[vivekfan]

How can you twist a diene to get it into s-cis conformation (particularly in Diels-Alder Reaction)...I need some help visualizing. And also, could someone explain the difference between endo addition and exo addition because in my book it says endo addition means that the substituents point down relative to the carbon bridge and the endo addition means that the two substituents (on the dienophile) point toward the carbon bridge...but I am not sure what this means.