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Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)

Monday, September 15, 2005
Doris Dallinger
Karl-Franzens-University Graz

Cyclocondensation of Hydrazine Derivatives with Alkyl Dihalides or Ditosylates

Rajender S. Varma and Yuhong Ju from the U.S. Environmental Protection Agency have reported (Tetrahedron Lett. 2005, 46, 6011. ) on the direct syntheses of 4,5-dihydro-pyrazole, pyrazolidine and 1,2-dihydrophthalazine derivatives by double alkylation of hydrazine compounds with alkyl dihalides or tosylates. Using water as solvent, the reaction could be performed under microwave heating at 120 °C for 20 minutes without the need of a phase transfer catalyst, making this protocol more environmentally friendly.

Synthesis of Flavones and Chromones

Utilizing the microwave protocol shown below (Tetrahedron Lett. 2005, 46, 6315. ), George W. Kabalka and Arjun R. Mereddy from the University of Tennessee were able to synthesize a variety of functionalized flavones and chromones in very high yields by the cyclization of 1-(2-hydroxyaryl)-1,3-propanediones.

Ring-Opening of Epoxides

In a recent publication, Miquel A. Pericās and co-workers from Universitat de Barcelona (Org. Lett. 2005, 7, 3829. ) have described the investigation of fluoroepoxide 1 as a new, chiral resolution reagent. By applying microwave heating, the regioselective ring-opening of this reagent induced by α-chiral primary and secondary amines occurred in higher yields and less byproducts were detected, compared to conventional heating. Because of the use of readily available enantiopure epoxide 1, the resulting diastereomeric amine products 2 could be very easily identified by 19F, 1H, 13C NMR and by HPLC.

Metal-Free Intramolecular Alkyne Trimerizations

Steven V. Ley and his group from the University of Cambridge (Org. Biomol. Chem. 2005, 3365. ) have shown that a variety of oligomeric alkynes cyclotrimerized in a [2 + 2 + 2] manner to the corresponding arenes without the need of any metal catalyst if microwave heating is used. Furthermore, a flow process was applied for one example, pumping the sample through a glass coil in the microwave cavity.

D. Dallinger, Org. Chem. Highlights 2005, September 15.