Oximes isomerization

Benzaldehyde reacts with hydroxylamine in the presence of excess of sodium hydroxide to 3deld an oxime of low m.p. (a- or. gjM-benzaldoxime) which is stable to alkah, but is rapidly rearranged by acids to give an isomeric oxime of higha m.p. ( 3- or < i/r-benzaldoxlme) ... 

In the following year Hantzsch and Werner published their theory, by which the greater number of isomeric oximes both of aldehydes and ketones have found a satisfactory explanation. 

But, these processes can give rise to modified nitro or nitroso compounds or their isomeric oximes. 

Problem 15.33 Symmetrical ketones, R,C=0, form a single oxime, but aldehydes and unsymmetrical ketones may form two isomeric oximes. Explain. 

On the other hand, vV-aryl-//-nitrosoaminonitriles undergo interesting secondary reactions. Under acidic conditions, salts of the sydnonimes form (Eq. 13) under basic conditions, a rearrangement to an isomeric oxime takes place, which, on treatment with phosgene in pyridine leads to 4-substituted-3-cyano-l,2,4-oxadiazolinones (Eq. 14) [26b],... 

Photolysis of 3/3-acetoxy-5a-pregnan-20/3-yl nitrite2 (1) in benzene gave an isomeric oxime shown to be (2) by the reaction outlined in the sequel ... 

Two alternative routes lead to 2-alkyltriazole 1-oxides from a-dicarbonyl compounds (Scheme 50, routes A and B). Unsymmetrical dicarbonyl compounds frequently, but not invariably, give rise to two isomeric hydrazones and two isomeric oximes and hence two isomeric 1,2,3-triazole 1-oxides (81JCS(P1)503). 2-Phenyltriazole 1-oxide is obtained from glyoxal via route A. However, 2-methyl-triazole 1-oxide is prepared from glyoxal by route B in a one-pot process under neutral conditions. 2-Benzyltriazole 1-oxide is obtained similarly. 2,5-Dimethyltriazole 1-oxides are accessible through both routes (86ACS(B)262). 

In the past, an important problem was distinguishing nitrones from the isomeric oxime O-ethers113. Among other possibilities the difference in dipole moments2 is also quite... 

Hranisavljevic-Jakovljevic, M., Pejkovic-Tadic, I., and Stojiljkovic, A., Thin-layer chromatography of isomeric oximes, J. Chromatogr., 12, 70, 1963 Chem Abs., 60, 7d, 1964. 

Beckmann rearrangement. The standard Beckmann rearrangement of A -3-keto steroids is complicated by the (act that the corresponding oximes consist of syn-and anti-isomers (lb and la), but the former isomers undergo rearrangement in the presence of acid much more readily than the an/i-isomers. However, the tosylates of the isomeric oximes equilibrate in methanol very readily in the presence of acid. Thus treatment of the tosylated oxime mixture with cone. HCl at 50° results in 3-azalactams (2) in 85-95% yield. ... 

Meyer (17) in the following year then expressed the view that the products of the action of nitrous acid on the ketones were not true nitroso compounds (4) but isomeric oximes (5), and he considered their reduction to be analogous to the conversion of a nitro to amino group, rather than that of ketone to pinacol. Thereafter the Treadwell formulation of pyrazines was abandoned. 

Isomerism of Ketoximes.—In the case of aromatic aldehydes they all yield these stereo-isomeric oximes. With the aromatic ketones the condition is different and some yield stereo-isomers and some do not. With symmetrical aromatic ketones in which the two radicals are alike such as di-phenyl ketone, CgHb—CO—C6H,5 no different space relation of the hydroxyl group is possible as the two forms will be identical. When, however, the ketone is unsymmetrical, i.e.j the two radicals are unlike, as in mixed aromatic-aliphatic ketones such as phenyl methyl ketone, CeHs—CO—CH3, or in unsymmetrical di-aryl ketones such as phenyl tolyl ketone, CeHs—CO—C6H4—CHz then the two stereo forms are possible though both forms are not known in all cases. 

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 ... 

One of the isomeric oximes A rearranges in the presence of acid to give the amide B, In the presence of base, the oxime undergoes ring closure to give C, Suggest mechanisms for both... 

The sequence can be applied to unsymmetrical ketones. The isomeric oxime acetates of 2-methylcyclohexanone (7) and (8), lead to mixtures of stereoisomeric acetoxy ketones (9) and (10) in 41-51% yield. Less than 1% of the tertiary acetate (11) is obtained. This result contrasts with acetoxylation of the ketone with lead tetraacetate which gives a substantial amount of the tertiary acetate as well as (9) and (10). 

Oxidation of 1,6-anhydro-a-D-galactofuranose (30) with Pt02 afforded the ketone 31, which was treated with hydroxylamine to give a 3 1 mixture of isomeric oximes 32. Hy-drogenolysis of 32 in the presence of Raney nickel followed by treatment with CbzCl afforded the carbamates 33 and 34. Acid hydrolysis of 33 afforded 35, followed by hy-drogenolysis to give 4 (Scheme 6) " ... 

Several studies have been carried out on the nitration of phospholenes by N2O4 (Scheme 10). The 3,4-dimethyl-3-phospholenes 85 react with N2O4 to yield the rra .s-3,4-dinitro derivative 86 when R = H, Me, Et, CICH2CH2, Pr or Pr, but when R = Me or CICH2CH2 compounds 87 and 88 are formed . The picture for l-alkoxy-3-methyl-l-oxo-3-phospholenes (89) appears to be more complex in some respects, the products then being mainly the rerr-alcohol 90 and the oxime 91 an explanation has been offered regarding the formation of the oximes 91 rather than the isomeric oximes. ... 

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