Stereoselective glycosylation

Trimethylsilyl triflate—silver perchlorate (1 1) also cataly2es the highly stereoselective glycosylation of P-naphthol to provide predominantiy the P-anomer. 

The above observations implied that a more viable strategy would be to achieve stereoselective glycosylation of the more readily soluble acyclic diene to... 

Stereoselective Glycosylations Employing Glycosyl Bromides and Applications... 

Scheme 4.74 Sialyl xanthate in regioselective and stereoselective glycosylations [398].

Scheme 15 Stereoselective glycosylations by (a) a remote C-4 O-acyl group, (b) remote C-4, C-6 O-acyl groups, (c) a remote C-3 O-acyl group.

Scheme 3.12 Stereoselective glycosylation of resin-bound glycosyl acceptor.

This strategy appears to be very attractive because of the possibility of completely solubilizing the support in most of the common solvents. From a chemical perspective, that property allows one to benefit from all the solvent conditions used in classical solution chemistry. This could prove to be very advantageous, especially to obtain stereoselective glycosylation without neighboring-group assistance. Moreover, isolation and purification of the polymer is easily achieved by precipitation usually in diethyl ether or methyl-tert-butyl ether (MTBE) and recrystallisation from ethanol. One major drawback of this type of support is its tendency to solidify at low temperature, thus limiting the variety of temperature conditions. 

Approaches to Deoxy Oligosaccharides of Antibiotics and Cytostatics by Stereoselective Glycosylations... 

Due to the biological roles of glycolipids, many papers have been devoted to their syntheses over the last ten years. The coupling of a fully protected carbohydrate donor to a lipid acceptor requires efficient and highly stereoselective glycosylation methods because lipid derivatives often have low reactivity. A few examples of glycosphingolipids syntheses will be discussed below as well as multistep preparations of other amphiphilic carbohydrates designed as biochemical mimetics, surfactants or liquid crystals. 

The Fmoc group, which is very useful in peptide synthesis [32], has proved an efficient tool in glycopeptide chemistry [10], Because the Fmoc protection of the amino function is rather stable to acids, it can be combined with fert-butyl-type protecting groups and exposed to stereoselective glycosylations that require a more or less acidic milieu. 

H. Kunz and W. Sager, Stereoselective glycosylation of alcohols and silyl ethers using glucosyl-fluorides and boron trifluoride etherate, Helv. Chim. Acta 68 283 (1985). 

Y. Itoh and T. Ogawa, Highly stereoselective glycosylation of W-acetylneuraminic acid aided by a phenylthio substituent as a stereocontrolling auxiliary. Tetrahedron Lett. 29 3981 (1988). 

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. 

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