Ultrasound-Assisted Diels-Alder Reaction

Modem technology has influenced these reactions. Ultrasound assisted versions of these reactions has been reported. Ultrasound irradiation facilitated the Diels-Alder reaction of dimethylhydrazone 94 with 95. The resultant pyridine 96 are afforded in shorter reaction times and increased yields. 

The use of ultrasonic (US) radiation (typical range 20 to 850 kHz) to accelerate Diels-Alder reactions is undergoing continuous expansion. There is a parallelism between the ultrasonic and high pressure-assisted reactions. Ultrasonic radiations induce cavitation, that is, the formation and the collapse of microbubbles inside the liquid phase which is accompanied by the local generation of high temperature and high pressure [29]. Snyder and coworkers [30] published the first ultrasound-assisted Diels-Alder reactions that involved the cycloadditions of o-quinone 37 with appropriate dienes 38 to synthesize abietanoid diterpenes A-C (Scheme 4.7) isolated from the traditional Chinese medicine, Dan Shen, prepared from the roots of Salvia miltiorrhiza Bunge. 

Diels-Alder Reaction Facilitated by Physical and Chemical Methods 155 Table 4.8 o-Quinone ultrasound-assisted Diels-Alder reactions... 

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