Glycals donors

A conceptually new direct oxidative glycosylation with glycal donors, employing a reagent combination of triflic anhydride and diphenyl sulfoxide, has recently been reported by Gin [83], This new 3-glycosylation method works very well with hindered hydroxy nucleophiles, including sterically shielded carbohydrate hydroxy systems, and can be run on large scales. 

Di Bussolo, V, Liu, J, Huffman, L G, Gin, D Y, Acetamidoglycosylation with glycal donors a one-pot glycosidic coupling with direct installation of the natural C(2)-A-acetylamino functionality, Angew. Chem. Int. Ed., 39, 204-207, 2000. 

Honda, E, Gin, D Y, C-2 Hydroxyglycosylation with glycal donors. Probing the mechanism of sulfonium-mediated oxygen transfer to glycal enol ethers, J. Am. Chem. Soc., 124, 7343-7352, 2002. 

Within the present article, a few selected examples are provided ofboth classical and more recent glycosylation methods applied to the synthesis of sLcL sLe", or of sulfated Lewis determinants. Recent procedures involve the use of glycal donors as developed by Danishefsky and his associates, of glycosyl fluorides according to Mukaiyama and Nicolaou, of sulfoxide donors as reported by Kahne, and enzyme-catalyzed oligosaccharide syntheses as developed by the schools of Whitesides, Wong, and Paulson. ... 

Valeria DB, Kim Y, Gin DY. Direct oxidative glycosylations with glycal donors. J Am Chem Soc 1998 120 13515-13516. 

Glycals have been widely used as glycosyl donors, especially for the synthesis of 2-deoxyglycosides 45b 47 In addition, they can be readily converted into 2,3-unsaturated... 

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. 

The glycals are easily converted into the 1,2-dihalo-derivatives, which in principle can act as glycosyl donors. However, these derivatives have not found wide application in glycoside synthesis, mainly because of the low facial selectivity in the initial addition of the electrophilic species [143-145]. In an example of a successful application, 2-deoxy-2-bromo-a-D-glucopyranosyl bromide [146] has been shown to give predominantly the 2-deoxy-P-D-glucopyranosides in silver-triflate-promoted reactions with alcohols. 

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