Tether-Directed O-Glycosylation

The concerted nature of the reaction was further probed by performing the glycosylation in the presence of 1 equiv of methanol [89a, c]. In the case of mixed acetal 248, the 

Stork and co-workers investigated silyl acetals as the tether-directing group for preparing the difficult )8-mannopyranoside linkage [90]. The desired mixed silyl acetal precursor was initially prepared in a stepwise fashion analogous to that described earlier, by 

Bols has also investigated the use of silyl acetals in intramolecular 0-glycosylation [92]. In early work, he showed that a-glucosides could be readily prepared stereospe-cifically by attaching the glycosyl acceptor through a dimethylsilyl acetal to the 2-posi- 

The use of bis-ester tethers in remote intramolecular glycosylation has been investigated by a number of groups. In contrast to the short tethers used by Stork, Hindsgaul, and Ogawa, these more remote tethers provide a much less predictable outcome and as a result they remain less synthetically useful. Another difference between the short tethers and remote bis-ester tethers is that the reacting alcohol functionality is not masked in the tether but is already free to react as soon as the anomeric center is activated. 

By changing the anchoring sites of the bis-ester template, it is possible to reverse completely the regioselectivity of the coupling [97 b]. For example, when a phthalyl tether was used to link the 2-OH of a glucosyl acceptor to the 6-OH of a mannosyl donor to 

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