Glycal-derived 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. 

To obtain this type of D-glucosamine donor in a stereocontrolled manner, another glycal derivative, namely 3-0-benzyl-2-deoxy-5,6-O-isopropyli-dene-D-araW o-l,4-anhydro-hex-l-enitol (151), was chosen as starting material because it is easily prepared from D-mannose and because the j8-face of the double bond is very hindered.79 As anticipated, azido-phenylselenyla-tion of 151 afforded an a//3 mixture of phenyl 2-azido-2-deoxy-l-seleno-D-glucofuranosides 152 in high yield. In order to obtain the glucosamine equivalent in pyranose form, compound 152 was treated under acidic conditions in the presence of mercury acetate to effect simultaneous cleavage of... 

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

Scheine 6 Using glycal-derived polymer-bound thioethyl glucosyl donors in the synthesis of a >4) linked... 

Addition of 2,4-dimethylbenzenesulfenyl chloride to the glycal derived from tetra-O-acetyl-neuraminic acid methyl ester, followed by reaction with sodium methylthiolate, furnished the 2-methylthio-3-thiosialic acid derivative 14. Its use as a glycosyl donor is referred to in Chapter 4, and the azido phenylselenation of a furanoid glycal is covered in Chapter 9. 

O-Picoloyl glycal derivative 22 has also been successfully used as the glycosyl donor on a palladium-catalyzed allylation reaction with various N-heterocyclic nucleophiles leading to p-ALglycosyl derivatives type 28, (Scheme 11). ... 

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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