Allyl glycoside latent

A novel latent-active glycosylation strategy was reported. This strategy is based on a rhodium-catalyzed isomerization of substituted allyl glycosides followed by a Lewis acid-mediated glycosylation reaction (Scheme 46).76 77... 

The allyl glycoside 323, can be considered a latent [177] form of a glycosyl donor which can be efficiently isomerized to the active vinyl glycoside, 324. The isomerization reaction was performed by a rhodium catalyst obtained by treating the Wilkinson s catalyst, (Ph3)P3RhCl,... 

SCHEME 22. Allyl glycosides in the active—latent strategy for oligosaccharide synthesis.147... 

Early on, random glycosylation of unprotected disaccharides, involving the nonselec-tive coupling of a fucosyl donor, was used to produce three sub-libraries of a-fucosylated disaccharides in one step (136). Nearly statistical mixtures of all possible trisaccharides were obtained after chromatography. Linear and branched trisaccharide libraries were prepared by the latent-active glycosylation (137, 138). This concept made use of the convenient access to both glycosyl donors and acceptors from common allyl glycoside precursors. 

An additional example was provided with the synthesis of p-(l (-linked tetra-saccharide 113 (Scheme 23). Toward this goal, allylic intermediate 107 was used as both precursor to the active donor form 108 and latent acceptor 109. Compotmd 108 was obtained by isomerization into its vinylic state using this time, butyl lithium in refluxing THF (88% yield) while 109 was obtained by de-O-acetylated under usual Zemplen conditions (NaOMe, MeOH). It is noteworthy to mention that the 4-0-acetate in 107 resisted the harsher isomerization conditions. As described for disaccharide 106 above, the acid-catalyzed (TMSOTf or NlS/TfOH) glycosidation occurred uneventfully to provide intermediate disaccharide 110 in 78% (a/p = 1 8) and 73% (a/p = 3 1) yields, respectively. Reiteration of the process provided tetrasaccharide 113 in 83% yield using TMSOTf in acetonitrile (a/p = 1 8). 

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