Neber rearrangement of oxime sulfonates

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>. 

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

The Neber rearrangement of oxime 0-sulfonates to 2//-azirines (or a-amino ketones, after aqueous acid workup) has been reviewed, together with the modified Neber , involving /V,/V,A-trimethylhydrazonium iodides.With an excess of base, the a-amino acetal can be formed from the 2//-azirine via the unstable 2-alkoxy aziridine. 

It should be remembered that the historically first pathway to 2H-azirines is the Neber rearrangement, in which oxime sulfonates of type 17 are transformed to a-aminoketones 18 by interaction with base, for example ... 

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