From Anomeric Acetates

The 3-deoxyhexoses were obtained crystalline after separation on a column of cellulose, a rather tedious operation recently separation of these sugars by fractional crystallization has been reported (41). From the major unsaturated ester 27, 3-deoxy-D-ribo- and -arabino-hexose were shown to have been produced respectively in the ratio 66 34, and from ester 28 in the ratio 24 76. Saturation of the enol grouping of the esters 27 and 28 gives, therefore, the 1,2-cis products preferentially. However, some hydrogenolysis of the anomeric acetate group accompanies simple saturation of the double bond and poses another separation problem. 

Apart from glycosyl acetate and benzoate donors, more elaborate O-carbonyl derivatives offer potential for distinct modes of anomeric leaving group activation. Many of these donors (Scheme 3.21) involve activation of an anomeric O-carbonyl derivative 134 at a remote functional group (Y). The adivated remote functionality, in turn,... 

HI hi acetic acid allows the reduction of jS-peracetates of the higher sugar N-acetylneuraminic acid to the corresponding anomeric deoxy compounds.277 At room temperature this method gave exclusively the a-anomer, whereas at —20 °C a 4 1 a [i ratio resulted. This may be explained by thermodynamic and kinetic protonation of ester enolates generated in situ from anomeric iodide in a manner reminiscent of previous reductions of 2-iodo sugar lactones.278... 

The most widely used method for the formation and isolation of the unstable forms of the poly-O-acylglycosyl halides is that of Schlubach," in which the stable form of a poly-O-acylglyeosyl bromide is caused to react with freshly prepared, activated silver chloride a metathetical reaction occurs, accompanied by a Walden inversion to give the unstable form of the chloride. When a method which gives only the stable form of the halide is employed, the yields from either of the anomeric acetates are usually the same. However, Stacey and his coworkers have found that crystalline tetra-O-acetyl-a-L-arabinopyranose, on treatment with hydrogen bromide in glacial acetic acid, gives tri-0-acetyl-/3-L-arabinopyranosyl bromide in 84% yield, whereas the sirupy tetra-0-acetyl-/3-L-arabinopyranose under identical conditions affords only a 12% yield. 

Thioglycosides are readUy prepared using a variety of methods, but mostly by nucleophilic substi-mtions at the anomeric center. They are prepared most commonly from anomeric l,2-franj -acetates... 

The endocyclic and acetal intermediates were excluded since experiments conducted in CD3OD showed that anomerization was accompanied by >98% exchange of the methoxyl group and solvent. Stereospecific formation of a glycopyranosidic product from the acetal was excluded by the fact that acidic treatment of it produced only furanosides, which were converted only slowly into pyranosides. The exocyclic intermediate was considered to be the most likely... 

Hydrolysis. The anomeric acetal amino sugars on treatment with SiO— from proline are similarly cleaved." all products in a synthetic approach to a-... 

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