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.

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G 25 °C NH Endo adduct N-H - 90 Furan Maleimide 90 °C NH H Exo adduct