Stereoselective glycosylations using glycosylation

Jona, H, Mandai, H, Chavasiri, W, Takeuchi, K, Mukaiyama, T, Protic acid catalyzed stereoselective glycosylation using glycosyl fluorides. Bull. Chem. Soc. Jpn., 75, 291-309, 2002. 

Mukaiyama, T, Jona, H, Takeuchi, K, Trifluoromethanesulfonic acid (TfOH)-catalyzed stereoselective glycosylation using glycosyl fluoride, Chem. Lett., 696-691, 2000. 

One aspect, besides the size of the ring that forms upon intramolecular glycosylation, governing the stereoselective outcome using this concept has been shown to be... 

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. 

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). 

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. 

To achieve the synthesis of the branched tetrasaccharide 11, a further building block fucosyl donor 12 was chosen, which promised a highly stereoselective glycosylation outcome. By using the benzoyl galactosyl... 

H. Kunz Stereoselective syntheses using carbohydrates as chiral auxiliaries G. W. Hart Ubiquitous and temporal glycosylation of nuclear and cytoplasmic proteins... 

A novel strategy for stereoselective glycosylations has been reported in which a chiral auxiliary at the 2-position of a glycosyl donor is used (Scheme 126).189 Upon formation of an oxonium ion, participation of the nucleophilic moiety of the auxiliary is (g) selective and depedent on the chirality. 

SCHEME 3.2 Stereoselective glycosylation by using chiral auxiliaries. 

Takeuchi K, Tamura T, Mukaiyama T. The trityl tetrakis(penta-fluorophenyl)borate catalyzed stereoselective glycosylation using new glycosyl donor, 3,4,6-tri-0-benzyl-2-0-/j-toluoyl-beta-D-glucopyranosyl phenylcarbonate. Chem. Lett. 2000 2 122-123. 

Ito, Y, Ogawa, T, An efficient approach to stereoselective glycosylation of A-acetylneuraminic acid use of phenylselenyl group as a stereocontrolling auxiliary. Tetrahedron Lett., 28, 6221-6224, 1987. Lemieux, R U, Hendriks, K B, Stick, R V, James, K, Halide ion catalyzed glycosidation reactions. Syntheses of a-linked disaccharides, J. Am. Chem. Soc., 97, 4056-4062, 1975. 

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