Question

The reaction of (Z)-butenedioic acid (fumaric acid) with butadiene sulfone gives only one product. Draw the product of this reaction.

Answer 1

Butene sulfone on heating undergo Cheltropic elimination reaction and a 1,3-Butadiene results.

Fumaric acid a reactive dienophile react with this in situ generated diene (1, 3-Butadiene). This is the thermally induced [4+2] cycloaddition reaction well known as Diels-Alder reaction and it’s a highly stereospecific and stereoselective reaction giving a single product. The reaction proceeds through six membered cyclic transition state and the endo product results because of stabilization of this T.S. by secondary orbital overlap. The Endo product though thermodynamically less stable and kinetically stable the extraordinary stabilization of T.S. yields endo product not just major but exclusively.