Neber rearrangement mechanism

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,... 

RCH2CH(NCl2)R , where the product is also RCH(NH2)COR The mechanism of the Neber rearrangement is via an azirine intermediate 19 ... 

Aminooxindols (11/153) are rearranged to 3-cinnolinols 11/154, if treated with equimolar amounts of /-butyl hypochlorite [106] [107] [108]. Compound 11/153 is known as Neber s lactam , and is formed from 11/154 with zinc and sulfuric acid. The mechanism preferred for the 11/153 —> 11/154 transformation involves nitrene formation [106]. - As already mentioned, the Gabriel synthesis (11/155 11/157) is a method for synthesis of primary amines. But the side product is the ring enlarged hydrazide, compare Chapter IV... 

It was discussed by Neber and co-workers (294-298) that some oximes (34) can be rearranged to a-amino ketones (36). The oxime was first converted into its /7-toluenesulfonic ester (35), which rearranged with sodium or potassium ethoxide to produce the amino ketone, which in most cases underwent further condensation to the dihydropyrazine (or pyrazine). The mechanism has been studied carefully in the reaction of 2,4-dinitrobenzyl methyl ketone and the unstable intermediate isolated from the reaction has been assigned (299, 300) the structure (37). The method was not always successful but literature preparations (294-298) are recorded in Table II.6. 

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