Glycals, glycosidation reactions

Our strategy for synthesis of the oligosaccharide chains,27.28 calls for 2,6-dideoxyhexoses or the corresponding glycals to serve as precursors for both a- and p-glycosidation reactions. It a selective p-glycosidation protocol can be developed, then in principle any structural isomer or analogue of the natural product can be assembled from a common set of monosaccharide precursors.29... 

Scheme 4 Glycal-based glycosidation reactions, (a) E+ = I(collidine)2Cl04 [11], AModosuc-cinimide [12], PhSeCI [13], PhSCI [14]. (b) Danishefsky method [I5. ...

W. R. Roush, D. P. Sebesta, and R. A. James, Stereoselective synthesis of 2-deoxy-p-glycosides from glycal precursors. 2. Stereochemistry of glycosidation reactions of 2-thiophenyl- and 2-selenophenyl-a-D-g/uco-pyranosyl donors, Tetrahedron, 53 (1997) 8837-8852. 

Cleavage of Acetals. Acetals can be cleaved in analogy to ethers, providing a newly functionalized product (eq 10), or simply the parent ketone (eq 11). Glycals have also been converted to the iodopyrans with TMS-I, and glycosidation reactions have been conducted with this reagent. ... 

Oxathiins derived from the cyclic diacylthione (60) and glycals by a cycloaddition reaction yield glycosides on desulfurization, offering an alternative approach to glycosyl transfer <96AG(E)777>. 

C-Glycosides.1 Reaction of an aldehyde with the diene 2 catalyzed by (R)-l results in dihydropyrones (3) in 80-97% ee, which can be converted to the glycals... 

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