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

Wednesday, March 15, 2006
Doris Dallinger
Karl-Franzens-University Graz

Asymmetric Mannich Reaction

Carsten Bolm and Belén Rodríguez from the University of Aachen investigated thermal effects in the (S)-proline-catalyzed Mannich reaction (J. Org. Chem. 2006, 71, 2888. ). By applying microwave heating with simultaneous cooling, reaction times and, most importantly, catalyst loadings could be reduced. Similar results were also achieved by heating in an oil bath for 23 h at the same temperature (45-50 °C). The corresponding reduced aminoalcohols could be obtained in excellent yields and ee´s.

Aminocarbonylations of Aryl Halides in Water

Mats Larhed and co-workers from Uppsala University have reported on the synthesis of benzamides via aminocarbonylations of aryl chlorides, bromides and iodides using Mo(CO)6 as solid CO source and water as solvent (Organometallics 2006, 25, 1434. ). Intensive investigations have been made toward reaction conditions, especially in terms of reagent ratios. By either using the aryl halide or amine as limiting reagent, accompanied with the proper Pd-catalyst, hydroxycarbonylations could be suppressed as side reactions and even the otherwise unreactive aryl chlorides could be employed under these conditions.

Asymmetric Hydrogenation

An asymmetric hydrogenation step within the synthesis of the C-glycoside analogue of β-D-galactosyl hydroxylysine (2) was disclosed by Jan Kihlberg and co-workers from Umeĺ University (J. Org. Chem. 2006, 71, 1911. ). Using Burk´s catalyst and microwave heating (70 °C for 40 min) for the hydrogenation step, the corresponding product 1 was obtained in excellent yield and selectivity (>99% de) which could not be reached by conducting the reaction at room temperature and prolonged reaction time.

Synthesis of 1,5-Diarylpyrazoles

Paul Humphries and Jennifer Finefield from Pfizer, San Diego, have described the 2-step synthesis of pyrazoles using microwave heating for both steps (Tetrahedron Lett. 2006, 47, 2443. ). In the first step, the enol ketone precursor 1 was prepared from aryl methyl ketones and ethyl trifluoroacetate. For the second step, enole ketones were reacted with 4-methylphenyl hydrazine and silica-supported p-toluenesulfonic acid (Si-TsOH) to give the corresponding pyrazoles 2 in good to excellent yields. Compared to standard p-TsOH, higher yields could be achieved using Si-TsOH and additionally the reaction work-up was simplified.

D. Dallinger, Org. Chem. Highlights 2006, March 15.