ABSTRACT
This chapter discusses reactions that are perceived to proceed in a concerted manner. It describes the most useful types of the pericyclic reaction, Diels-Alder reactions have found widespread use in organic synthesis because of their ability to control relative, and often absolute, stereochemistry while forming two carbon-carbon bonds. The Diels-Alder reaction is stereoselective; the relative stereochemistry of the dienophile is maintained in the product through syn addition. Frontier molecular orbital theory, in addition to the prediction of endo-exo selectivity, can also be used to explain the role of the Lewis acid as a catalyst in the Diels-Alder reaction. The use of electrogenerated acids has also been advocated for labile dienophiles at low temperatures. Clay catalysts suspended in organic solvents have provided comparable stereoselectivities but faster rates than the analogous reaction in water. High pressure is often used in the Diels-Alder reaction to effect reluctant cycloadditions and the success has been attributed to the large negative activation volume of these reactions.
