Chapter 13, Problem 44P

When furan and maleimide undergo a Diels–Alder reaction at 25°C, the major product is the endo adduct G. When the reaction is carried out at 90°C, however, the major product is the exo isomer H. The endo adduct isomerizes to the exo adduct when it is heated to 90°C. Propose an explanation that will account for these results.