Base-Induced Cycloaddition of Tosylmethyl or (tert-Butoxycarbonyl)methyl Isocyanide to 1,4-Disubstituted 2,3-Dinitro-1,3-butadienes. Access to 2,3-Disubstituted 4-Ethynylpyrroles

The reactions of 1,4-disubstituted 23-dinitro-1,3-butadienes 1a-g in THF with isocyanides XCH(2)NC (TosMIC, X = tosyl; TBICA, X = COOBu(t)) and DBU furnish good to moderate yields of 2,3-disubstituted 4- ethynylpyrroles 5a-g or 6a-g. In the reaction of Ig with TosMIC and DBU the nitrovinyl pyrrole 7g and the 3,3'-dipyrrole 4g are also isolated as by-products. A mechanism involving sequential base-induced addition, elimination and cyclization steps is advanced to account for the formation of the isolated products.