Rearrangement of Acyloxonium Ions in Polyol Systems

Esters of acyclic 1,2,3-trioIs, and esters of cyclic (l,3/2)-trioIs react, via a neighboring-group reaction, with antimony pentachloride to give acyloxonium salts in good yield. Thus, glycerol triesters (27) 

As acyloxonium derivatives of polyols, for example, 28, are relatively sensitive, decomposition reactions tend to occur during high-temperature n.m.r. studies, and it is desirable to verify that the coalescence of peaks observed is reversed when the temperature is lowered. For the coalescence temperature to be low, the free energy of activation of the rearrangement should be low, and the frequency separation of the signals that coalesce should be small, because, by the Eyring equation, a relationship exists between the coalescence temperature and this frequency separation.  

The formation of acyloxonium derivatives (35) of (l,2/3)-cycIo-hexanetriol from tri-0-acetyI-(l,3/2)-cyclohexanetriol and antimony pentachloride does not proceed in a manner analogous to the pathway 30 - 31. In this triol system, in its favored conformation, all three substituents are equatorially disposed. The neighboring-group reaction leading to splitting out of an acetate anion requires, however, a diaxial orientation of the substituents reacting. It is understandable that this reaction should proceed considerably more slowly, because of the expenditure of energy required for conversion of the triol into its all-axial conformation. 

From the acetal 34, the fluoroborate salt of the ion 35 can be obtained by use of triphenylmethyl fluoroborate to split out hydride 

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

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