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

螺旋结构是蛋白质、核酸和低聚糖的关键结构特征。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。通过共价构象限制、金属络合、疏溶剂效应和分子内非键相互作用，在合成体系中形成螺旋结构。我们最近介绍了通过适当编码的寡杂环链的分子内自组织强制产生螺旋超结构的一般概念。因此，螺旋诱导是利用吡啶-嘧啶序列实现的，它被证明可以产生多达四圈的分子螺旋。在这项研究中，我们将这一概念扩展到各种编码的寡杂环系统，如吡啶-恶二唑、吡啶-吡喃噻吩、吡啶-喹啉和吡啶-苯并噻唑交替序列。以2-吡啶二羧酸和2,6-吡啶二羧酸为原料，制备了3、5、7、9聚吡啶-恶二唑交替链。在质子核磁共振谱中，9-mer末端的吡啶质子明显上移，表明其在CDCl_3中的螺旋自组织。另一方面，采用Stille偶联反应制备了吡啶-吡喃噻吩和吡啶-喹啉3-聚物，并通过x射线晶体学分析确定了它们在固态中的跨固体偏好。此外，还制备了吡啶-噻吩交替低聚物（3-聚合体、7-聚合体和11-聚合体）。通过x射线晶体学分析证实了3-mer在固态中的顺流偏好，同时观察到11-mer的核磁共振谱中末端噻吩质子信号有明显的上移。由此提出了吡啶-噻吩交替链的螺旋自组织稳定性。

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.