Diels-Alder cycloaddition of vinylpyrazoles. Synergy between microwave irradiation and solvent-free conditions

When subjected to microwave irradiation and solvent-free conditions vinylpyrazoles undergo Diels-Alder cycloadditions within 6-30 min to give acceptable yields of easily purified products. This methodology overcomes the most important disadvantages of the classical conditions and it permits the reaction to be extended to low reactive dienophiles, such as ethyl phenylpropiolate, not described by classical heating. The regiochemistry of the later reaction has been inferred by NOE experiments and molecular orbital calculations. Copyright (C) 1996 Elsevier Science Ltd