Displacement : of an anomeric

The composition of reaction mixtures obtained under classical Fischer-Zach conditions was determined by capillary gas chromatography [53] of per-O-silylated and per-O-acetylated components of the crude reaction mixtures, and some progress was made in explaining the mechanism behind typical side-product formation. The compounds identified included products of solvolytic displacement of an anomeric bromine by acetic acid and water as well as 2,3-unsaturated derivatives. Interestingly, there was a difference in the reaction outcome of pentopyranosyl versus hexopyranosyl substrates. In the former case, glycals were accompanied by acetylated 1,5-anhydropentitols in the latter, hex-l-enitols contained admixtures of peracetylated 2-deoxyhexopyranosides. 

Amino sugars. Displacement of an anomeric bromine atom by an acetamino group is achieved when glycosyl bromides are treated with silver carbonate in MeCN in the dark. 

Insoluble silver salts such as silver silicate favor direct Sn displacement of an anomeric bromide by alcohols [88]. When applied to 2-deoxy-pyranosyl bromides, this procedure can lead to 2 -deoxy-P-disaccharides with a good stereoselectivity. For instance (Scheme 32), bromide 141, where an ester group at 0-4 can also favor [85] P-glycoside formation, reacted with the sugar alcohol 142 in the presence of silver silicate to give the disaccharide 143 in 85% yield (P a 5.4 1) [89]. 

Displacement of the anomeric benzoyl group by (phenylthio) trimethylsilane proceeds under milder conditions than displacement of the acetoxy group. Zinc(II) iodide induces generation of an oxonium intermediate at room temperature, whereas TMSOTf promotes the displacement at 0 °C. 2.28 Substitution of the anomeric p-nitrobenzoyl group with (phenylthio)trimethyl-silane occurs in the presence of TMSOTf and gives product within 1 h (eq 12). ... 

MeCN, r.t., 24 h) gave, respectively, a mixture of fluorinated derivatives 287 (67%) and 290 (6%), and 290 (54%) and 287 (11%). In each case, the minor products were formed by C-5 epimerization of the starting compounds prior to the displacement, followed by fluorination. Reduction (NaBHJ of 287 and 290, followed by hydrolysis, gave an anomeric mixture of 5-deoxy-5-fluoro-L-idofuranose (from 287) and 5-deoxy-5-fluoro-D-glucofuranose... 

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