Glycosyl donors 1,2-anhydro sugars

Comprehensive reviews of the chemistry of both the glycals [105-107] and anhydro sugars [108,109] have appeared in the recent literature. This chapter will attempt to summarize what is known about their preparation and reactivity in the formation of the glycosidic linkage and highlight recent examples to emphasize practical and strategic considerations in the choice of glycosyl donors. 

Glycals can be glycosylated not only directly via electrophilic addition, cyclo addition, nucleophilic addition and rearrangement reactions but also indirectly by conversion into a range of other glycosyl donors. One of the most important classes of these glycal-derived donors, the 1,2-anhydro sugars, is discussed in detail in Section 1.3. 

An hydro Sugars as Glycosyl Donors 5.2.3.1 1,2-Anhydro Sugars... 

Scheme 5.37 illustrates solid-phase oligosaccharide synthesis using glycal-derived 1,2-anhydro sugars as glycosyl donors [105]. 

Anhydro sugars have proven to be versatile glycosyl donors for the preparation of many glycoconjugates. The 1,2-anhydro bridge is an epoxide that is also part of an acetal and is responsible for the unique reactivity of 1,2-anhydrohexopyranoses. The first well-defined member of this class was prepared by treatment of the partially functionalized pyra-nose (17) with ammonia in benzene, and is commonly referred to as BrigTs anhydride (O Scheme 15) [46]. 

The validity of the approach was first demonstrated by the synthesis of a linear tetrasaccharide [22] and a hexasaccharide 13 [23] as outlined in Scheme 3. Polymer-bound galactal 5 was converted to the 1,2-anhydro sugar 6 by epoxidation with 3,3-dimethyldioxirane [24], Polymer-bound 6 acted as a glycosyl donor when reacted with a solution of 7 in the presence of zinc chloride, resulting in the formation of disaccharide 8a. Upon repetition, this glycosylation procedure accommodated the secondary alcohol glycosyl acceptor 10 as well as disaccharide acceptor 12. Huor-idolysis with tetrabutylammonium fluoride (TBAF) was used to cleave the desired products from the polymeric support and furnish hexasaccharide 13 in 29% overall yield from 5 [16]. 

Conversion of 26 to the protected thioethyl glycosyl donors 27 was achieved through epoxidation with dimethyldioxirane to yield the 1,2-anhydro sugar, followed... 

Anhydro sugars were first obtained by multistep synthesis, the first report of such a derivatives being by Brigl.177 Their potential as glycosyl donors was subsequently explored in disaccharide synthesis, but at that stage they did not enjoy widespread use.178-181... 

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