Stereodirecting Effect of C3‐Ester Groups on the Glycosylation Stereochemistry of L‐Rhamnopyranose Thioglycoside Donors: Stereoselective Synthesis of α‐ and β‐L‐Rhamnopyranosides.

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 α‐selective glycosylation to afford a wide variety of α‐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 β‐stereoselectivity. Only or predominant β anomeric products are obtained when these L‐Rha donors couple with the primary or reactive secondary acceptors, while the β‐selectivity may decrease significantly when these donors react with less reactive secondary alcohols. The synthetic utility of the newly developed α‐ and β‐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. [ABSTRACT FROM AUTHOR]