Question

Question:Draw a stepwise mechanism for the Carroll rearrangement, a reaction that prepares aγ,δ-unsaturated carbonyl compound from aβ-keto ester and allylic alcohol in the presence ofbase.

Short answer

Answer

Step-by-step solution

Carroll rearrangement

The Carroll rearrangement involves the conversion of $\mathbf{\beta }$-keto allyl ester into $\mathbf{\alpha }$-allyl-$\mathbf{\beta }$ -carboxylic acid. It is accompanied by decarboxylation, which gives the final product, i.e., a $\mathbf{\gamma }\mathbf{,}\mathbf{\delta }$-unsaturated carbonyl system.

Mechanism for Carroll rearrangement

The starting materials are given below:

In the presence of a base, the hydroxyl group gets deprotonated. The oxide anion then attacks at the electrophilic carbonyl carbon center, one that is a part of the ester group.

Deprotonation of hydroxyl group and formation of new ester

The ester that is formed tautomerizes and then participates in a [3,3]-sigmatropic rearrangement. The mechanism is shown below:

Tautomerization and Carroll rearrangement