Conversion of Pyranose Glycals to Furanose Derivatives: A New Route to Oligofuranosides

Acetylated pyranose glycals have been converted through a convenient three-step process into protected furanose reducing sugars. Ozonolysis of 2,3,5-tri-O-acetyl-glucal or 2,3,5-tri-O-acetyl-galactal, followed by treatment with dimethyl sulfide and then hydrolysis gave respectively protected arabinofuranose (6) and lyxofuranose (7) derivatives. Conversion of these hemiacetals to oligosaccharides was explored using a number of methods. Activation of 6 or 7 in situ afforded glycosides in modest yield and stereoselectivity. Glycosylation of tetraacetates 16 and 18, obtained from 6 and 7, gave similar results. However, thioglycosides 17 and 19, also derived from 6 and 7, were found to be effective glycosyl donors, producing products in good to excellent yields and with high stereoselectivities. The method was also used to synthesize a disaccharide in which one residue contained uniform 13C enrichment.