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

螺旋是蛋白质、核酸和寡糖的关键结构特征。因此，螺旋结构在仿生和合成超分子体系中受到越来越多的关注也就不足为奇了。合成体系中的螺旋结构是通过共价构象限制、金属络合、疏溶剂效应和非键合分子内相互作用实现的。最近，我们介绍了一般概念，通过适当编码的寡杂环链的分子内自组织的螺旋超结构的强制生成。因此，使用吡啶-嘧啶序列实现了螺旋诱导，所述吡啶-嘧啶序列被示出产生多达四圈的分子螺旋。在本研究中，我们将这一概念扩展到各种编码的寡杂环系统，如交替的吡啶-恶二唑，吡啶-喹喔啉，吡啶-苯并噻二唑序列，从2-吡啶羧酸和2，6-吡啶二羧酸制备了未取代的3-mer，5-mer，7-mer和9-mer交替的吡啶-恶二唑链。在质子核磁共振谱中，观察到9-mer的末端吡啶质子的明显的高场位移，表明其在CDCl_3中的螺旋自组织。另一方面，通过Stille偶联反应制备了交替的吡啶-二硫代噻吩和吡啶-喹喔啉3-聚体，并通过X射线晶体学分析确定了它们在固态下的反式选择性。此外，还制备了交替吡啶-噻吩低聚物（3-mer，7-mer和11-mer）。通过X-射线晶体学分析证实了3-mer在固态下的顺式偏好，同时观察到11-mer的NMR谱中末端噻吩质子信号的显著向上场移位。因此，交替吡啶-噻吩链的螺旋自组织稳定性被建议。

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.