Oximes 3,3 sigmatropic rearrangement

Phenyl- and 2-(2-thienyl)-3,3-dimethyl-3//-pyrroles (58) were obtained by the reaction of the corresponding ketoximes 56 with acetylene catalyzed by MOH (M = Na, K) in DMSO. The reaction intermediate observed is the corresponding O-vinyl oxime 57 which undergoes [3,3] sigmatropic rearrangement and cyclization to products 58 (equation 24). The yield of the products obtained strongly depends on the structure of the ketoxime . 

Enehydroxylamines (102) are invoked as intermediates in the rearrangement of O-vinyl, acyl or aryl oximes (101) (equation 31). Varlamov and coworkers demonstrated that the heterocyclization of ketoximes (103) with acetylene in snper basic medium and in the presence of metal hydroxides proceeds by a [3,3]-sigmatropic rearrangement of the enehydroxylamine 105 of the corresponding oxime vinyl ethers 104 (equation 32). The unreactivity of 3-methyl-2-azabicyclo[3.3.1]nonan-9-one oxime (106) in the same reaction conditions was explained by its inability to isomerize to the corresponding enehydroxylamine. 

Lantos and colleagues reported a different imidazole synthesis via hetero-Cope rearrangement. Oximes 116 reacted with benzenecarboxidoyl chloride affording adducts 117 that readily underwent a [3,3]-sigmatropic rearrangement at high temperatures (equation 35) to substituted imidazoles 118. 

The [3,3]-sigmatropic rearrangement of O-aryl oximes was first described by Sheradsky and it has been used as an adequate route to benzofuran ring, a structural unit largely present in natural products. 

One potential approach for the preparation of benzofuran derivatives involves the acid-catalyzed cyclization of O-aryl oximes 1193. 39 (equation 36). The transformation is analogous to well-known Fisher indole synthesis. The reaction proceeds through a [3,3]-sigmatropic rearrangement of enehydroxylamine 120 to compound 121 followed... 

A new synthetic method of benzofuran was reported (equation 39). The [3,31-sigma-tropic rearrangement of Af-trifluoroacetyl enehydroxylamines 136 obtained in situ by acylation of oxime ethers 135 in the presence of trifluoroacetic anhydride lead to the synthesis of cyclic or acyclic dihydrobenzofurans 138. The effects of base and temperature on the reaction products were studied. A similar pathway to that of Fisher indolization was proposed. The acylimine formed by the [3,3]-sigmatropic rearrangement of the V-trifluoroacetyl enehydroxylamine 136 gave the dihydrobenzofuran 137 by an intramolecular cyclization or the benzofuran 138 after elimination. 

The reaction of aliphatic and aromatic ketone oximes 97 with a dialkyl carbonate 98 in the presence of K2CO3 at 180-190 °C yields 3-alkyl-4,5-disubstituted-2(3//)-oxazolones 104 in 22-48% yields. Mechanistically, it is proposed that N-alkylation of the initially formed oxime O-carbonate 99 yields 100, which affords the enamine 101 in the presence of base. A [3,3] sigmatropic rearrangement ensues to produce 102, which then cyclizes to 104. In cases where 97 contains two methylene groups in proximity to the C=N bond, one of which is benzylic, the above reaction sequence is regioselective for the benzylic methylene group (Fig. 5.25 Table 5.6, Fig. 5.26). ... 

A mixture of dienamides was prepared from the oxime of isophorone (equation 50). Metallated 2-propynyl alcohols react with trichloroacetonitrile to form dienamides after a (3,3)-sigmatropic rearrangement and suitable prototropic shifts649 thus the dienamides formed exist as a mixture of isomers (equation 51). 

There are other types of proton shift tautomers, such as phenol/dienone, nitroso/oxime and imine/enamine, but these are less often encountered. There are also valence tautomers that exhibit fluxional structures, which undergo rapid sigmatropic rearrangements. Molecules that exhibit this type of isomerism are very interesting, but are not often encountered in undergraduate courses. An example of a valence tautomer is illustrated by the Cope system. 

In general, synthesis of 4-isoxazolines is accomplished via three routes 1,3-dipolar cycloaddition of nitrones to alkynes or by the oximation of a,i -ethylenic ketones, a-alkynic ketones and aldehydes or from the selective reduction of isoxazolium salts. The nitrone (262) underwent tandem cyclo-addition-[2,3]sigmatropic rearrangement with allenyl sulfoxide (263). And it resulted in 4-isoxazoline (264) (Equation (46)) <89TL663>. 

Arylbenzo[fo]furans were synthesized by the [3,3]-sigmatropic rearrangement of oxime ethers <04OL1761>. 

Several 2-(2-thienyl) and 2-(3-thienyl)pyrrole-3-carboxylates were made by reaction of acetylthiophene oximes with methyl propiolate or dimethyl acetylene-dicarboxylate. <93JCR(S)210> This reaction is related to other sigmatropic rearrangements which have been used in pyrrole and indole synthesis. 

The regioselective synthesis of pyrroles via [3,3]- and [l,3]-sigmatropic rearrangements of 0-vinyl oximes formed by iridium-catalysed isomerization of 0-allyl oximes is such that when enolization is favoured, a [3,3]-rearrangement followed by a Paal-Knorr cyclization gives a 2,3,4-trisubstituted pyrrole and when enolization is disfavoured, a [l,3]-rearrangement occurs prior to enolization to give a 2,3,5-trisubstituted pyrrole after cyclization (Scheme 10). ... 

O-Propargyhc hydroxylamines 16 are converted to the isomeric 3,5-disubstituted isoxazohnes on treatment with K2CO3 [328]. The mechanism of this transformation is not yet elucidated. A possible explanation involves (2,3)-sigmatropic rearrangement of 16 to a N-aUenic hydroxylamine 17, which may further rearrange to a a,P-unsaturated oxime 18 in analogy to (1), cyclization gives rise to the isoxazoline system ... 

Cesium derivative of 2-acetylanthracene oxime upon short contact with acetylene (5 min) under mild conditions (80°C) selectively forms 0-vinyl oxime of 2-acetylanthracene (44% yield) [227]. The subsequent 3,3-sigmatropic rearrangement of this 0-vinyl oxime tautomer leads to 2-(2-anthryl)pyrrole in 24% yield (Scheme 1.54). The rearrangement is realized in DMSO in the absence of a base (120°C, 30 min). 

The formation of two isomeric triphenylpyrroles may be rationalized by cycU-zation (via [3,3]-sigmatropic rearrangement) of isomeric O-vinyl oximes, the primary adducts of benzyl phenyl oximes to a- and p-carbon atoms of phenylacetylene (Scheme 1.114). 

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