Imine salts from oximes

Oximes, Tosylhydrazones, and Related Derivatives.—Ketoximes afford enimides in refluxing acetic anhydride-pyridine.181 Theproduct (207) from 5a-cholestan-3-one oxime (206), for example, gave 3-acetylamino-5a-cholest-2-ene (208) in 93% yield after chromatography on alumina. Use of succinic instead of acetic anhydride, with pyridine, gave the enimide (209) which was stable to chromatography. A radical mechanism is proposed. Reduction of ketoximes by Cr11, V11, or Tiin salts in acetic anhydride affords the same enamides, by acetylation of the intermediate imines. The... 

Low-valent metal salts have been used to bring about reductive cleavage of oximes. Corey and Rich-man used chromium(II) acetate to convert O-acetyl ketoximes into imines, which were hydrolyzed to ketones. " Aqueous titanium(III) chloride and vanadium(II) salts also reduce oximes again, the imines are usually hydrolyzed in situ, but some hindered imines, such as compound (37), are isolable." A method of preventing hydrolysis is to carry out the reduction in anhydrous conditions in the presence of an acylating agent. The products of such reactions, when applied to oximes of enolizable ketones, are en-amides. For example, these ketoximes are converted into A/-formylenamines when heated in acetonitrile with anhydrous titanium(III) acetate and acetic formic anhydride cyclohexanone oxime gives the en-amide (38 97% Scheme 22)." This type of reduction has been used by Barton and coworkers to prepare enamides from steroidal oximes. They reported that the reaction could be performed by acetic... 

Indoles have been prepared from reactions of o-aminophenylketones with reactive , or stable " arsonium ylides. Oxo-stabilized ylides reacted with 2-chloro-oximes to give trans-5-acyl-A -isoxazolines, and isoxazoles have been obtained from reactive arsonium ylides and a-isonitrosoketones, and from triphenylarsonium methylide and nitrile oxides The latter ylide reacts similarly with nitrile imines to give pyrazoles. With triphenylarsonium benzylides and benzoylylides,benzene diazonium salts give 1,3,4,6-substituted 1,4-dihydro-1,2,4,5-tetrazines in a reaction in which initial coupling of the reagents is followed by a dimerisation. ... 

The mechanism of Gutknecht pyrazine synthesis has been studied and is well understood. Reduction of the a-oximino ketone affords an a-amino ketone. If the reduction is carried out under acidic conditions, the a-amino ketone may be isolated as an acid salt. These acid addition salts are entirely stable. In these salts the ketone carbonyl may be hydrated, and this is particularly true for a-amino aldehydes. However, as soon as the free base of the amine is generated, either from the salt or during reduction of the oxime if this is carried out under neutral or basic conditions, rapid bimolecular imine formation occurs, which is then followed by rapid intramolecular formation of a second imine to afford a dihydropyrazine. Oxidation to the pyrazine may occur spontaneously upon exposure to air, particularly in the presence of transition metals, and it is this facile aerobic oxidation that doubtless accounts for the isolation of pyrazines by early workers in the field. 

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