Tethered glycosylation

Yet another approach to the synthesis of (3-D-mannopyranosides is the use of tethered glycosylation, also known as intramolecular aglycon delivery. Two such approaches are shown next ... 

Vinylsamarium species are generated fr nation or alkylation may then be perform Si-tethered glycosyl sulfone has been effect radical intermediate is implicated. 

Doi T, Sugiki M, Yamada H, Takahashi T, Porco JA, Jr. Glycosidation of solid-supported glycosyl donors tethered by a trialkylsilane linker. Tetrahedron Lett 1999 40 2141-2144. 

Ziegler and coworkers prearranged a glycoside by employing a succinyl tether between C-6 of a mannosyl donor and C-3 of glucosyl acceptor [151,152]. They found that the nature of the glycosyl acceptor and the length of the tether affected the anomeric selectivity of the intramolecular mannosylation (Scheme 4.4d) [153]. 

Yet another approach uses peptides as tethers for intramolecular glycosylations via prearranged glycosides (Scheme 5.113) [326,327]. The regio- and anomeric selectivity of the intramolecular glycosylation depends on the amino acid sequence of the peptide, which links glycosyl donor and acceptor. 

In this approach, the glycosyl donor and the glycosyl acceptor are linked by the 2-OH of the donor and the free OH of the acceptor. It is one of the most predictable and reliable methods for achieving 1,2-cis stereocontrol. Acetals, mixed p-methoxybenzylacetals and silicon tethering have been widely used as well as iodonium mediated tethering acetals derived from vinyl, allyl and allenyl ethers. These methodologies have been revised.6,76... 

Skrydstrup, Beau and co-workers122 have adapted Stork s method to the SmI2-reduction of glycosyl pyridyl sulfones bearing a silicon-tethered unsaturated group at HO-C(2). An example is shown with the synthesis of methyl a-C-zso-maltoside 172 from alkyne 170 via the 5-exo-dig radical cyclization of 171 (Scheme 56).144... 

The most common posttranslational modifications, discussed in the following sections, include phosphorylation, sulfation, disulfide formation, N-methylation, O-methylation, S-methylation, N-acetylation, hydroxylation, glycosylation, ADP-ribosylation, prenylation, biotinylation, lipoylation, and phosphopan-tetheine tethering. Many of the posttranslational modifications are proven to be cross talks. Other modifications exist in a smaller extent and include oxidation of methionine, C-methylation, ubiquitylation, carboxylation, and amidation. These topics will not be covered in this chapter which is meant to focus primarily on the recent literature (2005-08). For a more complete coverage of all posttranslational modifications and earlier literature (up to 2005), the reader is referred to Professor Christopher T. Walsh s book Posttranslational Modification of Proteins Expanding Nature s Inventory ... 

An acetal tethered compound can easily be prepared by treatment of equimolar amounts of a 2-propenyl ether derivative of a saccharide with a sugar hydroxyl in the presence of a catalytic amount of acid. Activation of the anomeric thio moiety of the tethered compound with N-iodosuccinimide (NIS) in dichloromethane results in the formation of the p-linked disaccharide. In this reaction, no a-linked disaccharide is usually detected. It is of interest to note that when this reaction was performed in the presence of methanol, no methyl glycosides are obtained. This experiment indicates that the glycosylation proceeds via a concerted reaction and not a free anomeric oxocarbenium ion. 

---