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

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

Ethylene-Alkyne Cross-Metathesis

Maurizio Botta and his group from the University of Siena, Italy have employed microwave mediated ethylene-alkyne cross-metathesis using Grubbs 2 catalyst for the synthesis of enantioenriched 2-substituted butadiens (Tetrahedron: Asymmetry 2005, 16, 2893. ). Importantly, microwave heating was essential for the fast reaction of enantiomerically enriched alkynes with ethylene (closed vessel saturated with ethylene) under retention of configuration at the propargylic/allylic position. At atmospheric pressure, no reaction was observed.

Synthesis of Aryl Azides under Mild Conditions

In a recent publication, Xifu Liang and co-workers from LEO Pharma, Denmark, have reported on optimization studies for the synthesis of aryl azides from aryl halides using copper(I) catalysis (Synlett 2005, 2209. ). In a first screening several ligands have been tested, resulting in the selection of 1,2-diamino ligands a and b. In further optimizations, different solvents and catalyst/ligand ratios have been studied, reaching full conversion by using EtOH/H2O as solvent mixture, 10 mol % of CuI, 15 mol % of ligand b and irradiation at 100 C for 30 minutes.

Solid-Phase Synthesis of Natural and Non-Natural AHLs

Helen E. Blackwell and co-workers from the University of Wisconsin-Madison have described the synthesis of N-acyl L-homoserine lactones (AHLs) via a microwave-assisted solid-phase route (J. Am. Chem. Soc. 2005, 127, 12762. ). Both natural and non-natural AHLs could be obtained in high purities and short reaction times. Among the two libraries, a set of non-native AHLs has been found to have high inhibitory affinity for bacterial quorum sensing.

Synthesis of 4-, 5-, 6- and 7-Azaindoles

Nicolas Lachance and co-workers form the Merck Frosst Centre for Therapeutic Research in Quebec (Canada) have shown that imines/enamines 3 can be converted via a microwave-assisted intramolecular Heck reaction to the corresponding azaindoles 4 in good yields (Synthesis 2005, 2571. ). Several synthesis routes have been investigated, including the one-step reaction (route A) of aminopyridines 1 with ketones 2 or ketals respectively as well as the one-pot two-step reaction (route B) leading to azaindoles 4. In this way a small library of 17 examples was synthesized employing chloro-, bromo- and iodoaminopyridines with toleration of sensitive groups like ketones and esters.

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