Why is endo preferred

Here we are adding furan, the diene, to maleic anhydride, the dienophile. 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.

This approach leads to the zig-zag exo 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 curled-up endo product. In fact, as the diagram shows, the endo product is usually the favored one. A number of researchers attribute the prefernece to a "secondary molecular orbital interaction" between the diene and the dienophile, whereas others describe the interaction as a London dispersion interaction, in which the weak intermolecular attractions stabilise the transition state in one geometry.

The endo and exo products are really two different diastereomers. If you think about it, you can see that when two rings fuse together to make a third, four new stereocenters can be created. Since each chiral centre could have two possible configurations, there are sixteen possible stereoisomers that could result in the reaction shown above. That's a lot of structures. Just eight of them are shown below. Note that they occur in pairs of enantiomers. However, most of those diastereomers don't really occur.

Draw the other stereisomers of the product formed from the reaction between furan and maleic anhydride. Chiral catalysts have also been shown to control the enantioselectivity of a Diels-Alder reaction. The achiral oxazolidinone below relies on a chiral BOX ligand and coordination of a copper catalyst J. Iminium formation blocks one face of the dienophile so that approach of the diene results in endo selectivity and good to high enantioselectivity J.

A cobalt-mediated alkyne cyclization undergoes a ring closing and re-opening to set up a Type I Diels-Alder reaction J. Overman published a stereospecific synthesis of pumiliotoxin in as a racemic mixture. The following year, his group published a second iteration of the same core, but with a much shorter overall synthesis Te trahedron Lett. The forward synthesis showcases a Curtius rearrangement to build the requisite carbamate protected diene.

Susequent addition of crotonaldehyde and heat results in the cyclohexene core, which can undergo a Horner-Wadsworth-Emmons reaction and reduction to give an advanced intermediate.

Steric Effects. Electronic Effects Regioselectivity Electron-rich dienes paired with electron-poor dienophiles result in high selectivity for the "ortho products.

For practice, convince yourself the the following reaction yields the product shown. Rate of reactivity: diene. Rate of reactivity: dienophile. Vote count: 1. No votes so far! Be the first to rate this page. Tell us how we can improve this page in your own language if you prefer?

Close Animation controls: Display controls:. Click the structures and reaction arrows in sequence to view the 3D models and animations respectively The endo product must have a lower energy transition state not final structure! How useful was this page? Click on a star to rate it! As you found this page useful Follow us on social media!