Acetylated sugars, glycoside synthesis

The use of this reaction has been confined almost entirely to the synthesis of phenolic glycosides, by fusion of the phenol with the 0-acetylated sugar in the presence of anhydrous zinc chloride or p-toluenesulfonic acid as catalyst the former favors the formation of the a and the latter of the /3 anomerA Improved yields result from the removal, under diminished pressure, of the acetic acid produced in the reaction, as well as of any that may be added. ... 

In practice, anomerization is almost always applied to the glycoside acetates, and in almost all recorded instances the transformation is from /3-d to a-D. Many of the catalysts employed in the synthesis of acetylated sugars or in their condensation with aglycons (for example, zinc chloride or sulfuric acid) have been employed for the anomerization of the glycoside acetates. This list does not, however, include titanium tetrachloride, the best-known anomerizing agent. The theory of anomerization is dealt with in an earlier review in this Series. ... 

Synthesis of Monosaccharide Derivatives.—New approaches to the synthesis of glycosides, including the alkoxylation of N-furanosyl-lactams, the reaction of l-0-acetyl-2,3,5-tri-0-benzoyl-P-D-ribofuranose with cyclic amide acetals derived from vicinal diols (see Vol. 10, p. 16), the Lewis acid-catalysed alcoholysis of sugars with an activated anomeric centre (see Vol. 10, p. 11), and the glycosidation of glycopyranosyl halides catalysed by silver trifluoromethanesulphonate (triflate), have been reviewed. A general review of carbonyl compounds has also dealt with some aspects of glycoside synthesis. 

Syntheses of amino-sugar glycosides and disaccharides, including chromogenic substrates for A -acetyl-p-D-glucosaminidase, are covered in Chapter 3. Synthesis of protected 2-amino-2-deoxy-glycosyl azides is covered in Chapter 10. 2,3,6-Trideoxy-5-0-(4-nitrobenzoyl)-3-trifluoroacetamido-L-riho-hexofuranosyl bromide, a furanoid ristosamine derivative, was synthesized from L-rhamnal via intermediates reported previously (Vol.25, p.253) to effect glycosylations. ... 

The other two general reactions for the synthesis of glycosides require the fully acetylated sugar as starting-materiaL In the Koenigs-Knorr reaction, this is next converted into the acetylglyco l halide, whilst in the Helferich reaction it is used directly. 

Aryl glycosides of 2-amino-2-deoxy sugars, as might be expected, can be prepared by similar techniques as was illustrated by the synthesis of phenyl 3,4,6-tri-0-acetyl-2-acetamido-2-deoxy-p-D-glucoside and -D-galactoside by toluene -sulphonic acid catalysed reactions between phenol and the hexosamine penta-acetates These products were then anomerised using zinc chloride as catalyst to provide means of obtaining -anomers. 

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