Wittig rearrangement with vinyl ethers

The rearrangement of an ether 1 when treated with a strong base, e.g. an organo-lithium compound RLi, to give an alcohol 3 via the intermediate a-metallated ether 2, is called the Wittig rearrangement. The product obtained is a secondary or tertiary alcohol. R R can be alkyl, aryl and vinyl. Especially suitable substrates are ethers where the intermediate carbanion can be stabilized by one of the substituents R R e.g. benzyl or allyl ethers. 

An earlier example of this type of domino reaction was reported by Greeves and coworkers (Scheme 2.194) [443]. Treatment of either the ( )- or (Z)- allyl vinyl ether 2-870 with NaH initiates the [2,3]-Wittig rearrangement to afford 2-872 via 2-871. The subsequent oxy-Cope rearrangement led to the aldehyde 2-873, which was reduced with NaBH4 to give the alcohols 2-874. Both isomers of 2-870 predominantly generated the (/ )-xyu-product 2-874 in comparable ratios as the main product. 

A novel approach to stereocontrol in 2,3-Wittig rearrangements involves the use of chiral chromium tricarbonyl complexes (Table 8a). The reactions, which were carried out with racemic ethers, show excellent diastereoselectivity as a consequence of a preferred transition state in which the benzylic oxygen and the R -substituent and the vinylic grouping and Cr(CO)3 grouping adopt anti orientations (Lc). 

The [1,2]-Wittig rearrangement of ethers with alkyl lithiums yields alcohols via a [l,2]-shift, as shown in Scheme 7.15. The groups R, and R2 may be alkyl, aryl, or vinyl (Wittig and Ldhmann 1942). 

Ketals are converted to allyl vinyl ethers after Wittig olefination [14]. Wittig reactions permit the incorporation of various substituents into the allyhc terminal of the ether. Enol ethers have been conveniently prepared by cleavage of acetals with various Lewis acids. Cleavage of the ketals with the Lewis acid, for example, triethylsilyl triflate, in the presence of diisopropylethylamine in refluxing 1,2-di-chloroethane afforded the enol ethers (Eq. 3.1.9). The resulting enol ethers were then heated to effect the Claisen rearrangement without isolation. 

The [l,5]-anion relay/[2,3]-Wittig rearrangement of 3,3-bis(silyl) enol allyl ethers to vinyl bissilanes provides vinyl bissilanes, which can be transformed into trisubstituted vinylsilanes through a [l,4]-Brookrearrangement/alkylation protocol with electrophiles (Scheme 79). ... 

---