Homoallylic alcohols 2,3 -Wittig rearrangement

Transformation of allyl ethers into homoallylic alcohols by treatment with base. Also known as Still-Wittig rearrangement. Cf. Sommelet-Hauser rearrangement... 

The [2,3]-Wittig Rearrangement allows the synthesis of homoallylic alcohols by the base-induced rearrangement of allyl ethers at low temperatures. 

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 ... 

In the presence of HMPT the anion of 3 undergoes a [2,3]Wittig rearrangement to provide an unstable aldehyde, which is trapped by an alkyllithium to give (E)-homoallylic secondary alcohols (5). 

Z)-Trisubstitutedalkenes. Still and Mitra have described an efficient synthesis of alkenes of this type from allylic alcohols by a [2.3] sigmatropic Wittig rearrangement. The alcohol 2 is converted into the allyl stannylmethyl ether (3), which can be isolated if desired. Treatment with n-butyllithium results in tin-lithium exchange and rearrangement to the homoallylic alcohol 4 in 95% overall yield. When 3 is transmetalated and immediately quenched with cyclohexanone, 5 is obtained in 73% yield. 

Since the appropriate allylic ethers, as starting material, are generated normally from allyl alcohols, the 2,3-Wittig rearrangement permits easy construction of homoallyl alcohol systems (31) from allyl alcohols (28). Due to its sigmatropic nature the rearrangement possesses several stereochemical features... 

The Still-Wittig rearrangement of stannylated cis-ethers (11) gives trans homoallylic alcohols (12) selectively (equation 5). If the underlying alcohols (R = Me) are rearranged under Biichi s conditions (DMF acetal, xylene, reflux), trans selectivity is achieved, too, but yields are lower (41-49%). ... 

Unequalled in elegance is the double Still-Wittig rearrangement of (48), which gave the bis-(ZZ)-homoallylic alcohol (49) in 95% isomeric purity (equation 16). (Z)-Selectivity has also been noted for the Wittig rearrangement of an acid dianion (equation 17). ... 

Perfect anti selectivities may also result where they would not have been predicted on the basis of much less satisfactory model studies (cf. Table 5, entries 1 and 9). Pertinent examples are the Wittig rearrangements of the sterically demanding acid (85 equation 20) or of the oxazoline (87 equation 21). ° The highly syn,as selective synthesis of homoallylic alcohols (26) from isopropyl esters was discussed earlier in a different context (equation 12, Scheme 3). ... 

The conversion of the cyclopentenyl alcohol (92) into the homoallylic alcohol (93) occurs by a dia-stereoselective Still-Wittig rearrangement on the less hindered face of the molecule (equation 23). °... 

Complete chirality transfer from cw-ethers (96 equation 24) to homoallylic alcohols (97) was the cornerstone for numerous applications of the Wittig rearrangement in naturd product synthesis (Table 9). The predictability of the absolute configuration of the rearranged product on the basis of a suprafacial bond shift and the ready availability of optically active starting materials make these reactions attractive. 

Allylic alcohols with a trans double bond give product mixtures in the Still-Wittig rearrangement cf. equation 7 ). Advantageously, such substrates e.g. 98) can be converted to homoallylic alcohols with 100% chirality transfer by the Biichi rearrangement (equation 25). ... 

The first asymmetric total synthesis of (+)-astrophylline was accomplished in the laboratory of S. Blechert. The Still variant of the [2,3]-Wittig rearrangement was used to generate the 1,2-trans relationship between the substituents of the key cyclopentene intermediate. The tributylstannylmethyl ether substrate was transmetalated with n-BuLi, which initiated the desired [2,3]-sigmatropic shift to afford the expected homoallylic alcohol as a single enantiomer. 

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. 

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]. 

The [2,3]-Wittig rearrangement is a sigmatropic reaction which results in the construction of homoallylic alcohols 2 from allyl ethers 1 upon treatment with base at low temperatures. 

Transmetallation can be employed in order to avoid the use of strongly basic conditions. One such variant is the [2,3]-Wittig-Still rearrangement wherein stannyl ethers can be converted to homoallylic alcohols. Several examples of this tranformation in the synthesis of amino acid components of bioactive polyoxins have been reported by Ghosh. In their synthesis of 5-0-carbomylpolyoxamic acid, a bioactive amino acid nucleoside, E and Z-allylic stannyl ethers, such as 45, derived from an isopropylidene L-threitol derivative, were subjected to the [2,3]-Wittig-Still rearrangement. 

The stereocontrolled synthesis of homoallylic alcohols and unsaturated ketones via a benzotriazole-mediated [2,3]-Wittig rearrangement... 

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