An organic reaction between a substituted alkene and a conjugated diene to form a substituted cyclohexene derivative is said to be Diels-Alder reaction (specifically, [4+2 cycloaddition). The formation of new [tex]\sigma[/tex]-bonds is the driving force of this reaction, which is energetically more stable than the [tex]\pi[/tex]-bonds.
The reaction that takes place due to the heating of dimethyl maleate in presence of 3-sulfolene is as shown in the image.
On heating 3-sulfolene the compound breaks down to give sulfur dioxide gas and butadiene. This diene then reacts with dimethyl maleate which acts as a dienophiles and then these two diene and dienophiles will undergo a Diels-Alder reaction to form a new 6-membered cyclohexene ring.
Thus, the product formed due to the heating of dimethyl maleate in presence of 3-sulfolene is dimethyl cyclohex-4-ene-1,2-dicarboxylate.
