Anionic -sigmatropic rearrangements

C4-insertions by means of a sigmatropic rearrangement process have been described using either a thermal Cope reaction, anionic Claisen amide enolate and zwitterionic aza-Claisen rearrangements. 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

Sigmatropic rearrangements of anions of (V-allyl amines have also been observed and are known as aza-Wittig rearrangements.291 The reaction requires anion stabilizing substituents and is favored by (V-benzyl and by silyl or sulfenyl substituents... 

A final crucial step in this synthesis was an anionic [2,3]-sigmatropic rearrangement of an allylic ether in Step D-4 to introduce the C(l) carbon. 

The corresponding reaction of diethyl 1,2-propadienyl phosphonates (sulfones or sulfoxides) with N-phenylhydroxylamine afforded a-anionic N-phenylvinyloxyamine 159, which upon [3,3]-sigmatropic rearrangement led to anionic 2-(2 -oxoalkyl)phe-nylamine 160. Further cyclization provides an efficient synthesis of indole derivatives 161 [84]. 

Notice that the final result is a 1,3 charge affinity inversion Umpolung) of an allylic derivative via a FGI of a functional group of type E by a group of type A, followed by a [2,3]-sigmatropic rearrangement. If the intermediate allyl anion reacts with a carbonyl compound as the electrophile the result is then a 1,4-D system, such as ... 

Let us now consider a dissonant 1,6-dicarbonyl system, which provides a good example of a [3,3]-sigmatropic rearrangement. The "illogical disconnection" would lead to an a,p-unsaturated ketone and a "homoenolate" anion ... 

In the solution proposed by Evans [25], however, the unsaturated ketone is condensed with the anion of an alkyl allyl ether, followed by a [3,3]-sigmatropic oxy-Cope type rearrangement (Scheme 5.22). In the retrosynthetic sense this means a [3,3]-sigmatropic rearrangement of the bis-enol form of the 1,6-dicarbonyl system. 

The Cope, oxy-Cope, and anionic oxy-Cope rearrangements belong to the category of [3,3J-sigmatropic rearrangements. Since it is a concerted process, the arrow pushing here is only illustrative. Cf. Claisen rearrangement. 

Aminophenols from anilines, 35, 2 Anhydrides of aliphatic dibasic acids, Friedel-Crafts reaction with, 5, 5 Anion-assisted sigmatropic rearrangements, 43, 2 Anthracene homologs, synthesis of, 1, 6 Anti-Markownikoff hydration of alkenes, 13, 1... 

Aliphatic 3-aza-4-oxa-Cope rearrangement of ester-amide dienolates increases the synthetic utility of anionic [3,3]-sigmatropic rearrangement initiated by N—O bond cleavage. Treatment of the enehydroxylamine 94 with KHMDS in the presence of TMSCl at —80°C provided a mixture of iV,0-disilylated 95 and 0,0-disilylated 96 derivatives (equation 30). Both of these would, on [3,3]-sigmatropic rearrangement, provide the corresponding silyl ethers 97 and 98, from which 99 and 100 are obtained on workup. 

The [2,3] sigmatropic rearrangement pattern is also observed with anionic species. The most important case for synthetic purposes is the Wittig rearrangement, in which a strong base converts allylic ethers to a-allyl alkoxides.191... 

The close association between metal ions and p-benzoquinones catalyzes their Diels-Alder reactions with anthracenes. The efficiency of the metal cations correlates with their Lewis acidity171. A mechanism proceeding via radical-anions for a [3,3] sigmatropic rearrangement was established172. 

Epoxidation of enones on treatment with basic hydrogen peroxide or /-butyl hydroperoxide, or with bleach, might be viewed as another form of trapping the initially produced anion.99 In this case the enol-ate, e.g. (425 Scheme 57), attacks the oxygen of the hydroperoxide to eject hydroxide and yield the epoxy ketone (426)." Finally, the initial anion can also be trapped by a sigmatropic rearrangement, as in... 

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