Anomeric Activating Groups and Stereoselectivity

When considering the formation of C-glycosides by electrophilic substitution, several direct comparisons can be made to the similar reactions 

One concern applicable to all aspects of organic chemistry is the question of the stereochemical outcome of reactions. In the case of C-glycosides, since no anomeric effect exists, the stereochemistry is completely dictated by stereoelectronic effects. Additionally, it should be noted that neighboring group participation is not a predominant factor. The major consequence of these observations is that axially (a) substituted C-glycosides are far more accessible than the corresponding equatorial (p) isomers. 

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Generally, the combination of MPBT/Tf20 cannot activate disarmed thioglycosides at low temperature, but in some case when the more electron-donating jo-methoxyphenylthio group is used as the anomeric function, good yield and stereoselectivity can still be achieved (eq 3). ... 

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