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

Diels‐Alder reaction of 2‐( E ‐2‐nitroethenyl)‐1 H ‐pyrrole ( 2a ) with 1,4‐benzoquinone gave the desired benzo[ e ]indole‐6, 9(3 H )‐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 a phenylsul‐fonyl 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(3 H )‐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.