudodisaccharide entity found in the mixture, either free or glucosylated at C-6 and to a lesser extent at C-3, C-4, C-4′ C-6, and C-5′ C-6. Minor spirodioxanyl pseudodisaccharide components are di-β- d-fructopyranose 1,2′:2,1′-dianhydride, which has also been found glucosylated at C-5, α- d-fructopyranose β- d-fructopyranose 1,2′:2,1′-dianhydride, β- d-fructofuranose β- d-fructopyranose 1,2′:2,3′-dianhydride, and the 6,6′-diglucosylated α- d-fructofuranose β- d-fructofuranose 1,2′:2,1′-dianhydride. A 13C NMR examination of the higher mass oligomeric fraction fraction suggests that it may involve 6- O-isomaltooligoglycosyl α- d-fructofuranose β- d-fructopyranose 1,2′:2,1′-dianhydride as the main structural components. The reaction of sucrose in anhydrous HF is believed to proceed through initial selective protonic activation of the tertiary anomeric carbon atom of the fructose moiety, resulting in the quantitative formation of difructose dianhydrides, which subsequently suffer electrophilic substitution by glucopyranosyl oxocarbenium ions generated in a second step by action of the HF.