5-Nucleophiles. reactions with sugar

By far, the most challenging task in bond-forming reactions with sugar derivatives is to effect C—C branching. The 2 nucleophilic displacement reactions with carbon nucleo-... 

Deoxyhalogeno sugars are susceptible to nucleophilic attack, leading either to displacement, elimination, or anhydro-ring formation. The ease of displacement decreases in the order I > Br> Cl > F the iodo and bromo derivatives have, therefore, been especially utilized in such reactions, although several reactions with chlorodeoxy sugars have now been reported as a result of the increased availability of these compounds. The approach delineated in Section 11,1 (see p. 227) for predicting the reactivity of sulfonic esters can be expected also to be applicable, in an approximate and qualitative way,... 

The nucleophile for which increased reactivity is most critical is the fluoride ion [3,4], Water molecules bind tightly to this ion, and their presence dramatically reduces its effective nucleophilicity. A variety of fluoride ion sources have been used in an effort to improve product yields in deoxyfluoro sugar synthesis [26,34]. The yields of substitution and elimination products generated from reactions with fluoride ion from several sources are listed in Table 2 [26]. Currently, the most attractive source of fluoride ion is tris(dimethylanuno)-5ulfur (trimetbylsilyl)difluoride (TASF), which is soluble in a variety of oiganic solvents and produces an anhydrous fluoride ion [33]. 

The hydroxyl group on the anomeric carbon atom of the ring forms of sugars is reactive and can be replaced by another nucleophilic group such as -O-R from an alcohol. The product is a glycoside (Eq. 4-5). The reaction with methanol occurs readily with acid catalysis under dehydrating conditions, e.g., in 100% methanol. 

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