This is actually a reverse , also known as a decomposition reaction in general

This is actually a reverse , also known as a decomposition reaction in general. We know that from how one reactant generates two.

The mechanism for this is fairly intuitive if one realizes that once 1,3-butadiene is to be formed in the mechanism, it will imply breaking the two ##"C"-"S"## bonds in 3-sulfolene to generate 1,3-butadiene.

If you work through drawing this out, you would see that breaking these bonds will generate ##"SO"_2##: sulfur dioxide.

This works because (that is seen in the upper-right curved arrow).

CHALLENGE: Can you draw the forward reaction? Hint: Sulfur is the source of the donated electrons, instead of a ##pi## bond on a more typical dienophile.