Methylation-GC-MS analysis of arabinofuranose- and galactofuranose-containing structures: rapid synthesis of partially O-methylated alditol acetate standards

Arabinose and galactose were treatedwithMeOH containing traces of H2SO4 or HCl at 25◦C to givemixtures of their methyl alphaand beta-furanosides, as shown by 1D and 2D nuclear magnetic resonance (NMR). Oxidation of the Me alpha,beta-Araf mixture with NaIO4 preferentially oxidised the beta-isomer, to give pure Me alpha-Araf . Each product was progressively O-methylated using the Purdie reagent (MeI/Ag2O) at 25◦C and resulting mixtures of partially methylated glycosides (PMGs) were rapidly assayed by thin layer chromatography (TLC) first to favour higher yields of mono-O-methyl derivatives and later for products with higher degrees of methylation. The products were converted to complex mixtures of partiallyO-methylated alditol acetate derivatives (PMAAs) by successive hydrolysis, reduction with NaBD4, and acetylation. These can be used as gas chromatography-mass spectrometry (GC-MS) standards in methylation analysis of complex carbohydrates containing arabinofuranosyl and galactofuranosyl units. Of particular interest were the retention times and electron impactMS of the difficult to prepare alditol acetates of 5,6-Me2Gal, 2,5-Me2Gal, 2,5,6-Me3Gal, 3,5,6-Me3Gal, 5-MeAra, 2,5-Me2Ara, and 3,5-Me2Ara. The relative reactivities of hydroxyl groups for mixtures of Me alphaand Me beta-Galf were HO-2 > HO-3 > HO-6 > HO-5, that of Me alphaand Me beta-Araf HO-2 > HO-3 > HO-5, and that of Me alpha-Araf HO-2 > HO-3 ≥ HO-5.