Thioglycoside donors

This concept can indeed be transferred to oligosaccharide syntheses using thioglycoside donors, as shown in Scheme 9.9192... 

B. Mukhopadhyay, B. Collet, and R. A. Field, Glycosylation reactions with disarmed thioglycoside donors promoted by /V-iodosuccinimide and HC104-silica, Tetrahedron Lett., 46 (2005) 5923-5925. 

Wong and coworkers showed that 5-azido sialyl donors protected with O-acetyl ester are useful for a-selective glycosylations of primary hydroxyls (Scheme 4.6e) [169]. It was proposed that the linear and electron-withdrawing nature of the C-5 azido moiety stabilizes the reactive axial acetonitrile adduct to allow the incoming nucleophile to approach the a-face in an SN2-like fashion. In addition, a chemose-lective glycosylation method has been developed for the synthesis of NeuAca-(2 —> 9) NeuAc as thioglycoside donor for use in the subsequent glycosylations [169]. 

The sulfoxide method has been applied to the concept [319,374] of intramolecular aglycone delivery for the formation of [1-mannosidcs by means of a silylene linker. In the original work, the acceptor and a thioglycoside donor were joined by means of a silylene group before the oxidation to the sulfoxide [141]. However, it was later found that the preformed sulfoxide was tolerated by the chemistry for the introduction of the linker [286,375]. The intramolecular aglycone delivery step was shown to function effectively for the transfer of the donor to the 2-, 3- and 6-position of glucopyr-anosides, as exemplified in Scheme 4.64. 

The thioglycoside donor is dissolved in CH2C12 ( 0.1 M) and cooled to — 78 °C under an inert atmosphere. wCPBA (70wt%, 1.2 equiv) is then added portionwise with minimal exposure to the atmosphere. The reaction mixture is warmed to room temperature over lh, at which time TLC shows the dissappearance of starting material and the formation of more polar compounds. The sulfoxides are purified by column chromatography over silica. 

In the search for more efficient promoters for P-selective glycosylation, Seeberger and coworkers [510] explored coupling conditions used for thioglycoside donors. 

S. Oscarson, U. Tedebark, and D. Turek, Synthesis of colitose-containing oligosaccharide structures found in polysaccharides from Vibrio cholerae 0139 synonym Bengal using thioglycoside donors, Carbohydr. Res., 299 (1997) 159-164. 

SCHEME 5.3 Effect of (V,(V-diacetylation of a thioglycoside donor on sialylation. Bz, benzoyl TMS, trimethylsilyl Ts, toluenesulfonyl. 

Xin-Shan Ye and coworkers [62] carried out the one-pot synthesis of Gb3 and iGb3 using thioglycoside donors with A-(phenylthio)-e-caprolactam (20) and Tf20 as... 

The first total synthesis of sLex hexasaccharyl ceramide was reported by Akira Hasegawa s group in 1991 using thioglycoside donors, emphasizing on the utility... 

Here, also the 2-0-acetylated donor gave high yields of disaccharide product, but the benzoyl analogue was still the most effective. These thioglycoside donors with their easy access and excellent glycosylation properties hence show very... 

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