Sugars peracetates

Fully acetylated aldonic acids can be prepared by oxidation of aldehydo-sugar peracetates. Direct acetylation of certain aldonic acid salts is possible, and addition of cadmium salts, in particular, affords high yields of acetates. The synthesis can also be accomplished by deamination of the readily prepared, acetylated amides with nitrous acid or nitrosyl chloride. Examples of the various methods are given in Ref. 86. 

A novel class of -acyl- -glycosides has been prepared by reaction of sugar peracetates with mixtures of Oj- and -trimethyl-... 

Not surprisingly, trifluoromethanesulphonic acid can be used effectively as catalyst for the Fischer glycosidation of free sugars, and phosphorus oxychloride catalyses the reaction between sugar peracetates and alcohols. For aryl, alkyl and aralkyl D-glucoside esters /9-anomers (the kinetic products) are favoured if the reactions are carried out in benzene solution, whereas the more stable o-products predominate if no solvent is used. °... 

In the area of -glycosides with unsaturated aglycones allyl compounds can be made by use of sugar peracetates and allyltrimethylsilane and a Lewis acid, and corresponding pent-2,4-dienyl analogues such as compound (65) are obtainable similarly by... 

Sugar peracetates treated with raethyIthiotrimethylsilane and boron... 

Guanidine has been used for the very rapid deacetylation of sugar peracetates quantitatively benzoyl and pivaloyl groups were unaffected. Peracetylated sugars, on treatment with tributyltin methoxide, gave products selectively deacetylated at the anomeric centre. Equatorial anomers were deacetylated more quickly, and hence selectively, than their axial counterparts. Lipases obtained from the porcine pancreas and Candida have been used to remove ester groups selectively the 3-0-aoetyl derivative of 1,2 ... 

A final topic that has been considered (46) concerns the VCD in the CH stretohing region of the peracetates of sugars. Here, no hydroxyl hydrogen bonds can form. No VCD arises from the methyl on the acetate groups as demonstrated by deuteration. Most of the spectra have both positive and negative VCD con-... 

Closely similar results were obtained with various diphenyl dithioacetal acetates (7), with the unsubstituted diethyl dithioacetals (8), and with the aldehydo-pentose peracetates (9) and the tetra-O-acetylaldo-pentose dimethyl acetals (10). Subsequent work in other laboratories has shown the same general principles for the methyl 5-hexulosonates (11) and the pentononitrile tetraacetates (12), two examples where a full series of stereoisomers has been studied. Other workers have investigated isolated examples or partial series (13, 14, 15, 16, 17, 18), and parallel work by x-ray crystallography (19, 20, 21, 22) on acyclic sugar derivatives in the solid state has shown excellent correlation with the general principles outlined here for the molecules in solution. 

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... 

Related methods may be used to introduce amino functions. For example, thiocyanate 237, made from the 4,6-dimesylate, rearranges to the isothiocyanate 238,225 and 3,4-unsaturated 2-cyanates, easily made in situ from the corresponding carbamates, afford the A-bonded 4-isocyanates of the 2-enes.229 From both types of rearranged products aminodeoxy sugars may be accessed, and in this way perosamine peracetate (239) can be synthesized from the cyanate of a 3,4-unsaturated 2-ol, which is thermally rearranged prior to dihydroxylation and acetylation.229... 

From the synthetic standpoint, the acetolysis of glycoside derivatives to the corresponding peracetates, followed by deacetylation, has been used in the synthesis of a wide variety of derivatives of free sugars (see, for example, Refs. 22 and 37-41). 

The great use of acetolysis in polysaccharide chemistry is in degradation of the polysaccharides, imder mild conditions, to oligosaccharide per-acetates. More-drastic conditions afford monosaccharide peracetates, including those of the aldehydo forms of the sugars. " The many oligosaccharides obtained from the partial acetolysis of various polysaccharides are excellently documented in a book by Bailey, and the reader searching for particular examples is referred thereto. 

Another intriguing sugar-based metabolite from a cyanobacterium is cyclodextrin 33, isolated from a Hawaiian Tolypothrix byssoidea strain. A range of related metabolites were produced these proved inseparable and were analyzed as a mixture. However, acetylation of the mixture yielded peracetate derivatives that could be separated by HPLC and analysis of both the purified peracetate and the unmodified mixture was used in the elucidation of the major component 33. MS and extensive NMR analysis led to the structure shown. 2D NMR analysis was crucial in the determination of the individual sugar units, while their sequence was solved by difference nuclear overhauser effect (NOE) experiments. The configurations of all sugar units were presumed to be D on the basis of comparison with related cyclodextrins. The Tolypothrix cyclodextrins were found to block the activity of some of the other toxic metabolites produced by this cyanobacterium. " ... 

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