Benzylidene acetal fragmentation route to 6-deoxy sugars: direct reductive cleavage in the presence of ether protecting groups, permitting the efficient, highly stereocontrolled synthesis of beta-D-rhamnosides from D-mannosyl glycosyl donors. Total synthesis of alpha-D-Gal-(1-->3)-alpha-D-Rha-(1-->3)- beta-D-Rha-(1-->4)-beta-D-Glu-OMe, the repeating unit of the antigenic lipopolysaccharide from Escherichia hermannii ATCC 33650 and 33652.
An approach to the stereocontrolled synthesis of beta-d-rhamnopyranosides is described in which 2,3-O-benzyl or related 4,6-O-[alpha-(2-(2-iodophenyl)ethylthiocarbonyl)benzylidene]-mannosyl thioglycosides are first used to introduce the beta-d-mannopyranoside linkage in high yield and stereoselectivity. Following glycosylation, treatment with tributyltin hydride in toluene at reflux brings about reductive radical fragmentation directly to the 6-deoxy sugar in high yield. A variation of these donors bearing a carboxylated donor on O3 is a highly alpha-selective mannosyl and, after radical fragmentation, alpha-d-rhamnosyl donor. Using this stereoselective glycosylation/radical-fragmentation approach, a concise synthesis of the title tetrasaccharide is realized in which both the beta-d- and alpha-d-rhamnopyranosyl units are obtained in a single step by a double radical fragmentation of the modified benzylidene acetals.
Crich
David
D
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607-7061, USA. Dcrich@uic.edu
Yao
Qingjia
Q
D001597:Benzylidene Compounds / Q000737:chemistry*; D004925:Escherichia / Q000737:chemistry*; D006031:Glycosylation; D008070:Lipopolysaccharides / Q000737:chemistry*; D008362:Mannosides / Q000737:chemistry*; D015394:Molecular Structure; D009844:Oligosaccharides / Q000138:chemical synthesis* / Q000737:chemistry; D012210:Rhamnose / Q000138:chemical synthesis* / Q000737:chemistry
D001597:Benzylidene Compounds; D008070:Lipopolysaccharides; D008362:Mannosides; D009844:Oligosaccharides; D012210:Rhamnose