The Woodward-Hoffmann rules were first applied to this reaction, and in order for it to occur, one of its reactants must be in s-cis form.
The MacMillan catalyst serves to accelerate an asymmetric version of this reaction, and Seelig showed that this reaction could be catalyzed in vitro by certain ribozymes. The Woodward-Hoffmann rules were first applied to this reaction, and in order for it to occur, one of its reactants must be in s-cis form. Driven by the conversion of pi bonds into more stable sigma bonds, this reaction almost always favors the endo product, and its simplest form is the reaction of ethene with butadiene to yield cyclohexene.  For 10 points, name this doubly-eponymous reaction, a 4+2 cycloaddition between a diene and dienophile.
The MacMillan catalyst serves to accelerate an asymmetric version of this reaction, and Seelig showed that this reaction could be catalyzed in vitro by certain ribozymes.
 Diels-Alder reaction