Question

(a) In the light of your answer to problem to propose mechanism for the reactions below.

Short answer

A stable product will be formed via the formation of a non conjugated diene. This non-conjugated diene by loosing from it, will give the product shown above.

Step-by-step solution

Diels-Alder reaction

Diels-Alder reaction was discovered by the German chemists Otto Diels and Kurt-Alder in 1928 for which they got Nobel prize 1950. 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, the diene is

while the dienophile is

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.