Sigmatropic isomerization

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

The dienol is unstable, and two separate processes have been identified for ketonization. These are a 1,5-sigmatropic shift of hydrogen leading back to the enone and a base-catalyzed proton transfer which leads to the / ,y-enone. The deconjugated enone is formed because of the kinetic preference for reprotonation of the dienolate at the a carbon. Photochemical deconjugation is a synthetically useful way of effecting isomerization of a,) -unsaturated ketones and esters to the j ,y-isomers. 

The aza-Cope/Mannich reaction takes advantage of the facility with which a y,<5-unsaturated itninium ion, such as 6, participates in a [3,3] sigmatropic rearrangement to give an isomeric species which is suitably functionalized for an intramolecular and irreversible Mannich cyclization (see intermediate 7). The aza-Cope rearrangement substrate 6 is simply an unsaturated iminium ion which can be fashioned in a number of ways from a homoallylic... 

Interconversion of the four tautomeric azepines. either by [1,5]-H sigmatropic shifts, or under base catalysis, is common and almost always results in the formation of a 3//-azepine. For example, 1 //-azepine (1) in the presence of trimethylamine isomerizes rapidly to the 3H-tautomer 2 accompanied by formation of some polymer.9... 

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

When 1,5-dienes are heated, they isomerize, in a [3,3] sigmatropic rearrangement known as the Cope rearrangement (not to be confused with the Cope elimination reaction, 17-8)When the diene is symmetrical about the 3,4 bond, we have the unusual situation where a reaction gives a product identical with the starting material ... 

Early experiments [1] using (+)-apopinene and deuterium showed, however, that in the isomerized molecules the deuterium content was very low and the isomerization was much faster than deuterium incorporation into the allylic position. Therefore it seemed probable that isomerization takes place through an intramolecular hydrogen shift. A sigmatropic 1,3-hydrogen shift was suggested, in which the allylic endo-H shifted top shift) [2]. 

As shown in Scheme 4.3, the Fisher indolization was thought to involve (i) hydrolysis of diethyl dimethylamino acetal 11, (ii) formation ofhydrazone 22, (iii) isomerization ofhydrazone to me-hydrazine 23, and (iv) [3.3] sigmatropic rearrangement followed by ring closure to give indole 16b. Acetal 11 is stable in AcOH at room temperature, but can be readily hydrolyzed to aldehyde 19 at 100 °C, with subsequent cyclization to hemiaminal 20. Hemiaminal 20 was also formed readily... 

In another example using the isomeric amidoxime substrate 61, the formation of the expected [3,3]-rearrangement product 63 was not observed (Scheme 6.22). Instead the Z-adduct 62Z cyclized to oxadiazoline 64. Interestingly, the E-adduct 62E rearranged to hydroxypyrimidinone 60 and imidazole 66 instead of 63. The rearrangement of the substrate 62E was proposed to occur via intermediate 65 via a [l,3]-sigmatropic rearrangement which, after cyclization, led to the observed products 60 and 66. 

In many cases the transformations may be more complex than indicated by Eqs. (9.89)-(9.100). An example of this is the photochemistry of cis,cis-1,3-cyclooctadiene [Eq. (9.94)].<169) A close examination of this reaction indicates that bicyclo[4.2.0]oct-7-ene is formed but in low relative yields during the initial reaction (see Table 9.9). In addition, the cis,trans-1,3-cyclooctadiene is formed and then consumed as the reaction proceeds. Fonken showed that the bicyclooctene initially formed, however, was not from thermal isomerization of the cis,trans-diene. Still a third reaction was the 1,3 sigmatropic hydrogen shift to form the cis, cis-1,4-cyclooctadiene ... 

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