Design, Synthesis and Helical Self-Organization of Pai-Electronic Oligomers with Multi-Functions

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Helices are key structural features of proteins, nucleic acid, and oligosaccharides. It is therefore no surprise that helical structures have received increasing attention in biomimetic and synthetic supramolecular systems. Helical organization in synthetic system has been obtained through covalent conformational restrictions, metal complexation, solvophobic effects, and nonbonding intramolecular interactions. We have recently introduced general concept for the enforced generation of helical superstructures by intramolecular self-organization of a suitably encoded oligoheterocyclic strand. Thus, helical induction was achieved using pyridine-pyrimidine sequences, which were shown to generate molecular helices up to four turns. In this study we have extended this concept to a variety of encoded oligoheterocyclic systems such as alternating pyridine-oxadiazole, pyridine-pyradinothiophene, pyridine-quinoxaline, and pyridine-benzothiadiazole sequences.The unsubstituted 3-mer,5-mer,7-mer and 9-mer of alternating pyridine-oxadiazole strands were prepared from 2-pyridinecarboxylic acid and 2,6-pyridinedicarboxylic acid. In the proton NMR spectrum, distinct up-field shift was observed for the terminal pyridine protons of the 9-mer, indicating its helical self organization in CDCl_3. On the other hand, alternating pyridine-pyradinothiophene and pyridine-quinoxaline 3-mers were prepared using Stille coupling reaction, and their transoid preference in the solid state was established by X-ray crystallographic analysis. Moreover, alternating pyridine-thiophene oligomers (3-mer, 7-mer and 11-mer) were also prepared. The cisoid preference of the 3-mer in the solid state was confirmed by X-ray crystallographic analysis while the marked up-field shift for the terminal thiophene proton signals in the NMR spectrum of 11-mer was observed. Thus, helical self- organization sbility of the alternating pyridine-thiophene strand was suggested.