Benzoate esters oligosaccharide synthesis

With acetate, benzoate, and p-methoxybenzoate on C-2, glycosidation was nearly completely a-stereoselective (Table IV, no. 7, footnote c). It was not clear that any 6-isomer was formed. The solvent used in these cases made no difference to the stereoselectivity. It was obvious that solvent did not compete with C-2 ester in participation. As observed previously in other glycosidations (6) p-nitrobenzoate was a less effective participating group. These glycosidations proved to be very rapid and reaction was complete in less than half an hour when 10—20% excess alcohol was used. Application of the reaction to oligosaccharide synthesis has been successful but as always, the reaction tends to be slightly less a-selective with secondary alcohols. 

The solid-phase synthesis of oligosaccharides is usually performed using acid-resistant linkers and protective groups, because of the slightly acidic reaction conditions required for glycosylations (Section 16.3). Hydroxyl group protection is conveniently achieved by conversion into carboxylic esters, such as acetates, benzoates, or nitro-benzoates. Support-bound esters of primary or secondary aliphatic alcohols can be cleaved by treatment with alcoholates [97-99] (Table 7.8), with DBU in methanol, with hydrazine in DMF [100] or dioxane [101], or with ethylenediamine [102], provided that a linker resistant towards nucleophiles has been chosen. 

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