Journal
EUROPEAN JOURNAL OF ORGANIC CHEMISTRY
Volume 2019, Issue 37, Pages 6377-6382Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/ejoc.201901186
Keywords
Carbohydrates; Glycosides; Glycosylation; Stereoselective synthesis; Synthetic methods
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The tuning effect of C3-ester groups on the glycosylation stereochemistry of L-rhamnopyranose (L-Rha) ethyl thioglycoside donors is described. On one hand, the L-Rha thioglycoside donors carrying 3-O-arylcarbonyl or levulinoyl group undergo highly alpha-selective glycosylation to afford a wide variety of alpha-L-rhamnoside products in high chemical yields. On the other hand, the glycosylation of the 3-O-4-nitropicoloyl and 2-pyrazinecarbonyl group substituted L-Rha thioglycosides displays beta-stereoselectivity. Only or predominant beta anomeric products are obtained when these L-Rha donors couple with the primary or reactive secondary acceptors, while the beta-selectivity may decrease significantly when these donors react with less reactive secondary alcohols. The synthetic utility of the newly developed alpha- and beta-directing L-Rha donors 1h and 1e has been demonstrated by the efficient synthesis of a structurally unique trisaccharide 9, which is derived from the cell wall polysaccharide of Sphaerotilus natans.
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