A-stereoselective glycosylation

Starting from commercially available ethyl sorbate (64) dihydroxylation with standard AD-mixes afforded diol 65 with 80% ee in 61% chemical yield [94). Protection as the acetonide followed by ozonolysis led to the formation of key intermediate 66 in a four step sequence from diol 65 in 33% overall yield. Further transformations gave fluoroglycoside 67, which was then used in a stereoselective glycosylation. 

Over the past decade, our laboratory has been actively involved in developing methods for the preparation and applications of glycosyl iodide donors (36-49) including efficient a-stereoselective glycosylation protocols that work well with various sugars iodides (39-41). Recently, we have applied this methodology in the synthesis a-anomeric glycolipids. 

Tatsuta [107] reported that highly a-stereoselective glycosylation of phenyl 2-deoxy-3,4-0-isopropylidene-l-thio-D-fucopyranoside 158 could be accomplished because of its rigid and unusual boat conformation. 2-Deoxy-a-glycosides (161 versus 159) were obtained with a much lower selectivity from 2-deoxyfucosyl donors 160 in an usual (d) chair conformation (Scheme 37). It was also shown that... 

The above observations implied that a more viable strategy would be to achieve stereoselective glycosylation of the more readily soluble acyclic diene to... 

Stereoselective Clycosylations of Glycosyl Iodides General methods for a-selective glycosylation of glycosyl iodides via in situ anomerization have been established [197]. Armed glycosyl iodides undergo the reaction in the presence of tetrabutylam-monium iodide (TBAI) and Hiinig s base (diisopropyl ethylamine DIPEA) with... 

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