Isomerization Beckmann rearrangements

When treated with certain reagents, the ketoximes in particular undergo the Beckmann rearrangement to isomeric acid amides (p. 227). 

Zeolites have also been described as efficient catalysts for acylation,11 for the preparation of acetals,12 and proved to be useful for acetal hydrolysis13 or intramolecular lactonization of hydroxyalkanoic acids,14 to name a few examples of their application. A number of isomerizations and skeletal rearrangements promoted by these porous materials have also been reported. From these, we can underline two important industrial processes such as the isomerization of xylenes,2 and the Beckmann rearrangement of cyclohexanone oxime to e-caprolactam,15 which is an intermediate for polyamide manufacture. Other applications include the conversion of n-butane to isobutane,16 Fries rearrangement of phenyl esters,17 or the rearrangement of epoxides to carbonyl compounds.18... 

This is the first example of a direct coupling of a diazo compound in a / position of the thiophene nucleus. The Beckmann rearrangement with the ketoxime of 2-thienylketone was found to give poor yields of the aceto-2-thiopheneamide. Furthermore, the Schmidt reaction using hydrazoic and sulphuric acids was investigated and did give rise to aceto-2-thiopheneamide but probably as an eutectic with the isomeric 2-thenoylmethylamide. 

Beckmann rearrangement org chem An intramolecular change of a ketoxime into its isomeric amide when treated with phosphorus pentachloride. bek-man re-g ranj-mont ... 

Lee and coworkers ", studying the Beckmann rearrangement of 1-indanone oxime derivatives 240, observed that the pure E and Z oximes isomerize under mild acidic conditions such as silica gel (equation 72). In the presence of Brpnsted or Lewis acids as silica gel or AICI3 the high rotational barrier of C=N double bond would be lowered by the formation of a complex between the tosylate and AICI3 241. This fact makes the... 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

However, the stereochemistry of the oxime cannot be easily controlled and this may be a drawback for the synthetic utility of the Beckmann rearrangement. When a mixture of oximes is obtained from the ketone and when the isomerization of the oxime cannot be prevented during the rearrangement reaction, a mixture of amides is obtained. In other less favourable cases, the intended oxime cannot be obtained and the wrong amide will result from the rearrangement reaction. 

Pollini and colleagues converted D(—)-quinic acid in five steps into a chiral oxime 395, R = H in an enantiomeric pure form and subjected this oxime to a Beckmann rearrangement (equation 161). Even though the reaction lacked selectivity, 395 was useful in the synthesis of the chiral epoxide 396, a key intermediate in the synthesis of (—)-Balanol 397. The same authors also prepared the isomeric epoxide 398. 

Krow and colleagues investigated the migratory preferences of the Schmidt and Beckmann rearrangements in norcamphors. Two isomeric lactams are usually formed in the Schmidt reaction but one lactam is obtained almost exclusively in a Beckmann reaction (equation 178). 

The asymmetric synthesis of (+)-Codeine 432 devised by White and colleagues included a Beckmann rearrangement to introduce the nitrogen atom in the carbocyclic structure (equation 182). Even though two isomeric lactams 430 and 431 were obtained as a result of the rearrangement, the preferential migration of the bridgehead carbon atom produced 430 as the predominant isomer. The synthesis of the non-natural enantiomer of Codeine was completed after oxidation, olefin formation and reduction. 

One of the most famous examples of intramolecular attack of oxygen on the nitriUnm ion intermediate was observed in the Beckmann rearrangement of Erythromycin oxime derivatives and was used in the discovery and synthesis of the commercial macrolide antibiotic Azithromycin 464. In fact, the Beckmann rearrangement of Erythromycin A 9( )-oxime 460 produced only small amounts (5%) of the expected amide 463, along with two isomeric imino ethers (461 and 462) in a fair yield (38 and 43%) (equation 198). 

The reaction of ketone 243 with hydroxyamine resulted in the formation of oxime 244. Treatment of this oxime with polyphosphoric acid induced the Beckmann rearrangement of oxime 244 to form lactam 245 without detectable formation of isomeric lactam (Scheme 43) <1997JHG921>. Other examples of the Beckmann rearrangement of pyran and thiane derivatives have been reported <2004BML5907, 2004TL1051, 2005JOC10132>. 

Fusco and Rossi797 observed the rupture of the pyrazole ring when 4-nitrosopyrazoles (81) were treated with phosphorus pentachloride, forming isomeric chlorocyanoazines (82 and 83). The reaction is evidently similar to the second-order Beckmann rearrangement.798... 

Beckmann rearrangement. The standard Beckmann rearrangement of A -3-keto steroids is complicated by the (act that the corresponding oximes consist of syn-and anti-isomers (lb and la), but the former isomers undergo rearrangement in the presence of acid much more readily than the an/i-isomers. However, the tosylates of the isomeric oximes equilibrate in methanol very readily in the presence of acid. Thus treatment of the tosylated oxime mixture with cone. HCl at 50° results in 3-azalactams (2) in 85-95% yield. ... 

The photo-Beckmann rearrangements of the oximes of 5a- and 5/ -cholestan-6-ones preserve the stereochemical integrity at C(5), giving in each case an isomeric pair of lactams the rearrangement must occur without dissociation of the C(5)-C(6) bond. "... 

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