Thioglycosides stereoselectivity

I. Solvent-Dependent, Kinetically Controlled Stereoselective Synthesis of 3- and 4-Thioglycosides... 

The neighboring group participation mechanism requires two conditions a neighboring ester group and traas-configmation. For example, in the course of 3- and 4-thioglycoside synthesis, a solvent-dependent kinetically controlled stereoselective mechanism was found (Figure 8). 

II. Regio- and a-Stereoselective Sialyl Glycoside Syntheses Using Thioglycosides of Sialic Acids in Acetonitrile 359... 

In the following section, our recent development of the regio- and a-stereoselective sialyl glycoside syntheses using thioglycosides of sialic acids in acetonitrile is described. 

II. REGIO- AND a-STEREOSELECTIVE SIALYL GLYCOSIDE SYNTHESES USING THIOGLYCOSIDES OF SIALIC ACIDS IN ACETONITRILE... 

A. Hasegawa, H. Ohki, T. Nagahama, H. Ishida, and M. Kiso, A facile, large-scale preparation of the methyl 2-thioglycoside of W-acetylneuraminic acid, and its usefulness for the re-stereoselective synthesis of sialoglycosides, Carbohydr. Res. 212 211 (1991). 

In this case the substituent at C-3, is believed to account for the stereoselectivity by formation of the charged intermediate 33. Similar to this case is the HgCl2-promoted glycosylation [18] of a corresponding thioglycoside like 35 with the aglycon, component 34. This leads predominantly to the P, l-> 4-disaccharide 36. 

Recently, Wiesner et al. [108] succeeded in a stereoselective P-glycosylation in the D-ribo series by use of an anchimeric assistance from the 3 position. The 3-O-p-methoxy-benzoyl group is supposed to form the 1,3-acyloxonium ion intermediate 215 either starting from an at- or P-configurated ethyl thioglycoside precursor (214). 

M. Blanc-Muesser, J. Defaye, and H. Driguez, Stereoselective thioglycoside syntheses. Part 4. A new approach to 1,4-linked-thio-disaccharides and synthesis of thiomaltose, J. Chem. Soc., Perkin Trans. 1, (1982) 15-18. 

Another popular promoter for glycosylation with thioglycosides is the reagent dimethyl(methylthio)sulfonium triflate (DMTST).60 A comparison between DMTST and NIS/TfOH for the formation of sialoglycoconjugates demonstrated that the iodine system provided superior yields and anomeric selectivity in nearly all cases.61 The yields and stereoselectivity also increased with an increase in solvent polarity, suggesting that enhanced stabilization of the oxacarbenium intermediate, the rate-limiting step in glycosylation, was responsible for the increased... 

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