Glycosylation mannosides

P-Mannosides are difficult to introduce because the axial C-2 substituent of a mannosyl donor sterically and electronically disfavors nucleophilic attack from the P-face. P-Mannosides have been obtained by the direct substitution of a-glycosyl triflates, which are conveniently prepared by the treatment of an anomeric sulfoxide with triflic anhydride (Tf20) or thioglycosides with NIS (Scheme 4.3a)... 

In the critical area of (1-mannoside synthesis [317-321], the evidence strongly suggests that a-mannosyl triflate serves as a reservoir for a transient contact ion pair (CIP), which is the glycosylating species (Scheme 4.37), although the possibility of an SN2-like mechanism with an exploded transition state cannot be completely excluded [135]. In view of the probable operation of the contact ion-pair mechanism... 

Access to P-mannosides [209] is illustrated by the preparation of 179 from P-glucoside 178 by oxidation of the equatorial 2-OH followed by stereoselective reduction to give the axial alcohol an efficient indirect route to the a-mannosides [206] utilizes the P-thioglucoside 182, readily obtained from epoxide 173, proceeding via an oxidation-reduction protection sequence to give P-thiomannoside glycosyl donor 184, from which a-mannoside 185 can be stereoselectively prepared. 

The synthesis of (3-D-fructofuranosides is yet another useful application of this concept [293-295]. The latter 1,2-cis-glycosidic linkage is as difficult to establish as in the case of (3-mannosides. In an elegant synthesis of a-D-fucofuranose-containing disaccharides, Plusquellec and coworkers used the IAD concept via p-methoxybenzylidene acetals in combination with a glycosylation protocol via pentenyl glycosides. Here, the intermediate NIS-adduct could be isolated (Scheme 5.108) [295]. 

Scheme 3 Synthesis of P-D-mannosides and P-L-rhammnosides by glycosylation via locked anomeric configuration.

The potential of the mannuronate ester donors in the formation of the p-mannosidic linkage has been demonstrated with the construction of a mannuronic acid alginate pentamer using a convergent orthogonal glycosylation.64... 

Scheme 6.9 Formation of P-mannosides using glycosyl phosphates...

Routes to Relative Reactivity Data Relative reactivity relationships describe the ratio of products between two glycosyl donors for an acceptor. Ley first constructed such relationships for fully protected mannoside and rhamnoside donors to rationalize the results of his one-pot syntheses employing cyclic diketals.16... 

Scheme 65 Synthesis of an Octaantennary Mannoside Cluster by Reaction of a Glycosyl Isothiocyanate with a Dendritic Polyamine15001...

Other human O-glycosylation deficiencies that are not detectable by IEF of ApoC-III affect oligosaccharides in O-mannosidic linkage to a-dystroglycan in Walker-Warburg syndrome and muscle-eye-brain disease [2, 32], and fucosylated N- and... 

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