2,3]‐Wittig Rearrangement

Abstract

Abstract This reaction is a highly regioselective [2,3]‐sigmatropic rearrangement of the conjugate bases of allylic ethers (or benzylic ethers) to alcohols with concomitant C‐C bond formation. Therefore, this type of anionic [2,3]‐sigmatropic rearrangement is generally known as the [2,3]‐Wittig rearrangement. This rearrangement has been extended to α‐alkoxystannes and is referred to as the Still–Wittig [2,3]‐sigmatropic rearrangement. Analogously, the rearrangement of silyl anion from a silyl allylic ether is termed as the siloxy ‐[2,3] Wittig rearrangement and the rearrangement of an allylic amine in the presence of a strong base is called the aza ‐2,3‐Wittig rearrangement. The driving force for this rearrangement is probably the conversion of a moderately stabilized carbanion to a more stable alkoxide. It has been proposed that the breaking C‐O bond and the forming C‐C bond in the transition state are almost eclipsed, and the steric effect and electronic interaction at this point result in a high stereoselectivity. All the mechanistic features of the reaction have been discussed and it has been found that the tertiary bis ‐allyl ethers do not undergo the [2,3]‐Wittig rearrangement when treated with butyl lithium, but proceeds readily when treated with the combination of t ‐BuOK and t ‐BuLi. For the tertiary bis ‐allyl ether derived from (+)‐camphor, the rearrangement occurs via an endo ‐transition state to afford products of R ‐configuration. On the other hand, the substituent at either α‐ or γ‐position of the allylic moiety will depress the lithiation. In the case of 3‐furylmethyl ethers, the [2,3]‐Wittig rearrangement is indeed competed with [1,2]‐Wittig rearrangement, and the actual outcome depends on the bases applied. Overall, the [2,3]‐Wittig rearrangement is very common and widely used in organic synthesis.