Question

Question: In the light of your answer to problem to propose mechanism for the reactions below.

Short answer

This product will be formed via the formation of a non conjugated diene.

Step-by-step solution

Diels-Alder reaction

Diels-Alder reaction was discovered by the German chemists Otto Diels and Kurt-Alder in 1928. Diels-Alder reaction is an important organic reaction which includes a substituted alkene in the form of a dienophile and a conjugated diene. It proceeds via concerted mechanism. Diels alder reaction can be used to generate 6 membered rings since there is a simultaneous construction of two new Carbon-carbon bonds.

Diene and dienophile

Diene is an organic compound which comprises of two conjugated pi bonds (4 pi bonds) while dienophile is a diene loving compound containing 2 pi electrons. When the interaction between them, occurs a transition state without any additional energy barrier, is obtained.

Here in the question given, pyrone is the diene

while the alkyne is dienophile.

Mechanism

First of all, the Diels-Alder reaction of given diene and dienophile generates a product in which double bonds are not conjugated.

Diels-Alder reaction

so a more stable product can be formed by the loss of on heating.