Copper(I) Oxide‐Mediated Cyclization of o‐Haloaryl N‐Tosylhydrazones: Efficient Synthesis of Indazoles

An efficient synthesis of indazoles from readily accessible E/Z mixtures of o-haloaryl N-to-sylhydrazones has been developed. The thermo-induced isomerization of N-tosylhydrazones is discussed. A series of valuable indazole derivatives are prepared in good yields, and the method has been successfully applied to the synthesis of the bioactive compounds, lonidamine, AF-2785, axitinib, YC-1 and YD-3.