Acyloxonium rearrangement

Free Energies of Activation (AG ) of the Acyloxonium Rearrangement in l,2,3-Triols ... 

A hexachloroantimonate salt of the D-gluco acetoxonium ion is isolable only if the reaction is performed at low temperature (—10°) in a nonpolar solvent (such as carbon tetrachloride) from which the salt is precipitated, by virtue of its insolubility, as fast as it is formed. Under these conditions, the ion 59 has no time to undergo successive acyloxonium rearrangements to 60, 61, and 62, because the rate of precipitation is greater than the rate of rearrangement. The salt thereby obtained contains over 90% of the D-gluco derivative. ... 

In contrast, if 56,57, or 55 in dichloromethane is treated at 20° with antimony pentachloride, a rapid acyloxonium rearrangement of the D-gluco ion 59 ensues, first by neighboring-group participation to give the D-manno ion (60), and, subsequently, to the D-altro ion (61), and then to the D-ido ion (62). In the solution, an equilibrium 59 60 61 62 exists between the ions. The equilibrium com-... 

On treatment in carbon tetrachloride with antimony pentachloride, tri-0-acetyl-/3-D-xylopyranosyl chloride (69), tetra-0-acetyl-/3-D-xylo-pyranose (70), and tri-O-acetyl-a-D-xylopyranosyl chloride (71) give an acetoxonium salt that precipitates directly from the solution. This salt consists of a mixture of the D-xylo, B-lyxo, and D-arabino derivatives (72, 73, and 74, respectively). None of these three compounds crystallize out preferentially. Evidently, the D-xylo derivative 72 is formed initially, and is then transformed by reversible acyloxonium rearrangements into 73 and 74. Salts are not isolated when the reaction is conducted in dichloromethane, because of their high solubility in this solvent. 

The salt mixtures, obtained in comparable yields from 69, 70, and 71 (see Table III), have different compositions. The acyloxonium rearrangement proceeds at clearly different rates for the three compounds, as was recognized from the progressively diminished content of D-arabino derivative in the product. The /3-d chloride 69 reacts the fastest, and the a-D chloride 71, the slowest. The former has... 

The acyloxonium rearrangement provides a good preparative access to D-talose from D-galactose. Thus, tetra-0-acetyl-/3-D-galactopy-ranosyl chloride (90) is treated with antimony pentachloride to give the mixture of salts 91 and 92, which is hydrolyzed and the mixture of products acetylated. Penta-O-acetyl-a-D-talopyranose, which is present as the pure a-D anomer, crystallizes out from the mixture, whereas the D-galactose pentaacetate does not. The latter exists as a difficultly crystallizable, a,/3 mixture, because hydrolysis of 91 and acetylation of the product leads to an anomeric mixture. 

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