[4 + 2]-Cycloadditions of 3-amino-4-imino-4H-thieno[3,4-c][1]benzopyran with some selected dienophiles

Abstract

3-Amino-4-imino-4H-thieno[3,4-c][1]benzopyran 1 adds dimethyl maleate 2 and dimethyl acetylenedicarboxylate 6 in a [4 + 2] mode across the thiophene ring and reacts with 2,3-dichloro-1,4-naphthoquinone 12 by substitution of both chlorine atoms. All primary adducts, 3, 7 and 13, however, are unstable under the reaction conditions. The structures of the final products 4, 10 and 15 have been assigned from their spectral data and their formation has been rationalized in terms of transformations available to the assumed primary products.