2-Oxo- and 2-Oximinoglycosyl Halides: Versatile Glycosyl Donors for the Construction of .BETA.-D-Mannose and .BETA.-D-Mannosamine-containing Oligosaccharides.

Abstract

An overview is given on the use of 2-oxo- and 2-oximinoglycosyl bromides for the practical acquisition of β-D-mannose and β-D-mannosamine-containing oligosaccharides of biological importance: any mono- or di-saccharide can easily be converted by known methodology into its 2-hydroxyglycal ester; these, by simple treatment with NBS/methanol, afford the respective glycosulosyl bromides in high yield; alternately, they may be converted into oximinoglycosyl bromides in a three-step sequence comprising hydroxylaminolysis, benzoylation, and photobromination as to allow overall yields of around 50%. Due to a non-participating, strongly electron-withdrawing substituent next to the anomeric center, β-glycosidation is essentially stereospecific. The subsequent hydride reduction of the oxo and oximino functions is equally endowed with high, in most cases exclusive stereoselectivity for attack of the hydride ion from the α-face, i.e. opposite to the anomeric substituent, thus leading to β-D-manno-configurated hexose and hexosamine units. The present state of exploitation of this methodology for the straightforward synthesis of oligosaccharides with β-D-mannose and N-acetyl-β-D-mannosamine units is highlighted.