Acetoxonium salts

Autoxidation of compound 219 has been reported to occur in the presence of two molar equivalents of acetyl chloride and oxygen the product 221 was isolated in racemic form. Since no reaction occurred in the absence of acetyl chloride, the acetoxonium salt 220 is implicated as the species that undergoes the autoxidation. 

In the following discussion, the dioxolanylium (2) and dioxanylium ring-systems will be referred to simply as acyloxonium derivatives, as the carboxonium ion of the ring is formally derived from a carboxylic acid. Thus, system 2 is an acetoxonium salt when R = Me, and a benzoxonium salt when R = Ph. 

Evidence that the reaction of the diol ester 5 with antimony pen-tachloride genuinely involves a neighboring-group reaction is provided by the stereospecificity of this reaction with esters of cyclic diols. trans -l,2-Cyclopentanediol diacetate (14) and Irons-1,2-cyclo-hexanediol diacetate (17) react " with antimony pentachloride to give the cis-acetoxonium salts 15 and 18, whereas the cis-diol esters (16 and 19) merely give difficultly soluble adducts from which unchanged starting material can be recovered after hydrolytic treatment. 

The mixture of benzoxonium salts is subject, in various solvent media, to an equilibration reaction similar to that involving the acetoxonium salts 59 and 62. The equilibrium proportions of the four benzoxonium ions (see Table II) correspond approximately to those found for the four acetoxonium ions 59, 60, 61, and 62. The content of the n-gluco ion is somewhat greater, and that of the D-ido ion is proportionately lower, but, evidently, a change in the nature of the acyl group does not lead to substantial alteration in the equilibrium composition. 

The D-ido acetoxonium salt (62) exhibits both of the reactions characteristic of an ambident cation (see Section 1,3). With ethanol-pyridine, 62 reacts by the cis pathway to give the orthoester 65. It is noteworthy that, in pyridine solution, no equilibration takes place between the u-ido salt (62) and the o-gluco salt (59). After hydrolysis of solutions of these salts and subsequent further acetylation, the pure pentaacetates of D-idose or D-glucose, respectively, are recovered. It may be supposed that, in pyridine solution, a salt such as 62 is converted at once into the non-isolable pyridinium orthoester (67), whereby each further acetoxonium rearrangement is prevented. In pyridine-ethanol, the salt 62 evidently reacts initially to give the intermediate 67, which reacts further with ethanol to give the orthoester 65. 

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. 

Composition of Acetoxonium Salts Prepared from 69, 70, and 71, and Proportions of the Acetoxonium Ions 72 73 74 75 at Equilibrium... 

Equilibration studies of systems in nitromethane and in acetonitrile have been conducted with the acetoxonium-salt mixture obtained from 69. As indicated in Table III, in nitromethane, the a- and jS-D-arabino ions (74 and 75) are favored to the extent of 57%. The equilibrium composition corresponds approximately to the composition of the salt isolated from 69 at 25°. In acetonitrile, in contrast, the D xylo ion (72) is favored to the extent of 55%. The content (10-12%) of the D-lyxo ion (73) in the solution thus resembles that of the D-manno ion (60) in the corresponding rearrangement in the D-glucose series. 

Surprisingly, the equilibration studies consistently show a substantial proportion of the -D-arabino ion (75), as indicated by a corresponding proportion of tetra-0-acetyl-/3-D-arabinopyranose obtained after hydrolysis and subsequent analysis of the product mixture. The /3-D ion (75) cannot be formed through acetoxonium rearrangement of 72, but must arise through subsequent anomerization of the a-D ion (74) formed initially, because the acetoxonium salt functions as an effective catalyst for anomerization If solutions of anomerically pure aldopentopyranose tetraacetates in nitromethane are treated with 2-metiiyl-l,3-dioxolan-2-ylium hexachloroantimonate (2,R=Me), a rapid onset of anomeric equilibration is observed. Under these conditions, tetra-O-acetyl-D-arabinopyranose gives an a /3 ratio of 9 91, and tetra-O-acetyl-D-xylopyranose, an a /3 ratio of 96 4. 

A range of D-glucose derivatives substituted at C-6 can likewise be transformed into acetoxonium salts. These compounds resemble the D-xylose analogs, inasmuch as here, also, only two rearrangement steps are possible, from the D-gluco ion (83) to the D-manno ion (84) and the D-altro ion (85). When such compounds in carbon tetrachloride at 50° are treated with antimony pentachloride, the salts are immediately precipitated out. Under these conditions, it is evident that the equilibria 83 84 85 are established, and, in all... 

---