Enhanced stereoselectivity of alpha-mannosylation under thermodynamic control using trichloroacetimidates.
O-Specific polysaccharides of Vibrio cholerae O1, serotypes Inaba and Ogawa, consist of alpha-(1-->2)-linked N-(3-deoxy-L-glycero-tetronyl)perosamine (4-amino-4,6-dideoxy-D-mannose). The blockwise synthesis of larger fragments of such O-PSs involves oligosaccharide glycosyl donors that contain a nonparticipating 2-O-glycosyl group at the position vicinal to the anomeric center where the new glycosidic linkage is formed. Such glycosyl donors may bear at C-4 either a latent acylamino (e.g., azido) or the 3-deoxy-L-glycero-tetronamido group. While monosaccharide glycosyl donors, even those bearing a nonparticipating group at O-2 (e.g., methyl), and the 4-N-(3-deoxy-L-glycero-tetronyl) side chain form alpha-linked oligosaccharides with excellent stereoselectivity, alpha-mannosylation with analogous oligosaccharide donors in this series is adversely affected by the presence of the side chain. Consequently, the unwanted beta-product is formed in a considerable amount. Conducting the reaction at elevated temperature under thermodynamic control substantially enhances formation of the alpha-linked oligosaccharide. This effect is much more pronounced when glycosyl trichloroacetimidates, rather than thioglycosides or glycosyl chlorides, are used as glycosyl donors.
Hou
Shu-jie
SJ
NIDDK, LBC, National Institutes of Health, Bethesda, MD 20892-0815, USA.
Kovác
Pavol
P
D000081:Acetamides; D062845:Chloroacetates; D006031:Glycosylation; D009682:Magnetic Resonance Spectroscopy; D008358:Mannose / Q000737:chemistry*; D015394:Molecular Structure; D019081:O Antigens / Q000737:chemistry*; D013237:Stereoisomerism; D013816:Thermodynamics; D014238:Trichloroacetic Acid / Q000737:chemistry; D041241:Vibrio cholerae O1 / Q000737:chemistry*
D000081:Acetamides; D062845:Chloroacetates; D019081:O Antigens; D014238:Trichloroacetic Acid; C009528:trichloroacetamide; D008358:Mannose
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