| Overview: The Diels-Alder reaction combines a diene (a molecule with two alternating double bonds) and a dienophile (an alkene) to make rings and bicyclic compounds. The three double bonds in the two starting materials are converted into two new single bonds and one new double bond. Since this reaction forms two new carbon-carbon bonds in a single step, it is a very useful and powerful reaction (one which earned Otto Diels and Kurt Alder a Nobel prize in chemistry for discovering it).
Typically, the Diels-Alder reaction works best when either the diene is substituted with electron donating groups (like -OR, -NR2, etc) or when the dienophile is substituted with electron-withdrawing groups (like -NO2, -CN, -COR, etc). Conformational requirements of the diene One quirk of the Diels-Alder reaction is that the diene is required to be in the s-cis conformation in order for the Diels-Alder reaction to work. The s-cis conformation has both of the double bonds pointing on the same side of the carbon-carbon single bond that connects them. In solution, the carbon-carbon single bond in the diene that connects the two alkenes is constantly rotating, so at equilibrium there is usually some mixture of dienes in the s-trans conformation and some in the s-cis conformation. The ones that are at that moment in the s-trans conformation do not react, while the ones in the s-cis conformation can go on to react.  Because of the Diels-Alder's requirement for having the diene in a s-cis conformation, dienes in rings react particularly rapidly because they are "locked" in the s-cis conformation. Unlike dienes in open chains in which there is usually some proportion of the diene in the unreactive s-trans conformation, dienes in rings are held in the reactive conformation at all times by the constraints of the ring, making them react faster. Stereochemistry of Diels-Alder reaction What about the stereochemistry of the Diels-Alder reaction? If your dienophile is disubstituted (substituted twice), there is the possibility for stereochemistry in the product. In the Diels-Alder reaction, you end up with the stereochemistry that you started with. In other words, if the substituents started cis (on the same side) on the dienophile, they end up cis in the product. If they started trans (opposite sides) on the dienophile, they end up trans in the product. Formation of bicyclo products. When the diene is in a ring, the product of the Diels-Alder reaction is a bicyclo ring system (which can be somewhat intimidating to draw at first). A bicyclo ring system is a compound in which two rings share more than two carbons. There are two main bicyclo ring systems that you typically have to deal with, and these are the ones that come from the diene being in a five-membered ring system and the diene being in a six-membered ring system. (The nomenclature of bicyclic alkanes can be found elsewhere). When you make bicyclic products (that is, when the diene is in a ring), and you have a dienophile that is substituted, there are two possible products that you can form from the Diels-Alder reaction--the endo product, in which the substituent points down from the top of the bicyclic molecule, and the exo product, where the substituent points towards the top of the bicyclic molecule. In general, you form the endo product preferentially over the exo product in the Diels-Alder reaction. Example of the Diels-Alder reaction In this example, since the diene is in a six-membered ring, you make a bicyclo product. Since the dienophile is cis disubstituted, you get the endo stereochemistry with the two cyano (CN) substituents pointing away from the top of the bicyclo compound. | 
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