Sialidation Using an Auxiliary Group at

Particularly annoying is the competitive elimination which results from the deoxy center at C-3, giving the 2,3-dehydro derivative 7 as the major by-product. As critical problems, there are several disadvantages to the stereoselective synthesis of a-sialyl glycosides  

1) the anomeric center (C-2) of sialic acid is not only sterically hindered, but the reactivity is also electronically disfavored by the carboxylic acid function, which destabilizes the oxonium ion intermediate necessary for glycosidation  

2) the lack of a substituent at C-3 precludes the suitable neighboring participation leading to a-glycoside and 

These combined factors are considered to disfavor formation of the desired a- 

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