Glycosyl halide coupling

The application of diazo coupling is somewhat limited by the availability of the p-aminophenyl glycosides, particularly of those of the oligosaccharides. Their precursors, the p-nitrophenyl glycosides, are usually obtained by the condensation of a per-O-acetylated glycosyl halide with p-nitrophenol in ethanol (Michael reaction)19,20 or by the reaction of a per-O-acetylated sugar with p-nitrophenol in the presence of a Lewis acid catalyst (Helferich reaction).21 p-Nitrobenzyl 1-thioglycosides have also been prepared by the condensation of the... 

Activation of thioglucosides. Thioglucosides are useful for glycosidation since they are more reactive than glycosyl halides. They are known to be activated by methyl triflate, but this triflate is a potent carcinogen. Methylsulfenyl trifluoromethanesulfonate (1) is also an activator and has the advantage that it can be prepared in situ. Thus the 1-thio-p-D-glucopyranoside (p—2) couples with a-D-... 

Glycosyl halides are popular electrophilic coupling partners for C-glycoside synthesis. The area of a-monohalo ethers in synthesis has been reviewed [118]. Heteroatoms at the 2-position have been well tolerated, as can be seen from the examples shown in Scheme 71. Alkylation of 407 with the potassium salt of diethylmalonate... 

The coupling of sugar derivatives in solution has been used for the preparation of many individual disaccharides and trisaccharides, usually by the Koenigs Knorr 93,94) and Helferich (95-97) reactions. These are reactions of esterified glycosyl halides with alcohols, catalyzed by silver or mercury ions. However, yields have ordinarily been low, and a mixture of a and /S configurations is usually formed at C-1. (Application of these reactions to polymer formation is discussed below.)... 

The experiments in Scheme 3 did not discover reactivity differences between acetylated and benzylated sugars, these having been noted in Paulsen s article [23]. However, they did discover that such reactivity differences could be exploited synthetically. We are not aware of any attempts to carry out chemoselective couplings of differently protected glycosyl halides. However, we believe that the prospects for such coupling are poor, for reasons that will be discussed in Sect. 2.4. 

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