JCE0597 p582 endo- and exo-Stereochemistry in the Diels-Alder Reaction: Kinetic versus Thermodynamic Control

In these experiments, which were used in the problemsolving mode (1), the stereoselectivity of the Diels–Alder cycloaddition of N-phenylmaleimide to furan is deduced by the characteristic splitting patterns in the proton-NMR spectra. The relationship of coupling constants to dihedral angle, as described by the Karplus equation, is illustrated. The data can also be used to demonstrate the concept of kinetic versus thermodynamic control. Two recent papers in this Journal reported laboratory experiments illustrating the stereochemistry of the Diels– Alder reaction (2, 3). Pickering (2) described the reaction of maleic anhydride with cyclohexadiene, α-phellandrene, and furan. Cyclohexadiene and α-phellandrene gave the endo stereoisomer, whereas furan gave the exo stereoisomer. These experiments were used in the problem-solving mode and students were asked to decide, by comparing their data with that in the literature, which stereoisomer was formed. In another experiment, Harrison (3) used NMR to study the Diels–Alder reaction between norbornadiene and phencyclone (reaction 1). The stereochemistry of this reaction was established from the fact that the methylene protons lie in the shielding cone of the aromatic system.