Allyl shifts cyclohexadienones

The classical Claisen rearrangement is the first and slow step of the isomerization of allyl aryl ethers to orlho-a ly lated phenols (Figure 14.46). A cyclohexadienone A is formed in the actual rearrangement step, which is a [3,3]-sigmatropic rearrangement. Three valence electron pairs are shifted simultaneously in this step. Cyclohexadienone A, a nonaromatic compound, cannot be isolated and tautomerizes immediately to the aromatic and consequently more stable phenol B. 

In the Claisen rearrangement, an allyl aryl ether gives an o-allylphenol on heating. The mechanism here is not a Whitmore 1,2-shift, but is a concerted peri cyclic [3,3] sigmatropic rearrangement, that results in a cyclohexadienone, which, after tautomerisation, gives the phenol. 

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