Why in the reaction of maleic acid anhydride and furan, the thermodynamic, lower energy product is the exo product? Isn't that opposite to the endo rule? There is some steric hindrance between the furane moiety and the C-O-C bridge.
The endo rule is a kinetic one, generally put down to "secondary orbital interactions" stabilizing the endo transition state. However, this has no bearing on the thermodynamics of the process, since this interaction is no longer present in the product. So, under thermodynamic conditions, the endo-rule can be overruled. In this case, the C=C bridge is more sterically demanding than the ether bridge, and so the thermodynamic product is the one where substituents point away from the C=C bridge, so given enough heat this product will be predominant.