Question

a) write a mechanism for the Michael addition of dimethyl malonate to mesityl oxide in the presence of sodium methoxide/ methanol.
b) Write a mechanism for cyclization step in the synthesis of dimedone.
c) Write a mechanism for the decarboxylation step of this synthesis, showing the structure of the transition state.
I will really appreciate your answer.

Answer 1

Michael addition : It is reaction between an α,β-unsaturated carbonyl compound and a compound with an active methylene group ( malonic ester,acetoacetic ester, cyanoacetic esters, nitroparafins etc.) in the presence of a base like sodium ethoxide or a secondary amine

Mechanism : The enolate derived from the malonic ester attacks the β-carbon atom of α,β-unsaturated compounds in conjugated additon. Theresulting carbanion abstracts a proton from water to give the product .

Answer 2

Mechanism:

Base abstracts a proton from dimethyl malonate to form a more stable enolate ion.

This enolate ion reacts with mesityl oxide to form a new enolate ion. It abstracts a proton from methanol to form a diester.

Base abstracts a proton from diester compound to form a enolate ion. This ion can be cyclizes to form unstable tetrahedron intermediate of six-membered ring. It can be dissociated into form cyclic ester. This compound undergoes hydrolysis to form carboxylic acid derivative.

This carboxylic acid derivative undergoes decarboxylation through transition state to form dimedone.