Some cycloadditions are thermally allowed, especially Diels-Alder reactions, which are often key steps in synthetic organic chemistry. As a result, both ultrasound and microwave irradiations, particularly the latter, by virtue of their thermal effects, have been extensively applied in the search for further improvements. Our research group has recently embarked on an exploration of synthetically useful cycloadditions, where a combination of either microwaves or ultrasound in neoteric reaction media (e.g. ionic liquids) has proven to be of substantial benefit.

Thus, reactions of cyclopentadiene with electronically poor dienophiles conducted in a conventional ionic liquid, [HMI][BF4], are greatly accelerated (in some cases from 1 h to 5 min). Moreover, the ionic solvent could be recycled for five times without loss of efficiency. These cycloadditions have also been improved with ultrasound in terms of higher yields and shorter reaction times when compared to the corresponding silent reactions. Although the origin of this activation remains uncertain, Lewis acid catalysis caused by thermal decomposition of the ionic liquid could be ruled out.

Finally, reactions of enaminoesters with chlorosulfonyl isocyanate were significantly accelerated under microwaves. The process is relevant as it leads to a straightforward synthesis of isoorotic acid derivatives. Starting from carbohydrate-based enaminoesters, a series of isoorotate nucleosides could easily be prepared. The process is believed to occur by a sequential process via polar intermediates. The role of microwaves is discussed.

The authors would like to express their gratitude to the Ministry of Education and Science (Grants BQU2003-05946 and BQU2005-07676) and the EU Program Interreg-III (Project Labtrans/SP4.E44) for financial support.

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