Pergamon Tetrahedron Letters 41 (2000) 4101–4105 TETRAHEDRON LETTERS Sonochemical cycloadditions of o-quinones. The search for a cation radical pathway Martín Avalos, a Reyes Babiano, a José L. Bravo, a Noemí Cabello, a Pedro Cintas, a,* Michael B. Hursthouse, b José L. Jiménez, a Mark E. Light b and Juan C. Palacios a a Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Extremadura, E-06071 Badajoz, Spain b Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK Received 17 March 2000; accepted 5 April 2000 Abstract Diels–Alder cycloadditions of an o-quinone monoketal with different furans, under argon, are considerably accelerated by ultrasonic irradiation. Moreover, these sonochemical reactions are regiospecific and proceed with a high diastereoselectivity. The results can be ascribed to the chemical role of ultrasounds which may favor a single electron transfer mechanism with respect to ionic or nonpolar pathways. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: cation radicals; Diels–Alder reactions; furans; o-quinones. Activation of Diels–Alder reactions, especially by Lewis acids 1 or nonconventional irradiation techniques, 2 not only enhances the versatility of this powerful methodology, but also enables milder and more selective cycloadditions. Assuming that Diels–Alder reactions are sensitive to thermal and pressure effects, they would be susceptible to ultrasonic activation since the cavitational collapse is capable of creating extremely high local temperatures (∼4500 K), 3 though accompanied by modest pressures (a few kbars), under almost adiabatic conditions (the duration of the hot spots is less than 100 nanoseconds with cooling rates >10 10 K/s). 3,4 The conditions provided by cavitation would be comparable to those of flash thermolysis. Unfortunately, little success has so far been encountered in ultrasound-assisted cycloadditions. This fact can be interpreted by the hypothesis of true sonochemistry, 5 pioneered by Luche, stating that chemical effects of ultrasounds will occur only if an elemental process is the sonication sensitive step, or when the high energy species released after the collapse participate as reaction intermediates. Accordingly, ultrasounds may favor a radical process by virtue of their ability in promoting single electron transfers. However, nonvolatile substrates have no probability to penetrate into the bubble to undergo these extreme conditions, although they may experience the shock waves in the bulk medium. On the other hand, the formation of cation or anion radicals does require appropriate * Corresponding author. Tel/fax: (34)-924-271-149; e-mail: pecintas@unex.es (P. Cintas) 0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(00)00578-5 tetl 16857