Microwave Chemistry Highlights
Synthesis of Isonitriles
A recent publication by Andrea Porcheddu, Giampaolo Giacomelli and Marghareta Salaris from the University of Sassari, Italy (J. Org. Chem. 2005, 70, 2361. ) describes a simple and efficient method for the synthesis of both aliphatic and aromatic isonitriles in high yields. By employing 1.3-3.0 equivalents of a cheap dehydration agent such as 2,4,6-trichloro[1,3,5]triazine (cyanuric chloride, TCT) aliphatic and aromatic formamides have been transformed to their corresponding isonitriles in high yields at 50-100 °C within 3-10 minutes of controlled microwave irradiation.
Fluorous Synthesis of Biaryl-Substituted Proline Analogs
Wei Zhang and Christine Hiu-Tung Chen from Fluorous Technologies Inc. (Tetrahedron Lett. 2005, 46, 1807. ) have reported on a solution-phase synthesis of biaryl substituted proline analogs by a three component 1,3-dipolar cycloaddition and a Suzuki coupling. Use of controlled microwave irradiation in both steps and perfluoroalkylsulfonyl protected hydroxybenzaldehydes has enhanced both the reaction and separation processes for the synthesis of bicyclic proline analogs.
Copper-Free Palladium-Catalyzed Sonogashira-Type Coupling
The palladium catalyzed synthesis of diaryl acetylenes has been demonstrated by Ulrich S. Sorenson and Esteban Pombo-Villar from Novartis Pharma AG (Tetrahedron 2005, 61, 2697. ). A direct coupling of activated aryl- and heteroaryl bromides and iodides with 1-aryl-2-trimethylsilylacetylenes has been developed for the synthesis of diarylacetylenes, avoiding the use of copper (I) iodide as a cocatalyst. Microwave dielectric heating has shown improvement in reaction yields over the conventional oil bath heating.
Synthesis of 1,2,4-Oxadiazoles using Polymer-Supported Reagents
An expeditious synthesis of 1,2,4-oxadiazoles has been demonstrated by Yin Wang and co-workers at Abbott Labs (Org. Lett. 2005, 7, 925, ). The synthesis of 1,2,4-oxadiazoles from a variety of readily available carboxylic acids and amidoximes using solid-phase reagents and catalysts has been devised in two approaches with high yields and simplification of purification process. Microwave technology has been utilized for a rapid optimization of reaction conditions with reduction in reaction times from hours to minutes.