Homoallylic carbanions

Silyl homoallylic alcohols are obtained with high y-regioselection and E-stereoselection on reaction of chiral alkoxy- and aminomethyl-substituted a -silylallyl carbanions with aldehydes factors which influence the diastereomeric excess have been identified. 

A three-step protocol has been applied to obtain homoallylic carbanions from cyclopropylmethanol derivatives by regioselective opening of intermediate cyclopropyllithiums. ... 

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) °. 

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. 

A chiral allylic alcohol (3-carbanion equivalent has also been developed which utilizes a DIPT-modified E)-y-(dimethylphenylsilyl)allylboronate reagent. This method involves treating the product homoallylic alcohol with Dimethyl-dioxirane and subjecting the derived epoxide to an acid-catalyzed Peterson elimination. This sequence has been applied in the synthesis of the trioxadecalin ring system of the mycalamides (eq 10). ... 

Intramolecular addition of an a-sulfinyl carbanion to an isolated double bond occurs on treatment of medium-ring -homoallylic sulfoxides (58), (59), (60) and (61) with butyllithium (0.5 equiv. equations 16-19) < Kinetic data suggest that this process is a nucleophilic addition of a carbaruon (an a-lithiated sulfoxide) to a nonactivated double bond and takes place reattily, provided a suitable proton source is available. Normally the free sulfoxide is the proton donor species. ... 

The related (Z)-lithium dialkenylcuprates (147) derived from acetylene itself also react well with epoxides to provide a useful route to (2)-homoallylic alcohols the lack of reactivity with esters allows an easy access to lactones (148) by condensations between epoxy esters and this type of cuprate (Scheme 29). Likewise, the lower homologs (149) and (151), both of which are relatively easy to prepare in optically active forms, can be readily converted into homoallylic and bishomoallylic alcohols (150) and (152) respectively. An ester unit can also be incorporated into the cuprate functions thus, addition of a mixed lithium cuprate, RCuYLi , to ethyl propiolate gives the cuprates (153), which add to epoxides to give unexpectedly the (Z)-crotonates (154). Such isomerization is not uncommon with vinyl carbanions in general, and is obviously a limitation when isomeric mixtures are produced. 

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]. 

Figure 3.7 Structures of allylic, homoallylic, and homobenzylic carbanions.

The sjmthesis of (+)-chanoclavin I (582) has been achieved by a multistage route from the acetal (580) by way of the key intermediate (581) whose aliphatic double bond is readily cleaved by ozone. A third, convenient synthesis of the diterpenoid trachylo-bane (586) has been achieved by a reductive homoallylic cyclization. Reaction of the mesylate ester (583) with methylsulphinyl carbanion gives the ketone (584) and the alcohol (585). Oxidation of the crude product mixture with Jones reagent gave ketone (584) which can be reduced to trachylobane (586). ... 

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