Neighboring group participation glycosylation

Although glycosyl triflates have been demonstrated to be intermediates with a number of armed donors, and even with disarmed donors not capable of neighboring-group participation, such as the sulfonate esters, typical disarmed donors with esters in the 2-position function in the anticipated manner through anchimeric... 

Scheme 4.39 Glycosylation with neighboring-group participation.

When neighboring-group participation is a feature of the glycosylation reaction, the use of a base in this manner frequently results in the isolation of orthoesters rather than the desired glycosides. In the case of activation by triflic anhydride, it is possible to avoid this problem by simply omitting the base. Alternatively, with more sensitive substrates, the hindered base may be retained and boron trifluoride ethe-rate be added to promote the rearrangement of the orthoester to the glycoside, as in... 

This synthesis demonstrated that the neighboring-group participation effect on the stereoselectivity of glycosylation reactions can be extended to sohd-phase processes. In this case, milder and more practical cleavage conditions than previously discussed were established. The use of /V- bromosuccinimide as the thiophilic reagent in acetone/water or tetrahydrofuran/methanol permitted the release of oligosaccharides in form of lactols or 1-0-Me glycosides, respectively. The tetrasaccharide derivative was isolated in 34% yield from thiol resin 3 (80% yield per step). 

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