Homoallyl alcohols 1,3-sigmatropic rearrangements

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 ... 

Z)-Homoallylic alcohols. The tributylstannylmethyl ether of an allylic alcohol on treatment with n-butyllithium at —78° undergoes tin-lithium exchange followed by [2,3] sigmatropic rearrangement to a homoallylic alcohol. A valuable feature is that the (Z)-isomer is formed. An example is shown in equation (I). ... 

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. 

Asymmetric selenoxide elimination of the optically active vinyl selenoxides affords optically active allenes and cyclohexylidenes. On the other hand, asymmetric [2,3]sigmatropic rearrangement of allylic selenoxides, selenimides, and selenium ylides leads to the formation of the corresponding optically active allylic alcohols, amines, and homoallylic selenides, respectively. 

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... 

IJ-Sigmatropic Rearrangements Table 6 Synthesis of Syn Homoallylic Alcohols - ... 

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. 

A stereoselective base catalysed [2,3] sigmatropic rearrangement of allyl ethers to homoallylic alcohols (stereoselective) (see 1st edition). 

In obtaining all-tram bis-homo-farnesol (7), needed for the synthesis of Cig-JH, Still and co-workers (1979) made use of their former finding (Still and Mitra, 1978) on the [2,3]-sigmatropic rearrangement of alkoxyorganolithium reagents, which provides an efficient method for the preparation of Z-homoallylic alcohols. 

A relative of this reaction can also be used to prepare homoallylic alcohols. When benzaldehyde reacted with 2-methylbut-3-en-l-ol, a 78% yield of 465 was obtained as a 49/1 ElZ ) mixture. Tin-catalyzed addition of the alcohol substrate to the aldehyde followed by a [3,3]-sigmatropic rearrangement led to the products. Clearly, this is not the same reaction, but it is useful to compare it with the other reactions. 

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. 

Hydroboration of terminal allenylsilanes with 9-BBN occurs from the less hindered side to afford [1] the kinetic products with Z geometry, which undergo fast 1,3-sigmatropic rearrangements to thermodynamically more stable ( )-2-(trimethylsilyl) allylboranes. Condensation of isomer with aldehydes proceeds through six-membered transition state and on treatment with 2-aminoethanol [2] gives the desired silated homoallylic alcohols with high diastereomeric purity (Scheme 6.15). 

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