O-Acetylglycosyl halides

Glycosyl dithiocarbonates (xanthates) are attractive glycosyl donors because of their high reactivity in glycosylation reactions. Most procedures for the synthesis of dithiocarbonates involve reaction of per-O-acetylglycosyl halide 3 or 4 with... 

The allyl glycosides have been synthesized from allyl alcohol and the free carbohydrate, or the per-O-acetylglycosyl halide in the presence of acid, or mercury cyanide, as the catalyst. The condensation of the alcohol occurs with both anomers of the carbohydrate, and a mixture of the a- and /3-glycosides is obtained. The anomers can be separated by preferential, solvent extraction,12 or by chromatography on... 

Table II (which contains a comparison not only of the relative stabilities of the O-acetylglycosyl halides but also of O-benzoylglycosyl halides, O-acylated sugars, and O-acylated glycosides). However, it is apparent that the theory oversimplifies the situation. For example, Hassel and Ottar84 have pointed out that the formation of 1,5-m-O-acetylglycosyl halide may be achieved in S vl reaction mechanisms or in cases where the natures of the old and new substituents are sufficiently different. Tetra-0-acetyl-/3-D-glucopyranosyl chloride is such a substance, and would exist in the Cl conformation (L) since, according to the theory, the 1C conformation LI would be highly unstable. However, for replacement of...

It has been pointed out, with reference to Table II, that the strongly acidic conditions which are ordinarily employed to replace the Cl-acetoxy group of a sugar acetate by halogen have yielded only one of the two anomeric O-acetylglycosyl halides. In 1901, Fischer and Armstrong106 described diastereoisomers for the acetochloro and acetobromo derivatives of... 

Elementary analyses and molecular-weight determinations have indicated simply the loss of the elements of hydrogen bromide in the reaction of the O-acetylglycosyl halides with diethylamine. That this loss occurs with the creation of a double bond and the retention of the ring structure... 

Nucleophilic Displacement Reactions of Poly-O-acetylglycosyl Halides.. 39... 

In synthetic carbohydrate chemistry, the poly-O-acetylglycosyl halides first came into prominence as intermediates in the preparation of glycosides... 

Another numerical relationship involving the optical rotations of the poly-O-acetylglycosyl halides has been deduced by Brauns, who has summarized the results obtained after many years of precise observation on carefully purified materials. Brauns has shown that, in a number of cases, the differences of the specific rotations of the acetohalogeno derivatives of a sugar are directly proportional to the differences in the atomic radii of the halogen atoms. That is, for a given sugar the differences of the specific rotations (acetochloro — acetofluoro), (acetobromo — acetochloro), (ace-... 

The unstable forms of the poly-O-acetylglycosyl halides, that is, those of the /3-series, have in many cases been obtained crystalline. They are characterized by their extremely easy rearrangement to the stable forms most of them, with the exception of the fluorides, undergo rearrangement when dissolved in polar solvents. Thus, tetra-O-acetyl-jS-D-glucopyranosyl chloride rearranges rapidly, even in pure chloroform. The mechanism of this rearrangement has been discussed by Hassel and Ottar and by Lemieux. ... 

By extending the study outlined above, relation of the solvolytic reactivity of a number of poly-O-acetylglycosyl halides to their structure has been possible. The importance of steric factors was demonstrated by a comparison of reactivity of 3,4,6-tri-0-acetyl-2-0-trichloroacetyl-/3-D-... 

The first aeetylated glycosyl fiuoride derivative was prepared by Brauns in 1923 and, in subsequent papers, he explored the synthesis of a number of poly-O-acetylglycosyl fluorides. In addition, Brauns prepared the other poly-O-acetylglycosyl halides of the same carbohydrates and investigated the proportionality relations which exist between their optical rotations and the diameters of the respective halogen atoms. ... 

The cyclic acetals have been of great assistance in the preparation of, for example, methylated sugars, - deoxy sugars, and amino-deoxy sugars. The specific syntheses of many di- and tri-saccharides have been described - in these, a suitable acetal of the aldose is caused to react with a poly-O-acetylglycosyl halide or a bromo- or chloro-hydrin derivative. 

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