The sonochemical Diels-Alder reaction

At first sight, this prediction does not fit with experiment, making a reexamination of the published material necessary, even if attempts were probably made and, being negative, remained tmpublished. 

In the first report on an attempted sonochemical Diels-Alder addition, Grieco et al. indicate that the reaction shown in Eq. 7 does not suffer any change from sonication in organic solvents, or in water when the sodium salt of the diene is 

In a recently published study, the cycloaddition of cyclopentadiene and methyl vinyl ketone was examined in various solvents (Eq. 8). A sonochemical effect was observed in halomethane solution, with a yield enhancement of ca. 300%, and a change in the endolexo ratio from 11 to 19. 

However, it seems that these effects are due to a solvent sonolysis, producing hydrogen halides (p. 60), the actual catalyst of the cycloaddition. The conclusion of the authors was the absence of a direct sonochemical effect. This question will be discussed below. 

An important work was reported by Snyder et al o-Quinonic abietanoids from a Chinese medicinal plant can be prepared with a Diels-Alder reaction as the key step (Fig. 10). 

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