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

Thursday, March 15, 2007
Doris Dallinger, C. Oliver Kappe
Karl-Franzens-University Graz

Synthesis of Peptidomimetics via Cross-Metathesis

Stephen Caddick and co-workers from University College London were successful in the generation of peptidomimetics via stereoselective cross-metathesis (Org. Biomol. Chem. 2007, 5, ASAP. ). By applying Grubbs’ second generation catalyst 1 in the reaction of two different single amino acids, products which still have the length of two amino acids but with a modified main chain (lack of the amide bond) are obtained. This structural feature is of interest with regard to protein folding and protein-protein interactions. Notably, for those products which needed 60 min reaction time, the reaction mixture was degassed after 30 min to remove any dissolved ethene and subsequently subjected to irradiation for additional 30 min.

Copper in Charcoal Catalyzed Diaryl Ether Synthesis

The group of Bruce Lipshutz from University of California, Santa Barbara, has reported on the cross-coupling of (het)aryl bromides with phenols which was catalyzed by copper impregnated into charcoal (Cu/C) (Org. Lett. 2007, 9, 1098. ). If the aryl bromide substrate was used as the limiting reagent, it was discovered that active copper was released from the charcoal into solution. By simply adjusting the stoichiometry to phenol being the limiting reagent, this problem could be overcome with the advantage that less Cs2CO3 base and thus less solvent could be used. However, longer reaction times were necessary in these cases. When aryl chlorides were employed, the reaction time under microwave heating turned out to be too long (8 h) to be of practical use.

One-Pot Synthesis of Polysubstituted and Annulated Pyridines

Chao-Guo Yan and co-workers from Yangzhou University have developed a modified Kröhnke protocol for the synthesis of pyridines (Org. Biomol. Chem. 2007, 5, ASAP. ). This typically two-step procedure could be performed in one pot by four-component cyclocondensation of a pyridinium salt 1, aromatic aldehydes 2, acetophenones 3 and ammonium acetate and leads to polysubstituted pyridines 4. When acetophenone is replaced by cylic ketones 5, annulated pyridines 6 are obtained in very good yields. By applying microwave heating, less by-products are formed and only a simple filtration/recrystallization work-up is required.

Pd-Catalyzed Asymmetric Heck-Reactions

A library of sugar based phosphate-oxazoline ligands for use in asymmetric Pd-catalyzed Heck couplings was synthesized by the groups of Oscar Pàmies and Montserrat Diéguez from Universitat Rovira i Virgili, Tarragona (Chem. Eur. J. 2007, 13, ASAP. ). Performing the Heck reactions of 2,3-dihydrofuran (1, X = O), cyclopentene (1, X = CH) and 4,7-dihydro-1,3-dioxepin 2 with phenyl- or cyclohexenyl triflate employing either ligand 1 or ligand 2 in combination with a Pd-catalyst, provided the corresponding coupled products in excellent conversions, regio- (up to 98%) and enantioselectivities. Compared to conventional heating at 50 °C (where excellent conversions, regio- and enantioselectivities were also obtained) applying microwave heating at 70 °C accelerated the reactions from up to 2.5 days to only 10-45 minutes.

D. Dallinger, C. O. Kappe Org. Chem. Highlights 2007, March 15.