Regioselective 1,3-dipolar cycloaddition reactions of unsymmetrical munchnones (1,3-oxazolium-5-olates) with 2-and 3-nitroindoles. A new synthesis ofpyrrolo[3,4-b]indoles

The unsymmetrical mesoionic munchnones 13 (3-benzyl-2-methyl-4-phenyl-1,3-o xazolium-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-nitroin dole-1-carboxylate) in refluxing THF to afford in good to excellent yields the pyrrolo[3,4-b]indoles 15 (2-benzyl-1-methyl-3-phenyl-4-carboethoxy-2,4- dihydropyrrolo[3,4-b]indole) 16 (2-benzyl-3-methyl-1-phenyl-4-carboethoxydi hydropyrrolo[3,4-b]indole), 18 (2-benzyl-1-methyl-3-phenyl-4-(phenylsulfony l)-2,4-dihydropyrrolo[3,4-b]indole), and 19 (2-benzyl-3-methyl-1-phenyl-4-( phenylsulfonyl)-2,4-dihydropyrrolo[3,4]indole). In several cases the regioc hemistry, which is opposite to that predicted by FMO theory, is very high a nd leads essentially to a single pyrrolo[3,4-b]indole; e.g., 6 + 13 --> 19 in 74% yield. (C) 2000 Elsevier Science Ltd. All rights reserved.