Synthesis and thermally stable helix-dimer formation of amidohelicene oligomers

Optically active amidohelicene monomer to nonamer were synthesized in high yields by a two-directional method. The CD spectra in chloroform exhibited a large difference between dimer and the higher homologs, and vapor pressure osmometry studies revealed the formation of dimeric aggregates for the latter. It is noted that amidohelicene oligomers possessing two-atom linking groups between helicene and m-phenylene spacer formed helix-dimers in solution as were ethynylhelicene oligomers. The helix-dimer of the amidohelicene octamer in chloroform was very stable, and did not dissociate at 5 × 10^-8M on heating to 60 °C. The dissociation of the amidohelicene oligomers to random-coil state took place in hydrogen-bonding breaking solvents, DMSO or THF. The equilibrium between helix-dimer and random-coil changed by varying the ratio in the mixed solvents of chloroform and DMSO. Notably, the equilibriums were not affected by temperature for various mixtures of helix-dimer and random-coil. Thus, the sensitivity toward the environment was quite different between the amido- and ethynylhelicene oligomers.