Often, the margin is substantial one might see 90% endo product or greater in some cases, although the ratio is sometimes much lower. In fact, as the diagram shows, the endo product is usually the favored one. This approach leads to the curled-up endo product. In the other case, the two molecules can be directly on top of each other one molecule appears to be folded underneath the other. This approach leads to the zig-zag exo product. The two reactants can approach each other such that one appears to be trailing behind the other, and in this case they appear to be facing the same direction, as far as the orientation of the oxygen atoms goes. Here we are adding furan, the diene, to maleic anhydride, the dienophile. Let's look at these two modes of addition with real molecules. The endo product that results has a kind of C-shape. Two cyclic molecules approach each other so that there is maximum overlap between their faces. The exo product that results has a sort of Z-shape.Īn endo addition looks more like this, schematically. The two molecules approach with minimal overal between their faces they combine edge-on.
These two outcomes are called "exo" and "endo" addition.Īn exo addition looks something like this, schematically. There are different ways the two original rings can combine, leading to different stereochemical outcomes. When two cyclic structures combine in a Diels Alder reaction, a third ring is formed in between the original ones. Often, there are already rings in the molecules undergoing reaction, and a new one is being added. It allows the construction of six-membered rings, which are very common in biological small molecules which are frequently synthetic targets. The Diels Alder reaction is probably the most common cycloaddition.
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