Deprotonation 1,2 -Wittig rearrangement

Studies on the deprotonation and subsequent [1,4]-Wittig rearrangement of a-benzyloxyallylsilanes have shown that the two processes are separate rather than concerted events.74 The presence of anion-stabilizing groups on the migrating substituent could be detrimental to the success of the reaction. 

The transformation of deprotonated allyl ethers into homoallylic alcohols is the [2,3]-sigmatropic version of the [1,2]-Wittig Rearrangement, and is therefore termed [2,3]-Wittig Rearrangement ... 

Enantioselective deprotonation to yield a configurationally defined organolithium offers great potential for future asymmetric [2,3]-Wittig rearrangements. The first steps in this area were taken by Nakai and Tomooka, who treated 182 with r-BuLi in the presence of the bis-oxazoline ligand 114. The product 183 is formed in 89% ee.133... 

Chiral lithium bases have been used for enantioselective deprotonation to yield configurationally stable a-oxy carbanions. This holds potential for asymmetric [2,3]-Wittig rearrangement in stereoselective synthesis. Thus, treatment of propargylic ether 72 with (S,S)-3 in THF at — 70 °C to —15 °C afforded propargylic alcohol 73 in 82% yield and in 69% ee of the shown enantiomer96,97. This product was successfully employed as a precursor of (-l-)-Aristolactone (Scheme 55). 

We also observed similar phenomena in the reaction of silyl enol ethers with cation radicals derived from allylic sulfides. For example, oxidation of allyl phenyl sulfide (3) with ammonium hexanitratocerate (CAN) in the presence of silyl enol ether 4 gave a-phenylthio-Y,5-un-saturated ketone 5. In this reaction, silyl enol ether 4 reacts with cation radical of allyl phenyl sulfide CR3 to give sulfonium intermediate C3, and successive deprotonation and [2,3]-Wittig rearrangement affords a-phenylthio-Y,6-unsaturated ketone 5 (Scheme 2). Direct carbon-carbon bond formation is so difficult that nucleophiles attack the heteroatom of the cation radicals. 

Other Enantioselective Reactions. Enantioselective epoxide elimination by chiral bases has been demonstrated. More recently, the enantioselective [2,3]-Wittig rearrangement of a 13-membered propargylic ally lie ether has been performed using the lithium amide of (f ,f )-(l) as the base for deprotonation (eq 15). For this particular substrate, THF is a better solvent than ether, although pentane produces better results in a related transformation (eq 16). In fact, a change in solvent in this type of reaction has been shown to lead to a reversal of the stereoselectivity of the transformation. ... 

Double bond rearrangements in carbohydrate systems lead to various kinds of sugar transformations. The [2,3]-Wittig rearrangements [73] initiated by deprotonation and followed by... 

Eichinger, P. C. H., Bowie, J. H. Gas-phase carbanion rearrangements. Does the Wittig rearrangement occur for deprotonated vinyl ethers J. Chem. Soc., Perkin Trans. 2 1990, 1763-1768. 

As noted in Section 3.11.1, some of the earliest mechanistic work on the 2,3-Wittig rearrangement was carried out with allyl benzyl ethers. Selective benzyl deprotonation is typically effected with n-butyl-lithium in THF or THF-TMEDA at low temperature. Rautenstrauch s studies, summarized in Table 8, clearly showed that lower temperature favors 2,3- over 1,2-rearrangement, and established the potential... 

Allyl propargyl ethers are also good substrates for the Wittig rearrangement. Deprotonation of the propargyl position is relatively facile. 

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