Question

The major product formed in the following reaction is 

• A. A
• B. B
• C. C
• D. D

Hint:

In Diels–Alder reactions, electron-rich dienes react with electron-poor dienophiles.
The reaction is stereospecific and favors the {endo product} due to secondary orbital interactions.

Answer 1

The Correct Option is A

Step 1: Identify the type of reaction. 
The reaction involves a conjugated diene (1,3-butadiene) and a conjugated dienophile (benzoquinone derivative). 
Such a reaction under heating (100°C, toluene, 96 h) represents a Diels–Alder cycloaddition reaction. 
Step 2: Role of the reactants. 
- The benzoquinone acts as the dienophile due to the electron-withdrawing carbonyl groups. 
- The butadiene acts as the diene. 
The methoxy group on the quinone ring activates the double bond adjacent to it, guiding regioselectivity. 
Step 3: Mechanism and regioselectivity. 
A [4+2] cycloaddition occurs between the diene and the $\alpha,\beta$-unsaturated carbonyl system of the quinone. 
This forms a new six-membered ring fused to the quinone ring, yielding a bicyclic adduct with endo selectivity favored. 
\[ \text{Diene + Quinone} \rightarrow \text{endo-bicyclic adduct (A)} \] Step 4: Product identification. 
The product (A) corresponds to an endo-Diels–Alder adduct, where the new ring is fused to the quinone nucleus and retains both carbonyl groups. 
Step 5: Conclusion. 
The major product is endo adduct (A).