Oximes Beckmann transformation

Fragmentation of the intermediate or concerted formation of nitriles from the activated oxime (Scheme 9, pathway 3) this is the Beckmann fragmentation. In some circumstances this pathway becomes dominant, particularly when there are quaternary carbons adjacent to the oxime. This transformation has found particular utility in ring-cleavage processes (sometimes called abnormal or second-order Beckmann rearrangements). 

This so-called Beckmann transformation has been of great significance for the explanation of the constitution of the isomeric oximes. If- eg., phosphorus pentachloride is allowed to act on both of the above formulated stereoisomeric oximes of the brombenzophenone, the same compounds are not obtained from both, but two different ones, which, as follows from their saponification products, correspond on the one hand to the benzoyl derivative of bromaniline, and, on the other, to the brombenzoyl derivative of aniline ... 

Schopf sought to obtain direct evidence for the location of the side-chain at C-13 by subjecting dihydrocodeinone oxime to a Beckmann transformation, when, if the side-chain is located at C-13 [liv], an aldehyde [lv] would be the product, whereas if the point of attachment is C-14 [lvi] a ketone [lvii] would be formed. Neither the oxime nor its methyl ether yielded the desired proof directly, and it was only after a sequence of further degradations that the conclusion was drawn that the product was [lv], indicating C-13 as the point of attachment of the side-chain [85] (see Chap. X). 

Beckmann Transformation The Beckmann transformation of dihydrocodeinone oxime was investigated by Schbpf [68] in an attempt to determine the point of attachment of the carbon end of the ethanamine side-chain. If the point of attachment is C-13 [xliv] the product of transformation should be ail aldehyde [xi.v], whoroas if it is C-5 [xltx] the product should bo a... 

The Beckmann transformation of flavothebaone trimethyl ether oxime yields an isoxime containing four methoxyl groups, which on treatment with methanol and hydrochloric acid yields a substance yrith one additional methoxyl. Both the isoxime and the methyl-derivative remain unchanged on heating with methanolic potassium hydroxide for seven hours [2],... 

Beckmann transformation of the oxime of 7,14-cyclocodeinone in methanol or ethanol gives the mixed acetals (94 R = Me) and (94 ... 

Victor Meyer thought structural isomerism was possible in the case of the benzaldoximes, and only one oxime of benzophenone was known. Auwers and Meyer soon confirmed Hantzsch and Werner s theory by discovering two oximes of -chlorobenzophenone. In the case of ketoximes Hantzsch used the Beckmann transformation (see p. 833) in assigning the syti - and anti-configurations, but Meisenheimer s discovery (see p. 833) modified his conclusions. Moore says Hantzsch s series is really in configuration,... 

All known ketenes dimerize when heated or when allowed to stand at room temperature or below for a sufficient length of time. The dimers of ketoketenes undoubtedly are derivatives of cyclobutanedione. For example, the dimer of dimethylketene has a pleasant odor similar to that of ketones it yields two isomeric (cis and trans) glycols upon reduction and it yields mono- and di-oximes which give the expected products in the Beckmann transformation. ... 

The mechanism of the Schmidt reaction has not been established with certainty. Schmidt proposed a mechanism in which the hydrazoic acid is cleaved by the strong mineral acid to nitrogen and the imide radical (NH). This radical is supposed to add to the carbonyl group, followed by a rearrangement either directly or by a Beckmann transformation of an intermediate oxime to the amide. ... 

Reaction of nitromalon-bis-A -methylanilide (105) with sulfuric acid gives A -methylisatin- -oxime (107) and not 4-methylquinoxalin-3-one 1-oxide (108) as originally suggested. This transformation may involve a Beckmann-type rearrangement of the protonated aci-nitro compound (106) prior to dehydrative ring closure. ... 

Ozonolysis of alkene 446 in the presence of acetaldehyde afforded diketone 448 through the intermediacy of 447. Ring expansion through Beckmann rearrangement took place when bis-oxime 449 was mesylated and warmed in aqueous tetrahydrofuran (THF). The bis-lactam so formed gave piperidinediol 450 on reduction with lithium aluminium hydride, and this compound was transformed into ( )-sparteine by treatment with triphenylphosphine, CCI4, and triethylamine (Scheme 105) <20050BC1557>. 

The transformation of oximes to lactams (the Beckmann rearrangement) was one of the earliest such acid-catalyzed reactions to be reported with TS-1 (138) and TS-2 (247) catalysts. The rearrangement of cyclohexanone oxime to e-caprolactam proceeds with high selectivity in the presence of TS-1, with high catalyst stability (138,247). 

Dehydration of oximes to form nitriles is, again, a rare but intriguing reaction. It has, indeed, been demonstrated that cytochrome P450 (see Chapt. 3 in [50]) is able to transform butanal oxime (11.103, R = H, R = Pr, Fig. 11.14, also 11.72) to butanenitrile by a Beckmann-type dehydration [105] [133]. The reaction is inhibited by the presence of 02, and is catalyzed by cytochrome P450 in its reduced (ferrous) state. 

The previous referred inconveniences have prompted an increasing interest in the development of alternative, essentially neutral and more environmental-friendly catalysts to promote the rearrangement of O-unsubstituted oximes. The development of highly efficient and selective transformations and also of processes for catalyst recovery and its reuse are the aim of some of the more recent studies. Much of this work is being done in industry to improve current production processes and is the subject of new patent applications. During the last two decades environment concerns have led to the development of green, simple and cost-effective catalytic systems for the Beckmann rearrangement. 

A simplified mechanism for the Beckmann rearrangements and important related reactions is shown hi Scheme 9. Summarizing the mechanism section, the key step of the reaction is the migration of an a-carbon group to the electronically deficient nitrogen atom of the oxime. A nitrilium ion in some cases or an imidate in others are key intermediates in the reaction. Their destiny determines the course of the transformation. Basically, three different pathways may be possible and can be synthetically exploited ... 

When the nucleophile is already present as a part of the starting oxime (for example, a heteroatom or a C=C double bond), intramolecular trapping of the electrophilic intermediate is possible and a new cycle is formed. This transformation is usually referred to as a Beckmann Rearrangement-Cychzation reaction. Two modes of ring closure may be possible, depending on the oxime structure (equations 111 and 112) ... 

Beckmann fragmentation is frequently applied to cyclic oximes resulting in a ring-cleavage reaction. Normally, a nitrile-alkene compound is obtained from the oxime and further transformation is usual. 

---