Synthesis of2H‐benzo[g]furo/thieno/pyrrolo[2,3‐e]indazolesviaIntramolecular Dehydrogenation Photocyclization

Main observation and conclusion A catalyst‐, acid‐ and base‐free, environmental‐friendly method for synthesis of 2 H ‐benzo[ g ]furo[2,3‐ e ]indazoles, 2 H ‐benzo[ g ]thieno[2,3‐ e ]indazoles and 2 H ‐benzo[ g ]pyrrolo[2,3‐ e ]indazoles via UV light irradiation of 3‐phenyl‐4‐(2‐heteroaryl)pyrazoles (aryl = furanyl, thiophenyl and N ‐methylpyrrolyl) in EtOH/H 2 O at room temperature under argon atmosphere was described. Irradiation of 3‐(2‐hydroxyphenyl)‐4‐(2‐heteroaryl)pyrazoles showed a high chemo‐selectivity to obtain dehydrogenation product 2 H ‐benzo[ g ]furo/ thieno/pyrrolo[2,3‐ e ]indazols‐10‐ol. The mechanism of photocyclization was expounded through the process of 6π‐electroncyclization, [1,5]‐hydrogen shift, pyrazole tautomerism, 1,3‐eneamine tautomerism and evolution of H 2 .