Ring opening of 2-substituted 4-nitrothiophenes with pyrrolidine. Access to new functionalized nitro-unsaturated building blocks

The reaction conditions of the ring-opening processes of 3-nitrothiophene 7a and of 3-nitrobenzo[b]thiophene 7b with pyrrolidine and silver nitrate were optimized as well as those of the subsequent S-methylation of the ensuing silver enethiolates 8a and 8b to 4-methylthio-2-nitro-1-pyrrolidino-1,3-butadiene 9a and 1-(2-methylthiophenyl)-1-nitro-2-pyrrolidinoethylene 9b. Under such conditions 2-X-substituted 4-nitrothiophenes 7c–i consistently gave good yields of the corresponding 4-methylthio-2-nitro-1-pyrrolidino-4-X-1,3-butadienes 9c–i. The nitroenamine derivatives 9a–i were then reacted with p-tolylmagnesium bromide to furnish moderate to good yields of 4-methylthio-2-nitro-1-(p-tolyl)-4-X-1,3-butadienes 10a,c–i and 1-(2-methylthiophenyl)-1-nitro-2-(p-tolyl)ethylene 10b. Stereochemistry of the interesting building blocks 9a–i and 10a–i was assigned on the grounds of 1H NMR data and NOE experiments.