Carbanions homoallylic rearrangements

Ring-cleavage reactions of cyclobutylmethyl compounds via bomoallylic carbanions have been reported.23,24 The reaction was denoted as a y-homoallylic rearrangement 25 and occurs with H-shift. 

The pioneering work on thia-[2,3]-Wittig rearrangement was reported by Rautenstrauch in 1971 °. The reaction of allyl sulfides 137 with n-BuLi at —30°C for 1.5-4 h gave homoallyl sulfide or thiol 138 ([2,3]-product) exclusively (equation 81). In contrast, a similar reaction of vinyl sulfide 139 gave no [1,2]-rearrangement product 141 albeit a-thio-carbanion was generated as shown by its methylation to 140 (equation 82) °. 

Due to our engagement in carbanion rearrangements we were especially interested in the homoallylic system 4,4-dilithio-l-butene 33. In the case of homoallyllithium 29 ring-chain equilibrium strongly favors the open-chain lithiumorganic compound and the introduction of four methyl groups into the a- and -positions is necessary... 

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. 

Carbanions derived from a-sulfonyl ethers 5 undergo [2.3] sigmatropic rearrangement the resulting a-hydroxy sulfoties are subject to a further fragmentation, yielding an aryl sulfinic acid and an aldehyde which reacts further with excess alkyllithium Lo afford a secondary homoallylic alcohol61 66. 

The [2,3] -Wittig rearrangement is a special class of [23] -sigmatropic rearrangement which involves an a-oxy carbanion as the migrating terminus to afford a variety of homoallylic alcohols or allenic alcohols [Eq. (6)] [2b, 10]. 

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