Acyloxonium salts

Hints In the first step an intermediacy 1,2-acyloxonium salt is formed. The azido functionality is reduced. 

The mechanism is explained via the formation of the rather stable 1,2-acyloxonium salt 25 (neighbor group effect). 

Some partially esterified derivatives can also be obtained by using the decomposition of acyloxonium salts (see Section IV,5) as a result of a fair degree of stereospecificity, the preponderant isomer obtained is that having the axial hydroxyl group acylated and the equatorial one unsubstituted. An analogous regioselectivity is exhibited by the lithium aluminum hydride-aluminum chloride cleavage of 1,6-anhy-dro-2,3-0-benzylidene-/3-D-mannopyranose or its 4-methyl ether, which yields the 3-O-benzyl derivative only.677... 

The method used most frequently for preparation of acyloxonium salts is the neighboring-group reaction. The esterified chlorohydrin (4) reacts with antimony pentachloride (or, better, with silver fluoroborate) to give the salt 2. The fluoro or bromo analogs react similarly with boron trifluoride or silver fluoroborate. Even the ether 6 can be transformed into 2 by use of antimony pentachloride or triethyloxonium fluoroborate. ... 

The direction of opening of the dioxolanylium ring in unsym-metrical molecules is determined by the nature of the other groups present. With acyloxonium salts (25) of diols constrained by a con-formationally fixed decalin system, both reaction-pathways proceed stereoselectively. By the cis pathway (25 —> 26) that is involved... 

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)... 

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