Synthesis of fluorescent tetracyclic lactams by a “one pot” three steps palladium-catalyzed borylation, Suzuki coupling (BSC) and lactamization. DNA and polynucleotides binding studies

Abstract

Tetracyclic lactams (benzothieno[2,3-c]quinolones) were prepared by “one pot” three steps palladium-catalyzed borylation, Suzuki coupling (BSC) and lactamization, starting from ortho-haloanilines and alkyl 3-bromobenzo[b]thiophene-2-carboxylates. The former were used as the components to be borylated with pinacolborane, and the latter as the brominated component in the Suzuki coupling. The amidation occurred with loss of the alkyl alcohol, presumably in the Suzuki coupling product, giving the corresponding tetracyclic lactam. This constitutes a novel application of the BSC reaction using sterically hindered substrates. In this work studies of absorption and fluorescence in several solvents and in presence of salmon sperm DNA or synthetic double stranded (ds) heteropolynucleotides, poly(dA-dT)•(dA-dT) and poly(dG-dC)•(dG-dC), were performed. The binding constants values (Ki = 2.6x10^5- 4.5x10^5 M^-1) point to a high affinity of the lactams to DNA. It was shown that the intercalation is the prefered mode of binding and that the substituted new lactams (with F or OMe) exhibit a higher affinity for A-T regions. Quenching experiments with iodide show that the methoxylated lactam is the more intercalative in DNA. The same type of experiments using this compound bound to heteropolynucleotides show a very low accessibility (fa = 0.07) of the lactam in poly(dG-dC)•(dG-dC) to the quencher showing a large majority of intercalative binding while the high affinity for A-T regions together with a higher accessiblity (fa = 0.25) point to the possiblity of intercalative and groove binding.