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Stereoselective synthesis of 2-C-branched (acetylmethyl) oligosaccharides and glycoconjugates: Lewis acid-catalyzed glycosylation from 1,2-cyclopropaneacetylated sugars.

Authors
  • Tian, Qiang
  • Dong, Liang
  • Ma, Xiaofeng
  • Xu, Liyan
  • Hu, Changwei
  • Zou, Wei
  • Shao, Huawu
Type
Published Article
Journal
The Journal of Organic Chemistry
Publisher
American Chemical Society
Publication Date
Feb 18, 2011
Volume
76
Issue
4
Pages
1045–1053
Identifiers
DOI: 10.1021/jo1016579
PMID: 21247172
Source
Medline
License
Unknown

Abstract

1,2-Cyclopropaneacetylated sugars as glycosyl donors reacted with a series of glycosyl acceptors (monosaccharides, amino acids, and other alcohols) in the presence of Lewis acid to produce oligosaccharides and glycoconjugates containing 2-C-acetylmethylsugars. Galactosyl donor gave good to excellent α-selectivities with TMSOTf as a catalyst, whereas galactosyl donor offered moderate to good β-selectivities when BF(3)·Et(2)O was used as a catalyst. However, glucosyl donors produced β-exclusive selectivity under both conditions. The stereoselectivities of glycosylation depend on the reactivity of donor sugars and Lewis acid catalyst, which effectively dictated the glycosylation pathways. The evidence suggests that galactosyl donors (e.g., 7) can undergo S(N)1 pathway with a strong Lewis acid (TMSOTf) and S(N)2 pathway under BF(3)·Et(2)O, whereas the glucosyl donors (e.g., 8 and 10) followed S(N)2 pathway. The stereoselectivity was also consequential to the formation of a C2'-acetal intermediate formed via the 2-C-acetylmethyl group and the anomeric carbonium intermediate in glycosylation.

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