Oximes Neber rearrangement

One of the more important approaches to 1-azirines involves a similar base-induced cycloelimination reaction of a suitably functionalized ketone derivative (route c. Scheme 1). This reaction is analogous to route (b) (Scheme 1) used for the synthesis of aziridines wherein displacement of the leaving group at nitrogen is initiated by a -carbanionic center. An example of this cycloelimination involves the Neber rearrangement of oxime tosylate esters (357 X = OTs) to 1-azirines and subsequently to a-aminoketones (358) (71AHC-(13)45). The reaction has been demonstrated to be configurationally indiscriminate both syn and anti ketoxime tosylate esters afforded the same product mixture of a-aminoketones... 

The aminoketone 1, required as starting material, can be obtained by a Neber rearrangement from a A -tosylhydrazone. Another route to a-aminoketones starts with the nitrosation of an a-methylene carbonyl compound—often in situ—to give the more stable tautomeric oxime 7, which is then reduced in a subsequent step to yield 1 ... 

Unlike the Beckmann rearrangement, the outcome of the Neber rearrangement does not depend on the configuration of the starting oxime derivative E- as well as Z-oxime yield the same product. If the starting oxime derivative contains two different a-methylene groups, the reaction pathway is not determined by the configuration of the oxime, but rather by the relative acidity of the a-methylene protons the more acidic proton is abstracted preferentially. ... 

An a-amino ketone, obtained by the Neber rearrangement, can be further converted into an oxime tosylate, and then subjected to the Neber conditions a ,a -diamino ketones can be prepared by this route. 

Both oximes (10) and their ester (11) or ether derivatives can be used in the classical Beckmann rearrangement and the reaction usually proceeds under acidic or neutral conditions (although basic conditions may also be used). In sharp contrast, only 0-oxime esters can be used as starting materials for the Neber rearrangement and basic conditions are always necessary. The Neber rearrangement is not stereospecific, as the stereochemistry of the starting material E or Z) does not influence the outcome of the reaction. In... 

Although both the Beckmann and Neber reactions can use oxime derivatives as starting materials, O-unsubstituted oximes cannot undergo the Neber rearrangement. The latter occurs only in strongly alkaline reaction conditions while the former can also proceed in both acid and basic media. As a consequence, the Neber rearrangement will only be a possible side reaction of base-induced Beckmann rearrangements. 

The Neber rearrangement is usually performed with ketoxime tosylates but ketone trimethylhydrazonium halides (519), iV,iV-dichloro-5ec-alkyl amines (520), N-chloroimines (521) and A-chloroimidates (522) may also be precursors for the reaction. Only the Neber rearrangement of oxime derivatives will be analysed in this chapter. 

Nitro groups attached to the benzene ring of chromans or chromanones may be reduced catalytically to give the amines but 3-aminochromanones (698) are usually obtained from the oxime (697) of the chromanone by the Neber rearrangement. The free bases are rather unstable (69JMC277). 

The Neber rearrangement of oxime sulfonates has been considered to proceed via a nitrene pathway or an anion pathway. If the latter mechanism is operative, the use of a certain chiral base might result in the discrimination of two enantiotropic a-protons to furnish optically active a-amino ketones. Verification of this hypothesis was provided by realizing the asymmetric Neber rearrangement of simple oxime sulfonate 83,... 

One significant (and indeed the first) preparative pathway to this ring system is the Neber rearrangement of oxime sulfonates. Recently it has been shown that this process can be mildly influenced by the introduction of chiral auxiliaries. Thus, rearrangement of the tosyl oxime 186 (formed in situ from the oxime 185) in the presence of catalytic amounts of the chiral quaternary... 

Another significant preparative pathway to the 2i/-azirine system is the Neber rearrangement of oxime sulfonates. The presence of strong electron-withdrawing groups in the a-position to the oxime increases the acidity of those protons, and thus favors the cycloelimination reaction under mild conditions. The Neber reaction occurs either through an internal concerted nucleophilic displacement or via a vinyl nitrene (Scheme 213) <2001EJ02401>. 

The Neber rearrangement involves the base-catalyzed conversion of oxime tosylates, quaternary salts of hydrazones or /V-chloroimines to a-amino ketones via isolable azirine intermediates (equation... 

In the synthesis of various heterocycles from a-amino ketones, pyrazine formation can be a complicating side reaction due to the tendency of the a-amino ketone to dimerize. One way of avoiding this problem would be to generate the a-amino ketone in a protected form, specifically, as an a-amino acetal. Such derivatives allow the manipulation of the amino moiety as desired. Accordingly, the azirine intermediates derived from oxime tosylates by the Neber rearrangement are subsequently treated with acidic ethanol to furnish the corresponding a-amino acetals (equation 65). ° ... 

The chemoenzymatic synthesis of a Ps adrenergic receptor agonist was developed by J.Y.L. Chung and co-workers. The key chiral 3-pyridylethanolamine intermediate was prepared via the Neber rearrangement of the ketoxime tosylate derived from 3-acetylpyridine. The oxime formation and the tosylation were carried out in a one-pot process using pyridine as the solvent. The solution of the ketoxime tosylate in ethanol was then cooled to 10 °C and potassium ethoxide was added. After the TsOK salt was removed from the reaction mixture, HCI gas was bubbled through the solution until the pH reached 2 and the 3-pyridylaminomethyl ketal was isolated as its di-HCI salt. 

Scheme 11.24 Asymmetric Neber rearrangement of oxime sulfonate 90...

The Neber rearrangement is the transition of an oxime / -toluenesulfonate into an amino ketone under the influence of an alkoxide, e.g. ... 

A methylene group adjacent to the oxime group is essential for occurrence of the Neber rearrangement. In oximes of unsymmetrical ketones it is the... 

The Neber rearrangement of oxime sulphonate esters (equation 28) is a useful method for the synthesis of a-aminoketones78. The reaction is base catalysed and involves abstraction of the most acidic proton a to the oxime functional group, as exemplified by the exclusive formation of compound 40 in this rearrangement (equation 29). 

Fewer mechanistic details are available for the Neber rearrangement than for the closely related Favorskii and Ramberg-Backlund reactions. Oxime tosylates, when treated initially with base and then with water, yield a-amino-ketones, viz-... 

This reaction, like all Neber rearrangements, is limited by availability of the appropriate oxime tosylate. Substrates in which the aryl group contain an electron-donating function are unstable, since they have a propensity to undergo Beckmann rearrangement. However, this difficulty can be resolved by subsequent conversion of the a-amino acetals. For example, catalytic hydrogenation of 2,2-diethoxy-2-(p-bromophenyl)ethylaraine yields the known parent compound, 2,2-diethoxy-2-phenylethylamine These two a-amino acetals readily undergo hydrolysis and should be protected from moisture. 

Oximes which are treated with tosyl chloride can also be used as substrates in the Neber rearrangement to a-amino ketones. The mixture shown in eq 26 was further subjected to reductive amination to 3ueld 14% of the CNS-active cis-lV,A(-dimethylated a-amino alcohol. 

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