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

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

“CuH in a Bottle” for Asymmetric Hydrosilylations

Bruce H. Lipshutz and Bryan A. Frieman from the University of California, Santa Barbara, have made investigations on [{(R)-(-)-DTBM-segphos}CuH] as a hydrosilylation reagent regarding stability and preparation (Angew. Chem. Int. Ed. 2005, 44, 6345. ). This reagent proved to be very stable over several weeks at room temperature (no decrease in ee´s) if prepared by using Cu(OAc)2∙H2O as copper source, and stored as stock solution in a bottle. Excellent ee´s and good conversions were obtained in several asymmetric hydrosilylations at room temperature, but also microwave heating could be applied for the reduction of enone 1. By using a substrate/catalyst ratio (S/C) of 1000:1, the reaction time could be reduced from several hours to only 10 minutes.

Glycosylations

Knud J. Jensen and co-workers from the Royal Veterinary and Agricultural University, Denmark, have described the microwave promoted synthesis of oligosaccharides (Org. Biomol. Chem. 2005, 3, 3966. ). Without the use of a strong Lewis acid, O-glycosylations could be accelerated from up to 31 hours to 5-40 minutes by applying microwave heating. Glycosylation reactions have been performed with trichloroacetimidate (2) or DISAL (3,5-dinitrosalicylate, 1) donors, respectively, in the latter case with α-selectivity for sterically hindered alcohols.

Synthesis of 3-Aminoimidazo[1,2-a]Pyridines

A novel one-step three-component reaction for the synthesis of 3-aminoimidazo[1,2-a]pyridines has been demonstrated by the group of Christopher Hulme from Eli Lilly and Company (Tetrahedron Lett. 2005, 46, 8355. ). In this multicomponent reaction, an aminopyridine 1, aldehyde 2 and TMS-CN (3) are reacted under microwave heating to give the desired products 4. By using TMS-CN as non-classical isonitrile equivalent, no further deprotection steps were necessary, making this method applicable for library synthesis.

Cross-Metathesis Reactions

In a recent publication Frédéric C. Bargiggia and William V. Murray from Johnson & Johnson Pharmaceuticals have reported on cross-coupling Metathesis between deactivated olefins (J. Org. Chem. 2005, 70, 9636. ). By applying microwave irradiation, reaction rates could be enhanced compared to classical heating, showing superior yields for the Hoveyda-Grubbs catalyst 1 than for 2nd generation Grubbs´ catalyst. No differences in yields or selectivities in homo- or hetero couplings, respectively have been observed by comparison studies of microwave- and oil-bath heating.

D. Dallinger, Org. Chem. Highlights 2005, November 15.
URL: https://www.organic-chemistry.org/Highlights/2005/15November.shtm