The First Efficient Synthesis and Optical Resolution of Monosubstituted Cyclotribenzylenes
作者:Carsten Schmuck、Wolfgang Wienand
DOI:10.1055/s-2002-23541
日期:——
A new and high yielding synthetic route to monosubstituted cyclotribenzylenes 6 via the cyclocondensation of benzene with a suitably monosubstituted diol 20, obtained from ozonolysis of the corresponding dibenzosuberene precursor 19, was developed for the first time! The dibenzosuberene itself could be readily prepared in large quantities from inexpensive starting materials in five steps. Using this synthetic approach, a mono bromosubstituted cyclotribenzylene 12a was synthesized on large scale. Another four monosubstituted cyclotribenzylenes 21-24 were also prepared either via bromine/lithium exchange followed by subsequent quenching with external electrophiles or a copper mediated reaction with cyanide. These molecules adopt a rigid crown conformation as shown by X-ray analysis and temperature dependent NMR studies. The barrier to inversion is quite high, requiring temperatures well above 120 °C before inversion takes place. Futhermore, such monosubstituted cyclotribenzylenes are planar chiral and after optical resolution, using HPLC, we were able to obtain the first planar chiral C1-symmetric cyclotribenzylenes in form of the optically pure enantiomers of 12a, the CD spectra of which are exact mirror images over the entire spectral range.
首次开发了一种新颖且高产率的合成路线,通过苯与适当单取代二醇20的环加成反应,制备单取代环三苯基烯6。该单取代二醇20源自相应二苯基烯前体19的臭氧化。二苯基烯本身可从廉价原料经过五步骤大量制备。运用此合成策略,实现了大规模制备单溴取代环三苯基烯12a。进一步通过溴锂交换后续用外部亲电试剂淬灭或铜介导的氰化反应,还合成了另外四种单取代环三苯基烯21至24。X射线分析与温度依赖性核磁共振研究表明,这些分子采取了刚性的冠形构象。翻转能垒较高,温度需远超120°C才发生翻转。此外,这类单取代环三苯基烯具有平面手性,经高效液相色谱光学拆分,我们首次获得了平面手性C1对称环三苯基烯,即光学纯的12a对映体,其圆二色谱在整个光谱范围内呈现精确的镜像关系。