Wittig-- and -Rearrangement

The [1,2]-Wittig rearrangement proceeds via a radical-pair dissociation-recombination mechanism, while the [2,3]-Wittig rearrangement is a concerted, thermally allowed sigmatropic process proceeding via an envelope-like transition state in which the substituents are pseudo-equatorial. 

Homo-allylic alcohol (Z)-substrate L r5 J envelope TS Homo-allylic alcohol 

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. 

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