Microwave-Promoted Hetero-Diels-Alder Reactions of 2(1H)-Pyrazinones in Ionic Liquid Doped Solvents and on Solid Support

Nadya Kaval, Erik Van der Eycken and C. Oliver Kappe, University of Leuven and Karl-Franzens-University Graz

Introduction



The readily available and broadly functionalized 2-azadiene system of the 2(1H)-pyrazinones easily undergo inter- and intramolecular Diels-Alder reactions with a wide range of dienophiles providing convenient routes to specifically substituted heterocycles.

The reactions in this series are known to be rather sluggish, requiring long reaction times, high temperatures and even sometimes high pressures. Since speed is generally recognized as an important factor in high-throughput synthesis, we have explored potential rate enhancements by controlled single-mode microwave irradiation (Emrys Synthesizer) in the presence of ionic liquids.

The results in solution phase were adapted to polystyrene supports using various acid labile linkers. This offers an efficient methodology for the separation of the resulting pyridines and pyridinones applying the concept of “traceless-linking”.

All steps in the solid-phase sequence were carried out under microwave irradiation. A detailed comparison between the microwave-assisted protocol and the conventional thermal method has been made for each reaction step.

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