Analysis of linkage positions in d-glucopyranosyl residues by the reductive-cleavage method

Abstract

The positions of linkage in the d-glucans amylose, cellulose, laminaran, pullulan were established by permethylation and subsequent reductive cleavage with triethylsilane and either trimethylsilyl trifluoromethanesulfonate (Me 3SiO 3SCF 3) or boron trifluoride etherate (BF 3 • Et 2O) as the catalyst. Reductive cleavages with Me 3SiO 3SCF 3 as the catalyst were performed by adding a stock solution of Et 3SiH and catalyst in dichloromethane to the previously dried, fully methylated polysaccharides. Subsequent acetylation was accomplished in situ by adding acetic anhydride, and, after extraction of the reaction mixture with aqueous sodium hydrogencarbonate, the dichloromethane solution was analyzed directly by gas-liquid chromatography-mass spectrometry. With Me 3SiO 3SCF 3 as the catalyst, all of these d-glucans gave the methylated 1,5-anhydro- d-glucitol derivatives expected. Reductive cleavages with BF 3 • Et 2O as the catalyst were carried out in two steps, with an intermediate reaction workup prior to acetylation of the product. With BF 3 • Et 2O as the catalyst, permethylated amylose gave the methylated 1,5-anhydro- d-glucitol derivatives expected, but permethylated cellulose and laminaran were not cleaved. Interestingly, BF 3 • Et 2O effectively catalyzed reductive cleavage of the α-(1→4) linkages in permethylated pullulan but not the α-(1→6) linkages, yielding a methylated disaccharide-anhydroalditol derivative containing the intact α-(1→6) linkage as a major product. The independent synthesis, and n.m.r. and mass-spectral characterization, of the methylated 1,5-anhydro- d-glucitol derivatives formed from these d-glucans by reductive cleavage are repored.