Solvent controlled divergent syntheses of polysubstituted pyrroles and pyrrolo[2,3-b]-1,4-thiazines

The reaction of 2-imidazolyl carbene-derived 2-arylthiocarbamoyl imidazolium salts with ethyl propiolate proceeded via the solvent dependent tandem [3+2] cycloaddition and subsequent ring transformation processes. While the reaction in butanone yielded 3-amino-2-vinylthiopyrroles as the major products, use of benzene as the solvent led to the predominant formation of pyrrolo[2,3-b][1,4]thiazines. This work disclosed a unique ring transformation of imidazoline-spiro-pyrrolines to pyrrolo[2,3-b][1,4]thiazines, which provides a simple route to both multifunctional pyrroles and pyrrolo[2,3-b][1,4]thiazines that are otherwise difficult to synthesize.