Glycosylations Glycosyl donors, introduction

M. Wilstermann and G. Magnusson, Synthesis of disaccharide glycosyl donors suitable for introduction of the /i-D-Galp-0 >3)-a- and -jS-D-GalpNAc groups, Carbohydr. Res., 272 (1995) 1-7. 

Introduction (relevant to this class of glycosyl donors/methods)... 

On a positive note, tuning the reactivity of glycosyl donors by selective introduction of different protecting and leaving groups has enabled highly efficient... 

A derivative of L-oliose (54L) suitable for glycosylation was obtained from L-fucose169 using the Barton deoxygenation method for introduction of the 2-deoxy function. Another approach for the synthesis of L-oleosyl synthons started from the 3,4-di-D-acetyl L-fucal (71), readily converted into the corresponding chloride (Scheme 24).138 This method was used for the synthesis of dTDP-p-L-oliose, a glycosyl donor in the construction of aclacinomycin A (72). 

A more successful approach was the introduction of the terminal p-tyveloside in the trisaccharide via its epimer, 3,6-dideoxy-D-r/Z>u-hexopyranose (paratose). The paratose glycosyl donor 145 was synthesized through a series of high-yielding steps starting from p-methoxyphenyl (1-D-glucopyranoside 144 (Scheme 41). 

In transferase-catalyzed reactions, the activated neuraminic acid serves as precursor for the introduction of the terminal NeuSAc residue into various oligosaccharides. This approach is particularly attractive because the chemical synthesis with NeuSAc glycosyl donors has two drawbacks the low yielding glycosylation reaction and the formation of anomeric mixtures. 

It was also found that introduction of any protecting group on the unprotected MOP glycosyl donors resulted in a significant decrease of the reactivity. This deactivation was considerable whenp-fluorobenzoates (FBz) were used as protecting groups, and it was applied to the synthesis of disaccharides, and to iterative oligosaccharide synthesis (O Scheme 80). 

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