Q.70. - Organic Chemistry

Question

Question: Draw a stepwise mechanism for the following reaction. (Hint: Two Michael reactions are needed.)

Step-by-Step Solution

Answer

Answer

Mechanism of the given reaction

Step 1: Michael reaction

The addition of a nucleophile to an  $\mathit{\alpha }\mathbf{,}\mathit{\beta }\mathbf{ }\mathit{u}\mathit{n}\mathit{s}\mathit{a}\mathit{t}\mathit{u}\mathit{r}\mathit{a}\mathit{t}\mathit{e}\mathit{d}\mathbf{ }\mathit{c}\mathit{a}\mathit{r}\mathit{b}\mathit{o}\mathit{n}\mathit{y}\mathit{l}$ to form a 1,4- addition product is called Michael addition.

The reaction is a conjugate addition reaction and is used to form mild C-C bonds.

The nucleophile can be a carbanion or any other nucleophile. It is the preferred reaction in unsaturated carbonyls due to steric hindrance in 1,2-addition products.

Step 2: Cyclization reactions

Reactions that cause the formation of a ring in the compound are called cyclization reactions. If the cyclization occurs within the same molecule, it is called internal cyclization.

These reactions are an important class of reactions in organic synthesis because the majority of the natural products contain a ring in their molecular skeleton or as a substituent.

Cyclization reactions are generally initiated by acids or bases by creating a reactive center in the molecular skeleton.

Step 3: Mechanism of the given reaction

The given reaction is initiated by the abstraction of a proton from the molecule.

This produces an electron-rich center that undergoes Michael addition with the unsaturated ester group forming a ring.

Another Michael addition leads to the formation of a bridged compound that is stabilized by proton abstraction from a water molecule.

Mechanism of the given reaction