Diels-Alder reactions of 2-(2-nitroethenyl)-1H-pyrroles and their oxygen and sulfur analogs with quinones

Diels-Alder reaction of 2-(E-2-nitroethenyl)-1H-pyrrole (2a) with 1,4-benzoquinone gave the desired benzo[e]indole-6, 9(3H)-dione (4a) in 10% yield versus a 26% yield (lit. 86% [5]) of the known N-methyl compound (4b) from the N-(or 1)-methyl compound (2b). Protection of the nitrogen of 2a with aphenylsulfonyl group (2c) gave a 9% yield of the corresponding N-(or 3)-phenylsulfonyl compound (4c). The reaction of 2b with 1,4-naphthoquinone gave in 6% yield (lit. 64% [5]) the known 3-methylnaphtho[2,3-e]-indole-6, 9(3H)-dione (6). The reaction of 2-(E-2-nitroethenyl)furan (8a) gave a small yield of the desired naphtho[2,1-b]furan-6, 9-dione (9a), recognized by comparing its NMR spectrum with that of 4b, The corresponding reaction of 2-(E-2-nitroethenyl)thiophene (8b) gave a 4% yield of naphtho[2,1-b]thiophene-6, 9-dione (9b), previously prepared in 24% yield [12] in a three-step procedure involving 2-ethenylthiophene. Introducing an electron-releasing 2-methyl substituent into 8a and 8b gave 12a and 12b, which, upon reaction with 1,4-benzoquinone, gave 2-methylnaphtho[2,1-b]furan-6, 9-dione (13a) and its sulfur analog (13b) in yields of 4 and 8%, respectively.