2,3-Wittig rearrangements allyl vinyl 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. 

As a technique for carbon-carbon c-bond formation, the [2,3] Wittig rearrangement of allylic ethers 1 offers many of the stereochemical features of the [3,3] Claisen rearrangement of allyl vinyl ethers (see Section D.1.6.3.1). 

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 importance of the base used in the [2,3]-Wittig reaction is made clear by the work of Mitchell and coworkers who have reported the rearrangements of silyl- and stannyl-substituted diallyl ethers. They found that the use of LDA on such substrates (38, 41, 43) leads to [2,3]-Wittig reaction while employing lithium diethylamide can result in an allyl-vinyl ether rearrangement instead. 

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. 

The Wittig rearrangement of allyl vinyl and benzyl vinyl ethers brought about by strong bases is not shown by benzyl vinyl sulphides. Base-catalysed rearrangement of bis(propargyl) sulphides involves first the formation of the... 

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). ... 

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