Diels-Alder reactions in the presence of a minimal amount of water

Diels-Alder reactions of neat reactive dienes and dienophiles are frequently vigorous and have a limited preparative value. It is possible to control the temperature of multi-gram scale Diels-Alder reactions by adding a minimal amount of water to neat reactants. In addition, the presence of water resulted in an increased reaction rate and formation of a higher purity product. The use of minimal amounts of other solvents, with lower heat capacities, did not give good results. Furthermore, some dienophiles, e.g. maleic anhydride, reacted poorly in a dilute water suspension or solution, but worked well in the presence of a minimal amount of water. This is an environmentally friendly, practical and efficient approach to preparation of Diels-Alder adducts on a multi-gram scale.