Synthesis of spiropyrans and merocyanine dyes based on 1-benzothieno[3,2-b]pyrrole

A convenient method was developed for the synthesis of photochromic spiropyrans and merocyanine dyes of the 1-benzothieno[3,2-b]pyrroline series based on the conventional condensation of the analog of Fischer’s salt, viz., 1,2,3,3-tetramethyl-3H-[1]benzothieno-[3,2-b]pyrrolium triflate, with aromatic 2-hydroxyaldehydes. Unlike Fischer’s salt, benzothieno-pyrrolium salts readily undergo the [1,5]-sigmatropic rearrangement giving rise to 2H-[1]benzo-thieno[3,2-b]pyrrole derivatives. The latter compounds were used for the synthesis of the first representatives of the previously unknown spiro compounds and merocyanine dyes. The long-wavelength maxima of merocyanines of the 2H-benzothienopyrrole series are batho-chromically shifted by more than 100 nm with respect to the absorption maxima of the “classical” 3H-benzothienopyrrole analogs.