Anhydro glycosyl donors

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 5.71 Structure of 1,6-anhydro glucose, and its use in the synthesis of selectively protected glycosyl donors.

The final example (Scheme 5.73) illustrates the efficient preparation of epoxide 212 from 2-iodo-l,6-anhydro-D-glucose 67 (see Scheme 5.5) [221,222]. This Cerny epoxide [109] is a useful and versatile precursor to a range of 2-substituted glucose derivatives 213, which can be elaborated further to suitable glycosyl donors. 

Preparation of 1,2-Anhydro-tri-O-Benzyl-a-D-Glucose and General Method for Its Use as a Glycosyl Donor in the Formation of fi-Glycosides... 

Anhydro derivatives of mono- and disaccharides have been particularly useful as glycosyl donors and acceptors for the production of dextran-type oligosaccharides [5]. 

The by-product could be avoided by choosing an alternative glycosyl donor, 61 [89], which, after coupling to 1,6-anhydro acceptor 45 and debenzylidenation, allowed the TEMPO-oxidation on a derivative with a protected 2,3-glycol... 

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]. 

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