The first really interesting reaction seen by Organic Chemistry students is the Diels-Alder [4+2] cycloaddition, an example of a concerted process where all the bond making and breaking take place concomitantly. The SN2 is also a concerted process, however the Diels-Alder reaction is a gem because of the high density of relative stereochemistry that can be established in a single synthetic step. The most interesting and unique of all organic reactions learned fall within the narrow category of sigmatropic rearrangements, intramolecular pericyclic processes wherein one σ-bond is exchanged for another σ-bond. Example reactions are the Cope and Claisen rearrangements.
The Diels-Alder reaction is a very powerful and selective means for introducing densely packed relative stereochemistry in one synthetic step. The term “relative stereochemistry” is used reiteratively to clearly indicate the typical reaction is not stereoselectively producing one enantiomer over another. Such an outcome would require the transition state taking place in an asymmetric environment. Part One revealed how the reaction is useful in the synthesis of simple six-membered rings, however the synthetic utility spans beyond this.
Few reactions have the capacity to introduce densely packed relative stereochemistry to a molecule in one single step as does the Diels-Alder reaction, a concerted [4+2] cycloaddition. This highly synthetically useful reaction permits facile entry into complex fused ring systems endemic to some natural products exhibiting medicinal properties. This article focuses on the rudimentary elements of the Diels-Alder reaction. Students interested in a more detailed understanding of the dynamics of the reaction are encouraged to engage in the suggested readings.
Remembering back to my first year of graduate school, I recall the first seminar I ever presented in front of an audience of people who knew a lot more than I did. We called the seminars “chalk talks.” Unlike PowerPoint presentations, all the material, including the strategy for presentation, had to be completely memorized. When ready to start, one had only a piece of chalk, an empty blackboard and 50 minutes of time.