Sigmatropic reactions 2,3 -Wittig rearrangement

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... 

Rearrangement reactions (Miscellaneous) (see also Electrocyclic rearrangements, Sigmatropic rearrangements, Wittig rearrangement)... 

The [2,3]-Wittig Rearrangement is a [2,3]-sigmatropic reaction, a thermal isomerization that proceeds through a six-electron, five-membered cyclic transition state. A general scheme for [2,3]-sigmatropic reactions is given here ... 

These rearrangement reactions are interpretable in terms of [2.3] sigmatropic shifts of the intermediate ylides. A number of such rearrangements of open-chain systems have been described, involving sulfonium ylides [43] [44] [45], ammonium ylides [46] [57], anions in a-position to oxygen (Wittig rearrangement) [48] [49], and fluorenyl carbanions [50]. 

Axial ViS. equatorial selectivities of Wittig rearrangements were studied with the f-butylcyclohexane derivatives (89 equation 22) (Table 8). The lithio dianion of acid (89a) prefers equatorial attack. The Bu 02C (89b) and MesSiCsC substituted ethers (89c) undergo nonstereoselective rearrangements. The Still-Wittig reaction of (89d) is the only one among all known 2,3-sigmatropic shifts on this skeleton that prefers axial attack (67 33). 

The l,5-dien-3-ols produced by 2,3-Wittig rearrangement of diallylic ethers can serve as pivotal intermediates for further elaboration through tandem or sequential 3,3-sigmatropic reactions. " The three sequences examined to date are summarized in Scheme 10. 

A related reaction is the [2,3] Wittig rearrangement.33,37 This goes via a live-membered transition state - we shall not go into any more detail about that - but it too is a useful reaction both for making homoallylic alcohols and because of the stereocontrol that can be achieved in the process. Allylic ether 150 gives38 only the diastereoisomer shown of alcohol 152. The [2,3]-sigmatropic rearrangement 151 creates an -alkenc at the expense of a Z-alkene and two new chiral centres at the expense of one. The immediate product of the [2,3]-shift is an oxyanion instead of a carbanion. 

Other common all-suprafacial [2,3]-sigmatropic rearrangements are the Mislow reaction 6.11 —> 6.12, and the anionic 2,3-Wittig rearrangement 6.110 —> 6.111 of the anions made from diallyl ethers by treatment with... 

The [2,3] Wittig rearrangement is a useful reaction involving a [2,3] sigmatropic shift, and has been used to obtain homopropargylic alcohols from allylic propargylic ethers with high stereoselectivity. The reaction has been modified by Marshall to provide a useful route to optically active allenic alcohols. 

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