Wittig reaction with unprotected sugars

Application of the Wittig reaction in the carbohydrate field is accompanied by certain difficulties. A correct choice of the initial sugar components is the main problem, owing to the basicity of phosphoranes and, especially, to the drastically basic conditions employed with phosphonium ylides (2a). It is not surprising, therefore, that protected (acetalated and aeetylated) aldehydo sugars and resonance-stabilized phosphoranes were used at first,3-5 although partially protected, and even unprotected, aldoses were shown to be amenable to the reaction with various resonance-stabilized phosphoranes, thanks to the presence of the carbonyl form in the mobile equilibrium. The latter reactions, however, are extremely complicated (see Section IV, p. 284). 

While the use of allylic ethers provides convenient routes to the formation of ketones and aldehydes, the Wittig reaction, as illustrated in Figure 7.14, may be utilized in the formation of ester functionalized C-glycosides. This approach is either a or (S selective and works with benzylated, acetylated and unprotected sugars. A particular advantage is that reaction intermediates can be intercepted and utilized. Moreover, glycosidic activation is not required for this type of reaction. 

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