Secondary allylic ethers 2,3 -Wittig rearrangment

As noted previously, the [2.3] Wittig rearrangement of zirconium ester enolates demonstrates a strong preference for Z-oleftn formation. For secondary allylic ethers 16, the preferential formation of Z-i v -products suggests that rearrangement occurs through an Ra transition stale conformation in order to minimize the RA-G steric interaction62,6a. 

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. 

Benzyl methyl ether or allyl methyl ethers can be selectively metalated at the benzylic/allylic position by treatment with BuLi or sBuLi in THF at -40 °C to -80 C, and the resulting organolithium compounds react with primary and secondary alkyl halides, epoxides, aldehydes, or other electrophiles to yield the expected products [187, 252, 253]. With allyl ethers mixtures of a- and y-alkylated products can result [254], but transmetalation of the lithiated allyl ethers with indium yields y-metalated enol ethers, which are attacked by electrophiles at the a position (Scheme 5.29). Ethers with ft hydrogen usually undergo rapid elimination when treated with strong bases, and cannot be readily C-alkylated (last reaction, Scheme 5.29). Metalation of benzyl ethers at room temperature can also lead to metalation of the arene [255] (Section 5.3.11) or to Wittig rearrangement [256]. Epoxides have been lithiated and silylated by treatment with sBuLi at -90 °C in the presence of a diamine and a silyl chloride [257]. 

The [2.3] Wittig rearrangement of secondary ethers (e.g., 11) stabilized by /X-donor substituents G yields E-olefins for most substrates12 31-33. The preferential formation of -olefins is attributed to rearrangement through a transition state conformation in which the allylic substituent Ra adopts an exo orientation, presumably to minimize A13 interaction with the olefinic substituent. 

Propynyl ethers of both E- and Z-secondary allylic alcohols (29, 31) undergo [2,3] Wittig rearrangement with the expected (E anti, Z->syn) diastereoselectivity34-37,39. 

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