Regioselective 1,3-Dipolar Cycloaddition Reactions of Unsymmetrical Münchnones (1,3-Oxazolium-5-olates) with 2- and 3-Nitroindoles. A New Synthesis of Pyrrolo[3,4-b]indoles |
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Authors: | Gordon W Gribble Erin T Pelkey Wendy M Simon Hernando A Trujillo |
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Institution: | Department of Chemistry, Dartmouth College, Hanover, NH 03755, USA |
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Abstract: | The unsymmetrical mesoionic münchnones 13 (3-benzyl-2-methyl-4-phenyl-1,3-oxazolium-5-olate) and 14 (3-benzyl-4-methyl-2-phenyl-1,3-oxazolium-5-olate) react with the N-protected 2- and 3-nitroindoles 1 (ethyl 2-nitroindole-1-carboxylate), 6 (3-nitro-1-(phenylsulfonyl)indole), and 17 (ethyl 3-nitroindole-1-carboxylate) in refluxing THF to afford in good to excellent yields the pyrrolo3,4-b]indoles 15 (2-benzyl-1-methyl-3-phenyl-4-carboethoxy-2,4-dihydropyrrolo3,4-b]indole), 16 (2-benzyl-3-methyl-1-phenyl-4-carboethoxy-2,4-dihydropyrrolo3,4-b]indole), 18 (2-benzyl-1-methyl-3-phenyl-4-(phenylsulfonyl)-2,4-dihydropyrrolo3,4-b]indole), and 19 (2-benzyl-3-methyl-1-phenyl-4-(phenylsulfonyl)-2,4-dihydropyrrolo3,4-b]indole). In several cases the regiochemistry, which is opposite to that predicted by FMO theory, is very high and leads essentially to a single pyrrolo3,4-b]indole; e.g., 6+13→19 in 74% yield. |
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Keywords: | münchnone 2-nitroindole 3-nitroindole 1 3-dipolar cycloaddition 1 3-oxazolium-5-olates pyrrolo[3 4-b]indole |
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