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

Abstract

A catalyst-, acid-and base-free, environmental-friendly method for synthesis of 2H-benzo[g]furo[2,3-e]indazoles, 2H-benzo[g]thieno[2,3-e]indazoles and 2H-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 2H-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 .

Photoinduced Annulation of N‐Phenylbenzamides for the Synthesis of Phenanthridin‐6(5H)‐Ones

“…Based on the experimental results and previous work, [23–27] the mechanism for the photoinduced intramolecular annulation of N ‐phenylbenzamide was proposed and shown in Scheme 5. In the beginning, it is believed that amide‐1 a is tautomerized into imine‐1 a in the presence of methanesulfonic acid.…”

“…[20] Photoinduced 6π-electrocyclization annulation is one of the good strategies for the synthesis of complex polyheterocyclic compounds, stereo compounds and fused heterocyclic aromatic hydrocarbons. [21] [22] Previously, our group developed a photo-induced 6π-electrocyclization of 3-(furan-2-yl)À 2-phenylpyridine, [23] 4-(furan-2-yl)À 3phenyl-1H-pyrazole, [24] 3-(furan-2-yl)À 4-phenyl-2Hchromen-2-one, [25] 3-(furan-2-yl)À 2-phenyl-4H-chromen-4-one [26] and 2-(furan-2-yl)À 1-phenylbutane-1,3dione [27] for the synthesis of complex polyheterocyclic aromatics, e. g. benzo…”

“…Photoinduced 6 π ‐electrocyclization annulation is one of the good strategies for the synthesis of complex polyheterocyclic compounds, stereo compounds and fused heterocyclic aromatic hydrocarbons. [21] [22] Previously, our group developed a photo‐induced 6 π ‐electrocyclization of 3‐(furan‐2‐yl)−2‐phenylpyridine, [23] 4‐(furan‐2‐yl)−3‐phenyl‐1 H ‐pyrazole, [24] 3‐(furan‐2‐yl)−4‐phenyl‐2 H ‐chromen‐2‐one, [25] 3‐(furan‐2‐yl)−2‐phenyl‐4 H ‐chromen‐4‐one [26] and 2‐(furan‐2‐yl)−1‐phenylbutane‐1,3‐dione [27] for the synthesis of complex polyheterocyclic aromatics, e. g. benzo[ h ]furo[2,3‐ f ]quinoline, 2 H ‐benzo[ g ]furo [2,3‐ e ]indazole, 4 H ‐furo[2′,3′:3,4]naphtho[2,1‐ c ]chromen‐4‐one, 13 H ‐benzo[ c ]furo[2,3‐ a ]xanthen‐13‐one, and 1‐(5‐hydroxynaphtho[2,1‐ b ]furan‐4‐yl)ethan‐1‐one.…”