Glycosylations, block syntheses

The synthesis is an excellent example of the use of thioglycosides as glycosyl donors in the block synthesis of complex oligosaccharides as well as the use of partial glycosylation of diols to achieve short paths to the target. 

As an alternative to chemical sialylations, glycosyl transferases have been used to accomplish sialylations, particularly of N-linked glycopeptide building blocks (see also Section 6.3.1.1.5). The 2,3- and 2,6-sialyltransferases have been easily expressed in preparative amounts. Their use in glycopeptide building block synthesis is described in Section 6.3.3.6. 

Block syntheses of hexa- and nonasaccharide have been recorded using conventional glycosyl halide block synthesis but based upon chemistry used to synthesize the tri-sacharide building units [50],... 

SCHEME S.9 Block synthesis by sequential glycosylations with different t3rpes of glycosyl donors. 

Support-derived trisaccharide building block 79 was used in a block synthesis of dodecasaccharide 82 (Scheme 13 p. 23). Reaction of support-bound glycosyl acceptor 78 with 79 furnished hexasaccharide 81. Removal of a silyl protecting group and coupling with trisaccharide donor 79 was repeated twice, followed by photolytic cleavage of the linker to furnish dodecasaccharide 82 in 10% yield from 73 [64]. 

A new block synthesis of peptides can be applied with 0-glycosylated derivatives, and in this way the 82-mer diptericin 70 has been made by methods involving the coupling of cysteine to a benzylthio ester. Solid phase methods were used to make serglycin glycopeptides e.g. 71. ... 

Most remarkable, however, is the relative indifference of the system to steric bulk at 02 as recorded for the 2-0-silyl ethers and subsequently for glycosidic substituents. A block synthesis of a (3-(l— 2)-tetramannan (Scheme 16), with a first coupling to a 2-0-TBDMS protected donor and a second to a 2-0-glycosyl donor, illustrates this phenomenon very nicely [103]. 

A review of block synthesis of oligosaccharides describes glycosylation approaches and protecting group strategies and contains material of relevance in disaccharide synthesis. 

Hence, they are perfectly fitted to be used in block synthesis of oligosaccharides, where stable oligosaccharide donors are a prerequisite. Their stability also makes them very apt in synthetic schemes where a number of chemical modifications are performed just prior to the glycosylations, as, e.g., in various types of... 

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