作者:Edzard M. Geertsema、Anne Marie Schoevaars、Auke Meetsma、Ben L. Feringa
DOI:10.1039/b609271c
日期:——
The concept of bisthioxanthylidene biscrown ethers as potential stereodivergent chiral ligands in asymmetric synthesis is introduced. Substituted bisthioxanthylidenes may be chiral and can exist as stable enantiomers due to their folded structure. As a result, both a right-handed helix (P) and left-handed helix (M) are present in this type of molecule. This offers the unique possibility to construct two crown ether moieties, attached to the same molecule, of which one exhibits (P)-helicity and the other (M)-helicity. When the crown ether moieties differ in size they can be complexed selectively with a base containing a cation of appropriate diameter. In this manner the (P)-helix and the (M)-helix can be activated selectively to serve as a chiral environment for base catalyzed asymmetric synthesis. Thus, we envisioned the new concept of a single chiral ligand to separately synthesize two enantiomers of a chiral product just by varying the added base. For this purpose, four new bisthioxanthylidene monocrown ethers and two new bisthioxanthylidene biscrown ethers were synthesized. Two biscrowns and two monocrowns were separated into their respective enantiomers (HPLC) and optical data (UV and CD) were collected to ensure stability of enantiomers at ambient temperatures. Ion complexation of one mono- and two biscrown ethers with potassium and sodium cations was investigated.
介绍了双硫杂蒽亚基双冠醚作为不对称合成中潜在的立体发散手性配体的概念。取代的双硫代亚甲基可能是手性的,并且由于其折叠结构而可以作为稳定的对映体存在。因此,此类分子中同时存在右旋螺旋 (P) 和左旋螺旋 (M)。这提供了构建连接到同一分子的两个冠醚部分的独特可能性,其中一个表现出(P)-螺旋性,另一个表现出(M)-螺旋性。当冠醚部分尺寸不同时,它们可以选择性地与含有适当直径的阳离子的碱络合。以这种方式,(P)-螺旋和(M)-螺旋可以被选择性地激活以充当碱催化不对称合成的手性环境。因此,我们设想了单一手性配体的新概念,仅通过改变添加的碱基即可单独合成手性产物的两种对映体。为此,合成了四种新的双冠醚和两种新的双冠醚。将两个双冠和两个单冠分离成各自的对映体 (HPLC),并收集光学数据(UV 和 CD)以确保对映体在环境温度下的稳定性。研究了一种单冠醚和两种双冠醚与钾和钠阳离子的离子络合。