Widjaja Tien, Fitjer Lutz, Pal Aritra, Schmidt Hans-Georg, Noltemeyer Mathias, Diedrich Christian, Grimme Stefan
Institut für Organische und Biomolekulare Chemie, Universität Göttingen, Tammannstr 2, 37077, Göttingen, Germany.
J Org Chem. 2007 Nov 23;72(24):9264-77. doi: 10.1021/jo7017558. Epub 2007 Nov 1.
The pseudohelical hydrocarbons (R)-6, (S)-7, and (R)-8 and the helical hydrocarbon (P)-9, formally derived from the helical hydrocarbon (P)-4 by stepwise replacement of each of the four-membered rings by a five-membered ring, have been prepared. Their optical rotations vary systematically, both in magnitude and sign. Of the extremes, (P)-4 represents the usual case of a right-handed dextrorotatory helix, while (P)-9 represents the unusual case of a right-handed levorotatory helix. To rationalize these facts, DFT calculations of the rotatory power of (P)-helices of three-, four-, and five-membered rings have been performed. The results show a very good agreement with the experimental data for the rigid helices of three-membered rings and always show the correct sign and order of magnitude for the flexible helices of four- and five-membered rings for which Boltzmann-averaged optical rotations of up to six conformers had to be used. Within the conformers of the latter, a set of large dihedral angles for the bonds of the inner sphere correspond to a high specific rotation, and a set of small dihedral angles correspond to a low specific rotation. As a consequence, the Boltzmann-averaged values markedly depend on the geometry and weight of the conformers involved.
已制备出假螺旋烃（R）-6、（S）-7和（R）-8以及螺旋烃（P）-9，它们是通过用五元环逐步取代螺旋烃（P）-4中的每个四元环而正式衍生得到的。它们的旋光性在大小和符号上都有系统地变化。在这些极端情况中，（P）-4代表右手右旋螺旋的常见情况，而（P）-9代表右手左旋螺旋的不寻常情况。为了合理解释这些事实，已对三元、四元和五元环的（P）-螺旋的旋光能力进行了密度泛函理论（DFT）计算。结果表明，对于三元环的刚性螺旋，计算结果与实验数据非常吻合，对于四元和五元环的柔性螺旋，计算结果始终显示出正确的符号和量级，对于后者，必须使用多达六个构象异构体的玻尔兹曼平均旋光性。在后者的构象异构体中，内球键的一组大的二面角对应于高的比旋光度，而一组小的二面角对应于低的比旋光度。因此，玻尔兹曼平均值明显取决于所涉及构象异构体的几何形状和权重。
