Rearrangement para-Claisen

If both ortho positions bear substituents other than hydrogen, the allyl group will further migrate to the para position. This reaction is called the para-Claisen rearrangement. The formation of the para-substituted phenol can be explained by an initial Claisen rearrangement to an ortho-2l y intermediate which cannot tautomerize to an aromatic o-allylphenol, followed by a Cope rearrangement to the p-allyl intermediate which can tautomerize to the p-allylphenol e.g. 6 ... 

The Cope rearrangement is of great importance as a synthetic method e.g. for the construction of seven- and eight-membered carbocycles from 1,2-divinylcyclopropanes and 1,2-divinylcyclobutanes respectively (e.g. 11 12), and has found wide application in the synthesis of natural products. The second step of the para-Claisen rearrangement is also a Cope rearrangement reaction. 

The kinetics in solution pertaining to ethers is relegated primarily to the Claisen rearrangement. An excellent review of the kinetics of this rearrangement appeared in 1963. Discussion of this topic will be abbreviated for this reason. The reaction scheme for the ortho- and para-Claisen rearrangement is illustrated below with allyl phenyl ether... 

The ortho-Claisen rearrangement is characterised by the shift of the double bond in the allylic group. This allylic shift is absent in the para-Claisen rearrangement [1]. 

Figure 2. Mechanism of the para-Claisen rearrangement of ortho substituted allyl phenyl ethers.

In the studies of syrrngm, an active component in traditional Chinese medicine, it was shown that 4-hydroxy-3,5-drmethoxybenzoic acid 167 reacted with allyl bromide under basic conditions to produce a mixture of O- and C-allylated compounds 168, 169. After the mixture was subjected to heating at about 200 °C, a para-Claisen rearrangement took place to form the main product 169 in 71% yield (equation 76) . 

The Claisen rearrangement of allyl 2,6-dimethallylphenyl ether produces both 4-allyl-2,6-dimethallylphenol and 2-allyl-4,6-dimethallylphenol. If the reaction is interrupted before completion, some methallyl 2-allyl-6-methallylphe-nyl ether can be isolated. Explain the formation of each of these products, and discuss the mechanism of the para Claisen rearrangement in view of these results. 

When = H, the dienone (2), usually known as the orf/zo-dienone, rapidly tautomerizes to the 2-(prop-2-enyl)-phenol (3). This overall process is known as the orf/zo-Claisen rearrangement. If H, the dienone (2) undergoes a Cope rearrangement to give a 4-(prop-2-enyl)-cyclohexa-2,5-dienone (4), usually termed the para-dienone. The para-dienone (4) rapidly tautomerizes to the para-substituted phenol (5), and this overall transformation is termed the para-Claisen rearrangement (Scheme 3.22). 

Our last example, also due to Schmidt, is of an unusual concerted para Claisen rearrangement. Direct para rearrangement does not occur with phenyl allyl ethers [e.g., (192)] because the transition state (197) would be... 

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