16.26 : [3,3]-sigmatrope Umlagerung von Allylvinylethern: Claisen-Umlagerung

The Claisen rearrangement is an oxy-variant of the Cope reaction in which one of the saturated carbons of a 1,5-diene is replaced by an oxygen atom.

A simple example is the thermal isomerization of allyl vinyl ethers to γ,δ-unsaturated carbonyl compounds.

The equilibrium strongly favors the product since the rearrangement yields a compound with a stable carbon–oxygen bond.

Like the Cope rearrangement, the mechanism follows a concerted reorganization of six electrons, preferably via a chair-like transition state.

The reaction is highly stereoselective. For example, isomerization of substituted allyl vinyl ethers favors the stereoisomer where the double bonds adopt an E configuration.

Note that the product is derived from a chair-like transition state, where the substituent occupies an equatorial position.

In allyl aryl ethers, the rearrangement gives an unsaturated ketone intermediate, which tautomerizes into a stable enol form, restoring aromaticity to the ring.

If both ortho positions are occupied, the reaction proceeds via two sequential Claisen rearrangements to give the p-allyl product.