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Copper-Catalyzed Domino Annulation Approaches to the Synthesis of Benzoxazoles under Microwave-Accelerated and Conventional Thermal Conditions
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Current address: Department of Chemistry and Biology, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3.
Abstract
Two domino annulation approaches for benzoxazole synthesis have been developed. In the first approach, copper-catalyzed intermolecular cross-coupling of 1,2-dihaloarenes with primary amides initially forms the Ar−N bond of the benzoxazole ring, followed by copper-catalyzed intramolecular cyclization to form the Ar−O bond. Benzoxazoles were formed in good yields for the reaction of 1,2-dibromobenzene, but the reaction was not regioselective for the reaction of 3,4-dibromotoluene. Furthermore, the method is limited by the availability of 1,2-dihaloarenes. As a result of these limitations, an alternative more versatile one-pot domino annulation strategy was developed involving reaction of 2-bromoanilines with acyl chlorides in the presence of Cs2CO3, catalytic CuI, and the non-acylatable ligand 1,10-phenanthroline. Under these conditions initial acylation of the aniline is followed by copper-catalyzed intramolecular cyclization of the resultant 2-haloanilide to form the Ar−O bond of the benzoxazole ring. Optimized conditions using microwave irradiation achieved much shorter reaction times than conventional heating (i.e., 210 °C for 15 min versus 95 °C for 24 h) and were applied to the synthesis of a small library of benzoxazoles. These copper-catalyzed approaches complement existing strategies for benzoxazole synthesis, which typically utilize 2-aminopheonls as precursors.
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History
- Published In Issue May 02, 2008
- Article ASAPApril 01, 2008
- Received: October 2, 2007
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