Acetoxonium intermediate

Jackson and Hayward59hydroxyl group in l,4 3,6-dianhydro-D-glucitol is preferentially sulfonylated prior to conversion into the mixed ester is incorrect. They based their conclusions on studies of the rate of replacement of p-tolylsulfonyloxy group by iodide in the three dianhydro stereoisomers. However, their proof of structure was questioned by Lemieux and Mclnnes59[Pg.247]

When enantiomerically pure frani-2-acetoxycyclohexyl tosylate is solvolyzed, the product is racemic fran.y-diacetate. This result is consistent with the proposed mechanism, because the acetoxonium intermediate is achiral and can only give rise to racemic material. Additional evidence for this interpretation comes from the isolation of a cyclic orthoester when the solvolysis is carried out in ethanol, where the acetoxonium ion is captured by the solvent. 

The isomerization of simple allyl acetates proceeds by 1,3-migration via a six-membered cyclic acetoxonium intermediate and is most efficiently catalyzed by cationic gold(I)-NHC complexes. The rearrangement is carried out either by refluxing in dichloroethane or with microwave heating. By contrast, the allylic rearrangement of Baylis-Hillman acetates already occurs at room temperature in the presence of AuCl and AgOTf. ... 

In this reaction isomerization occurs via a 1,3-acetoxonium intermediate (85), as shown in Scheme 26. 

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

Angyal and Murdoch127 argued that both pentaacetates would be expected to be formed on hydrolysis of the intermediate, acetoxonium ion 48, but King and Allbutt135 have shown that such ions are hydrolyzed to give mainly the axial acetate. Thus, the 1,2,4,5,6-pentaace-tate would be obtained136 from 48. 

Dolby, L. J., Schwarz, M. J. The mechanism of the Prins reaction. IV. Evidence against acetoxonium ion intermediates. J. Org. Chem. 

Similar principles have been applied to the preparation of previously unavailable acetals of D-allopyranose.55 In this procedure, penta-0-acetyl-/3-D-allopyranose was first converted into the intermediate, O-acetylated /3-D-allopyranosyl chloride, which reacted with sodium borohydride in 1,2-dimethoxyethane, by way of the postulated acetoxonium ion, to give diastereoisomeric 1,2-O-ethylidene-a-D-allopyranoses. 

A wide range of acetals may be prepared by allowing dialkyl-cadmium derivatives to react with the intermediate 1,2-acetoxonium ion.56 The bulky, dialkylcadmium presumably approaches from the least-hindered side (exo), leading to the formation of only one isomer. 

By reduction of the intermediate acetoxonium ion, Hodge and coworkers77 prepared (and separated) diastereoisomeric forms of per-acetates of 1,2-O-benzylidene-, 1,2-O-ethylidene-, and 1,2 4,6-di-O-ethylidene-a-D-glucopyranose. For the 1,2-O-ethylidene ring, the exo methine proton resonates at 8 4.87-4.83, and the endo at 8 5.47-5.24 (signals as quartets in benzene-d6). 

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. 

These results can be explained by the participation of the trans acetoxy group in the ionization process. The assistance provided by the acetoxy carbonyl group facilitates the ionization of the tosylate group, accounting for the rate enhancement. The acetoxonium ion intermediate is subsequently opened by nucleophilic attack with inversion at one of the two equivalent carbons, leading to the observed trans product. ... 

Orthoacetates have been obtained in good yields from 2,3,4,6-tetra-O-acetyl-a-D-glucopyranosyl bromide and alcohols (e.g, cholesterol) in THF in the presence of silver salicylate. The presence of an intermediate oxonium ion (116), which loses THF in forming the acetoxonium ion intermediate, was inferred from... 

Two of the four possible diastereoisomeric 2,3 5,6-di-0-ethylidene-j3-D-allofuranoses have been isolated following acid-catalysed condensation of D-allose with acetaldehyde. Examination of the diastereoisomers and derivatives thereof by H n.m.r. spectroscopy established the configuration of the C-2 atom in the 2,3-O-ethylidene ring as R and as either R or S in the other acetal ring. Condensation of 3-0-benzyl-D-allose with acetaldehyde gave, after acetylation, 1,2-di-0-acetyl-3-0-benzyl-4,6-0-ethylidene-)8-D-allopyranose, 5,6-di-O-acetyl-3-0-benzyl-l,2-0-(jR)-ethylidene-a-D-allofuranose, and two diastereoisomeric 3-0-benzyl-l,2(R) 5,6-di-0-ethylidene-a-D-allofuranoses. An alternative route to related diastereoisomeric 1,2-O-ethylidene-a-D-allopyranoses, via reduction of acetoxonium-ion intermediates, is shown in Scheme 21, while acid-catalysed condensation of methyl 4,6-0-benzylidene-a-D-allopyranoside... 

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