Glycosylation steps

SCHEME 17. Synthesis of the glycon of Sokodoside B, involving H2S04-Si02-promoted glycosylation steps. 

Trisaccharide 80 was synthesized by a similar approach, in which a rhamnoside was formed in 86% yield, transformed into its trichloroacetimidate, submitted to the second glycosylation step promoted by H2S04-silica (84% yield), and subsequently deprotected to 80.91... 

A one-pot technique, combining two or more glycosylation steps based on activation of one donor over another, have also been developed [182,189-191]. This one-pot technique is virtually a variation of a simple stepwise selective activation strategy, which allows further improvement in the efficiency of the synthesis by avoiding the necessity for isolation (and purification) of the intermediates. 

Even controlled-pore glass (CPG) could be successfully employed as solid support with (9-glycosyl trichloroacetimidates as glycosyl donors. Thus, limitations of solvents and reaction temperatures in the glycosylation step, as experienced with the Merrifield resin, are restricted to those observed in solution-phase synthesis. Therefore, regio- and stereocontrol of the glycosylation reactions should be available from well-established solution-phase methodologies. 

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. 

The second step (glycosylation step) should consist of a sterically uniform, high-yielding glycosyl transfer to the acceptor however, by a truly catalytic process, where diastereocontrol may be derived from the glycosyl donor anomeric configuration (by inversion or retention), by anchimeric assistance, by influence of the solvent, by thermodynamics, or by any other effects. 

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