Acetoxyl rearrangements

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]. 

Asymmetric epoxidation of 10a under standard conditions yields the crystalline epoxy alcohol 2a in 95% ee (91% chemical yield). Treatment of 9a with thioanisol in 0.5N NaOH, in rerf-butyl alcohol solution, gives -after protection of the hydroxyl groups as benzyl ethers- the sulfide a (60% overall yield) through an epoxide ringopening process involving a Payne rearrangement. Since the sulfide could not be hydrolysed to the aldehyde 7a without epimerisation at the a-position, it was acetoxylated in 71% yield under the conditions shown in the synthetic sequence (8a... 

Oxidation with lead tetraacetate is a far less selective process.490,491 Studied mainly in the oxidation of cycloalkenes, it gives stereoisomeric 1,2-diol diacetates, but side reactions (allylic acetoxylation, skeletal rearrangement) often occur. A change in reaction conditions in the oxidation of cyclopentadiene allows the synthesis of different isomeric mono- and diesters.492... 

A simple and efficient procedure for the direct oxidation of C-3 of l,4-benzodiazepin-2-ones, applicable to the preparation of the anxiolytic agents oxazepam and lorazepam, has been developed that represents an improvement over the well-established Polonovsky rearrangement of the N-4 oxide <20060PD1192>. Iodine in AcOH at 65 °C catalyzed acetoxylation in a reaction that involved iodination at C-3 followed by a rapid nucleophilic displacement by KOAc. The liberated HI was recycled to iodine by inclusion of a stoichiometric oxidant, with K2S2O8 being the optimal compromise of cost, availability, and efficiency. 

On treatment with acetic anhydride in pyridine, the yellow tetraketone (68), the yellow triketone (66), and the diketone (67) afforded monoacetyl, diacetyl, and triacetyl derivatives, respectively. All these derivatives contained a tertiary acetoxyl group. However, comparisons of 1H-NMR spectra of the parent ketones with those of the acetyl derivatives indicated that a rearrangement of the molecule or a major conformational change must have occurred. On the basis of extensive H-NMR spectral analysis, structure 69 was proposed for the diacetyl derivative which was derived from the triketone 66. Structure 69 was justified on the basis of conformational arguments. 

The rearrangement products of the A-oxides (112) and (115)79 provide another comparison of the relative reactivities of the 2-methyl group and the 7-methylene hydrogens.28a 28b 29b In contrast to these results, the symmetrical IV-oxide (118) yielded only acetoxyl derivatives. 

Table 1 Acetoxylation of Sulfides from Sulfoxides via the Pumtnerer Rearrangement... 

A combination of the Pummerer rearrangement and the Ritter reaction occurs in the reaction of acetonitrile with methyl phenyl sulfoxide (equation 25) in a mixture of irifluoroacetic acid and its anhydride, although a substantial amount of the nonnal a-acetoxylation also occurs. Participation by amido groups is also possible, the interest here being largely in the construction of lactams via the intramolecular cycli-zation mode. Whereas Wolfe and his coworkers were unable to find conditions for the cyclization of S-phenylcysteinamide sulfoxides under Pummerer conditions, Kaneko found that variously substituted... 

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