Synthesis of the first thieno-δ-carboline: Fluorescence studies in solution and in lipid vesicles

The first thieno-δ-carboline (6,8,9-trimethyl-5H-pyrido[3,2-b]thieno[3,2-f]indole) was prepared in good yield (70%) by intramolecular palladium-assisted cyclization of an ortho-chlorodiarylamine. The latter was in turn selectively synthesized in high yield (90%) by C–N palladium-catalyzed cross-coupling of 3-bromo-2-chloropyridine with, the also prepared, 6-amino-2,3,7-trimethylbenzo[b]thiophene. Fluorescence studies in solution show that thieno-δ-carboline has a solvatochromic behaviour. Despite the low fluorescence quantum yields in solution, studies of its incorporation in lipid vesicles of DPPC, DOPE and DODAB indicate that thienocarboline is located mainly inside the lipid bilayer, exhibiting different behaviours in gel or liquid-crystalline phases. Our studies are useful for the incorporation of thienocarboline in liposomes and for controlled drug release assays, due to its biological activity.