Why is the exo product formed in the Diels–Alder reaction between furan and maleic anhydride?

Question

For most Diels–Alder reactions, the major product is *endo* because there are favourable interactions between the newly forming pi bond and the electron withdrawing groups of the dienophile. Why is the reaction between furan and maleic anhydride an exception? From what I understand, the carbonyl groups of the anhydride are electron withdrawing.

Answer 1

In general, the endo product is the kinetic product (forms fasters) and the exo product is the thermodynamic product (it is more stable). In principle, any diels-alder reaction can be made to favor an exo product by raising the temperature high enough that the diels alder is reversible. Reversible conditions favor the thermodynamic product. In practice, it seems many diels alder reactions give the kinetic product regardless of temperature, but I'm not sure if this is 100% true.

Why would a reaction with furan be especially reversible? Perhaps because Furan is stabilized by aromaticity, which makes the the diels-alder adduct relatively unstable relative to the aromatic reactant. Also, in general a 2.2.1 bicycle has some nasty angle strain that a typical cyclohexene product would not have. Those are some possible reasons.