Fluorescent Short‐Stranded Helical Foldamers Based on L‐shaped Dibenzopyrrolo[1,2‐ a ][1,8]naphthyridine

M/P Helicity Switching and Chiral Amplification in Double-Helical Monometallofoldamers

Short-stranded double-helical monometallofoldamers capable of M/P-switching were constructed by the complexation of two strands, each with two L-shaped units linked by a 2,2'-bipyridine, with a Zn(II) cation. The helix terminals of the "double-helical form" folded by π-π interactions can unfold in solution to equilibrate with the "open forms" that are favored at higher temperatures. Interestingly, the helical chirality of the monometallofoldamers with chiral side chains induced a single-handed helix sense and controlled M/P-switching depending on achiral solvent stimuli. For instance, the (M)-helicity was favored in nonpolarized solvents (toluene, hexane, Et2O), whereas the (P)-helicity was favored in Lewis basic solvents (acetone, DMSO). Circular dichroism (CD) and rotating-frame overhauser enhancement spectroscopy (ROESY) measurements revealed that the conformational change of the chiral side chains due to interaction of Lewis basic solvents with the double helices induced helicity bias. These novel double-helical monometallofoldamers possessed a stable helical structure and exhibited switchable chiroptical properties (gabs ∼ 10-3-10-2). In addition, the chiral strand exhibited chiral transfer and amplification abilities through the formation of chiral heteroleptic double-helical monometallofoldamers when mixed with an achiral strand.

Spacer-Dependent Cooperativity of Helicity in Fluorescent Bishelical Foldamers Based on L-Shaped Dibenzopyrrolo[1,2-a][1,8]naphthyridine

For the construction of helical foldamers composed of π-frameworks, the choice of appropriate π-π stacking units and π-spacers connecting them is important. The transfer of helicity between the minimal helix structural units is also an essential factor in the construction of homochiral helical foldamers. Tetramers 4 a-4 d, which have four L-shaped dibenzopyrrolo[1,2-a]naphthyridine units, were synthesized to investigate the interplay and cooperativity of the helical structures. Tetramer 4 a bridged with a biphenyl unit formed a homochiral bishelical structure with π-π stacking between the L-shaped units (3.3 Å), consisting only of (P,P)- and (M,M)-enantiomers without the (P,M)-diastereomer, owing to interplay through the axial chirality of biphenyl unit in the solid state. Similarly, in solution, thermodynamic stabilization of the two helix formations worked cooperatively to favor the bishelical form of 4 a. Furthermore, bishelical foldamer 4 a emitted intense fluorescence (Φ=0.86).

Fused chromeno and quinolino[1,8]naphthyridines: Synthesis and biological evaluation as topoisomerase I inhibitors and antiproliferative agents

The synthesis of 1,8-naphthyridine derivatives fused with other heterocycles, such as chromenes and quinolines, as well as their behaviour as topoisomerase I inhibitors is studied. The preparation is carried out through a direct and simple process as an intramolecular [4 + 2] cycloaddition reaction between functionalized aldimines, obtained by the condensation of 2-aminopyridine and unsaturated aldehydes, and olefins. In particular, while no clear inhibitory activity is observed for chromeno[4,3-b][1,8]naphthyridine fused heterocycles, a very different result is observed for quinolino[4,3-b][1,8]naphthyridine derivatives. Experimental assays indicated that quinolino[4,3-b][1,8]naphthyridines inhibited the topoisomerase I enzymatic reaction behaving like a poison, as occurs with the natural TopI inhibitor, camptothecin. Furthermore, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549), human ovarian carcinoma (SKOV3), and on non-cancerous lung fibroblasts cell line (MRC5) was also screened.

Erbium anchored iminodiacetic acid (IDA) functionalized CoFe2O4 nano particles: an efficient magnetically isolable nanocomposite for the facile synthesis of 1,8-naphthyridines

We report a novel magnetic catalyst for the synthesis of biologically important compounds.