Stereoselective glycosylations using intermediate

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. 

Park J, Kawatkar S, Kim J-H, Boons G-J (2007) Stereoselective glycosylations of 2-azido-2-deoxy-glucosidses using intermediate sulfonium ions. Org Lett 9 1959-1962... 

Example use of ortho esters as intermediates in regio- and stereoselective glycosylations. 

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). 

The presence of an equatorially substituted cyclic sulfonium intermediate 7 that controls the stereoselectivity of glycosylation was confirmed by nuclear magnetic resonance (NMR) spectroscopy. Combined use of this methodology to introduce a-glycosides and traditional /3-glycoside protocols provides a potential route to one-pot multistep glycosylations and automated syntheses for a wide variety of oligosaccharides. 

---