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Paper | Regular issue | Vol 81, No. 7, 2010, pp.1651-1659
Published online, 28th April, 2010
DOI: 10.3987/COM-10-11950
■ Microwave-Assisted Selective Synthesis of 2H-Indazoles via Double Sonogashira Coupling of 3,4-Diiodopyrazoles and Bergman–Masamune Cycloaromatization

Hayato Ichikawa,* Haruhiko Ohfune, and Yoshihide Usami

*Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1, Izumi-cho, Narashino, Chiba 275-8575, Japan

Abstract

The microwave-assisted double Sonogashira coupling of 3,4-diiodo-1-trityl and 1-phenylpyrazole with terminal acetylene took only three minutes. Dialkynylpyrazoles, the coupling products, were heated at 240 °C in the presence of 1,4-cyclohexadiene to obtain 2H-2-trityl and 2-phenylindazoles, respectively. This synthetic route to 2H-indazole, which was achieved via cyclization to form the 6-membered ring of dialkynylpyrazole, is a novel procedure.