NMR and CD Spectral Features of Thiaheterohelicenes. Effect of Intramolecular Overlap of Aromatic Rings

NMR and CD spectral features of a series of thiaheterohelicenes containing alternant thiophene and benzene rings were examined as associated with their helical structures. The terminal ring protons of the thiaheterohelicenes reflected the characteristics of intramolecular overlap of aromatic rings in 1H NMR spectra. With the progression of the helix a sudden chemical shift of the β proton in the end thiophene ring to the higher field occurred, in contrast to carbohelicenes in which the drift of the upfield shift of the corresponding terminal ring proton was gradually suppressed. The CD spectra of thiaheterohelicenes were quite different from those of carbohelicenes. The intensities of the α and β absorption bands weakened with the progressive annelation, whereas the intensity of the p absorption band gradually increased, accompanied by a successive increase in the anisotropy factor of the band. These facts suggest that the structures of thiaheterohelicenes differ from those of carbohelicenes in having a peculiar distortion of the helix.