Trisaccharides synthesis

Dehydrativeglycosylation of 1-hydroxy donors with Ph2S0/Tf20 in conjunction with thioglycoside acceptors opens the way for sequential double glycosylation, one-pot procedures for trisaccharide synthesis, as exemplified by the efficient one-pot synthesis of the a-Gal epitope and a hyaluronan trisaccharide [566]. This study also shows the potential of selenoglycoside as acceptors in dehydrative glycosylation (Scheme 4.112). 

Rather early it became evident that the NIS glycosy-lation of an axial 4-OH group, even in a blociced 3-amino sugar li)ce daunosamine or rhodosamine, could not be effected efficiently 6 ). Consequently, a trisaccharide synthesis was required that allowed facile inversion of a precursor structure subsequent to the advantageous use of NIS glycosylation steps. 

This reaction sequence was not meant at the outset to add another daunosaminide synthesis to the number reported in the literature (2S.). However, with only six straightforward steps from D-glucose and an overall yield of approx. 20% it may indee constitute a rather favorable alternative. The main purpose for the development of this sequence resides in the advantageous incorporation of the precursor for the unit A into the trisaccharide synthesis. 

SCHEME 5.9 Use of an AI-Troc-protected donor in a one-pot trisaccharide synthesis. AgOTf, silver(I) triflate Cbz, benzyloxycarbonyl. 

The above results clearly demonstrate that our double RCM approach for C trisaccharide synthesis is a viable, elegant and efficient approach to this important class of compounds and should provide the means to quickly assemble substrates for further biological or conformational studies. The synthesis of more complex and biologically relevant structures is underway and will be reported in due course. The author would like to express his appreciation and thanks to the co-workers whose names appear in this article and in the references. Without their hard work and dedication to the art, the contents of these pages would only be ideas. 

DCC (1,3-dicyclohexylcarbodiimide) glycopeptide synthesis, 170-171 olefin alcohol esterification, 28, 30 p-C-trisaccharide synthesis, 44 1,3-Dicyclohexylcarbodiimide. See DCC (1,3-... 

Chenault, H K, Castro, A, Glycosyl transfer by isopropenyl glycosides trisaccharide synthesis in one pot by selective coupling of isopropenyl and n-pentenyl glycopyranosides. Tetrahedron Lett., 35, 9145-9148, 1994. 

Scheme 20.14 Trisaccharide synthesis employing a base sensitive linker.

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